Symposium EL01 : Emerging Material Platforms and Approaches for Plasmonics, Metamaterials and Metasurfaces

Over the last several years, there has been intensive worldwide effort to explore dynamic and reconfigurable nanophotonics, driven mainly by innovation at the component level. True two-dimensional optical phase array metasurfaces with reconfigurable elements represent a new opportunity for nanophotonics, and here we describe a design approach linking nanophotonic device and array-level architectural design. The results show a surprising ability to achieve near-ideal optical phased array performance for several functions, including beam steering and reconfigurable lens operation, even with the use of highly non-ideal individual nanophotonic devices. We will show experimental results for a ‘universal’ reconfigurable metasurface in which a single dynamically reconfigurable optical phased array aperture can be used to perform multiple functions.

The isolation of stable atomically thin two-dimensional (2D) materials on arbitrary substrates has led to a revolution in solid state physics and semiconductor device research over the past decade. A variety of other 2D materials (including semiconductors) with varying properties have been isolated raising the prospects for devices assembled by van der Waals forces.¹ A fundamental challenge in using 2D materials for opto-electronic devices is enhancing their interaction with light, ultimately responsible for higher performance and efficiency in the devices. In particular, for photovoltaics; inorganic materials (e.g., Si, GaAs and GaInP) can concurrently maximize absorption and carrier collection. But thin film absorbers have lacked the above ability often due to due to surface and interface recombination effects. In contrast, Van der Waals semiconductors have naturally passivated surfaces with electronically active edges that allows retention of high electronic quality down-to the atomically thin limit. First, I will show our recent work on photovoltaic devices from transition metal dichalcogenides of molybdenum and tungsten such as MoS₂, WSe₂ etc.^2-4 as well as more recent work on high open circuit voltage devices.
Next we will focus on the subject of strong light-matter coupling in excitonic 2D semiconductors. Visible spectrum band-gaps with strong excitonic absorption makes transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as attractive candidates for investigating light matter interaction and applications as absorbing media in opto-electronics.^{3, 5} Further, the excitonic features become more prominent as the layers are thinned down and dominant in the monolayer limit where the TMDCs transition into direct band-gap semiconductors with strong photoluminescence. In addition, TMDCs are known to have very large values of optical constants which allows strong light trapping even in ultrathin samples.² We will present our recent work on the fundamental physics of light trapping in multi-layer TMDCs when coupled to plasmonic substrates. We systematically demonstrate via calculations and matching experiments that the presence of strong excitonic resonances in multilayers (< 20 nm thickness) combined with surface plasmon excitations of the nearby metals can achieve strongly coupled modes with apparent voided crossings in reflectance spectra. Further, we explore additional light confinement by patterning 1D arrays of rectangular resonators of varying widths and periods (100 nm to 500 nm). We observe newer and higher order modes appear with increasing TMDC thicknesses and widths. Simulated field profiles suggest that these modes range from mainly plasmonic to even hybrid nature as well as guided modes for longer wavelengths and thicker TMDCs. Further, the plasmonic mode exhibit strong dependence on 1D array grating period.
References:
1. Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. ACS Nano 2014, 8, (2), 1102–1120.
2. Jariwala, D.; Davoyan, A. R.; Tagliabue, G.; Sherrott, M. C.; Wong, J.; Atwater, H. A. Nano Letters 2016, 16, (9), 5482-5487.
3. Jariwala, D.; Davoyan, A. R.; Wong, J.; Atwater, H. A. ACS Photonics 2017, 4, 2692-2970.
4. Wong, J.; Jariwala, D.; Tagliabue, G.; Tat, K.; Davoyan, A. R.; Sherrott, M. C.; Atwater, H. A. ACS Nano 2017, 11, 7230–7240.
5. Brar, V. W.; Sherrott, M. C.; Jariwala, D. Chemical Society Reviews 2018, 47, (17), 6824-6844.

Plasmonic nanocircuits have the potential to open new routes in manipulating optical information beyond the diffraction limit and future quantum information processing technologies. For the realization of multi-functional plasmonic devices, it is of major importance to control properties of their supporting guided optical modes such as modal profiles, impedances and propagation constants. In this talk, several ultra-compact active plasmonic configurations are considered from the viewpoint of realizing important photonic functionalities, including on-chip detection of optical spin-orbit interactions and high-speed electrooptic switching and modulation.

Metalenses consist of a large number of optical nanoantennas which are capable of focusing the incoming wavefront of light [1-6]. We use a 60 × 60 dielectric achromatic metalens array to capture multidimensional optical information. The highest efficiency can be up to 74% at a wavelength of 420 nm, while the average efficiency is approximately 39% over the whole working bandwidth. The light field images and the depth information of objects can be determined by reorganizing the patches of sub-images and calculating the disparity of neighbor sub-images, respectively. The depth information can be used to optimize the patch sizes to render the all-in-focus images without artifacts. The smallest feature of objects that could be resolved in our system is 1.95 μm under the incoherent white light. Our work provides several advantages associated with light field imaging: elimination of chromatic aberration, polarization selectivity and compatibility of the semiconductor process. Considering the flexibility, the achromatic multiplexed metalens array with integrated functionalities may be promising for multifocusing microscopy, high-dimension quantum technology, hyperspectral microscopy, micro robotic vision, nomen automobile sensing, virtual and augmented reality (VR and AR), drones, and miniature personal security systems [7].
References
1 S. M. Wang, et al., Nature Comm. 8, 187 (2017).
2 B. H. Chen, et al., Nano Lett. 17, 6345 (2017).
3 S. M. Wang, et al., Nature Nanotechnology 13, 227 (2018).
4 V.-C. Su, C. H. Chu, G. Sun and D. P. Tsai, Optics Express 26, 13148 (2018).
5 M. L. Tseng, et al., Adv. Optical Mater. 6, 1800554 (2018).
6 H.H. Hsiao, et al., Adv. Optical Mater. 6, 1800031 (2018).
7 R. J. Lin, et al., Nature Nanotechnology, 14, 227 (2019)

Metasurfaces possess an outstanding ability to tailor the phase, amplitude and even spectral responses of light with unprecedented ultrahigh spatial resolution, thus have attracted significant interests for several applications in optics. Here, we propose and experimentally demonstrate a novel meta-device that integrates color printing and computer-generated holograms within a single-layer dielectric silicon metasurface by modulating spectral and spatial responses at the subwavelength scale, simultaneously. With our design, such a metasurface appears as a microscopic color image under white light illumination, while encrypting two different holographic images that can be projected to the far field when illuminated with red and green laser beams. The metasurfaces consist of two types of meta-atoms made by amorphous silicon, each of them acts as a color filter under white light and provides a color channel for a specific wavelength to independently manipulate phase distributions by utilizing their orientations angles. Both the phase and spectral responses can be defined at a subwavelength scale simultaneously and independently. For the hologram design, we developed a modified parallel iterative Gerchberg-Saxton algorithm, which obtains holograms for arbitrary shapes to adapt “color-printing” indexed pattern. Such an algorithm is the key to the wavelength multiplexing holograms by utilizing the color filter property (wavelength selectivity) of the two designed meta-atoms. The method can further extend the design freedom of metasurfaces. By exploiting spectral and spatial control at the level of individual pixels formed by the meta-atoms, multiple sets of independent information can be introduced into a single-layer device that requires only a single lithography step. The additional complexity and enlarged information capacity are promising for novel applications such as information security and anti-counterfeiting.

Active tuning of nanophotonic devices has many potential applications. Such tuning can be achieved by changing either the shape or material properties of a structure. Phase-change materials, such as Germanium Antimony Tellurium (GST), are of particular interest as they can exhibit large and non-volatile changes in their refractive index. Pulsed laser illumination has effectively been used to induce phase transitions in GST-based optical devices. Such changes can cause large changes in the optical scattering properties in the near-infrared. However, electrical tuning of GST-based photonic antennas and metamaterials has remained elusive.
Here we present optical antennas and metasurfaces combining phase-change material and plasmonic structures. By inducing phase-changes electrically in optical antennas, we achieve reversible multi-level tuning of scattered light intensity by more than 30%. Metasurfaces, designed as a perfect absorber, show over a 3-fold enhancement of reflection in the visible wavelength range between phases. This work demonstrates a first GST-based active metasurfaces working in the visible wavelengths, showing the potential to develop randomly-accessible metamaterial platforms in which metamolecules can be individually controlled.

Photonic systems promise larger bandwidths, increased speeds, and reduced power consumption compared to their electronic counterparts. An optical diode analogous to an electrical diode represents the most fundamental device needed to realize such photonic platforms. Current optical diodes rely on the relatively weak magneto-optical effect, which requires high magnetic field strengths and long optical path lengths (> 100 µm) to break reciprocity. In order to miniaturize optical nonreciprocity and develop an optical diode that is scalable with current electronics (< 1 µm), we present a scheme based on Stimulated Raman Scattering (SRS) and discuss the design of an intrinsically chiral metasurface that supports diode-like behavior when combined with SRS.

Optical chirality in a dielectric metasurface requires coupled, non-orthogonal electric and magnetic dipole moments. Using full-field electromagnetic simulations, we investigate a dielectric metasurface comprised of cylinders that allow for the individual tuning of electric and magnetic Mie resonances by modifying their diameter and height, respectively. For a Si metasurface with a cylinder height of 600 nm and diameter of 500 nm organized into a square lattice with a period of 1.2 µm, transmission approaches unity at a wavelength of 1830 nm – a result of the overlap of the electric and magnetic dipole moments. Next, we break the orthogonality of electric and magnetic modes by introducing a notch into the center of the cylinder. We sweep notch depths from 50 nm to 300 nm with a notch diameter of 80 nm. The effective electric current loop that defines the magnetic dipole resonance is perturbed by the introduction of the notch, thus resulting in nonorthogonal dipole moments and a resonance in transmission at 1830 nm.By offsetting the notch in different directions in neighboring disks, we break four-fold rotational symmetry for the entire unit cell both geometrically and optically. This geometry exhibits a significant chiral response. For a metasurface array with a lattice period of 1.2 µm, cylinder height of 640 nm, diameter of 500 nm, and optimized notch positions, the difference in transmittance between L-CP and R-CP exceeds three orders of magnitude at a resonant wavelength of 1895.5 nm. This resonance exhibits a Q-factor exceeding 10⁴and can be optimized for even higher Q-factors by shifting the magnetic dipole moment via the height of the cylinders.

Next, we show how this chiral response can enable all-optical isolation through SRS. The probe beam frequency is set to the chiral resonance defined at 1895.5 nm. The pump frequency is Stokes-shifted by the frequency of the optical phonon (15.6 THz in Si) and is set to 1725.3 nm. Importantly, the pump handedness is fixed to L-CP and the probe must also be L-CP to transmit with amplification; a R-CP probe would be orthogonal to the L-CP pump and would not experience amplification. With probe illumination from the backwards direction, a L-CP probe will not experience amplification due to its orthogonality with the rotation of the pump, and a R-CP probe will be reflected by the chirality of the metasurface. This system results in a highly-asymmetric response, where transmission is enhanced for only the L-CP probe in the same direction as the pump illumination, thus resulting in near diode-like behavior for pump powers >2 MW/cm². For a pump power of 5 MW/cm², the L-CP probe in the forward direction experiences transmittance >2x more than any other probe handedness and direction. This asymmetry in transmission can be further enhanced by optimizing a geometry that minimizes polarization conversion, approaching values comparable with electronic diodes. This work presents both a general exploration of intrinsic chirality in metamaterials and an entirely new scheme for achieving optical isolation at the nanoscale.

The reflection of light from metallic mirrors results in a near-zero electric field at their surface. This precludes strong light-matter interaction between such mirrors and two-dimensional (2D) materials placed in direct contact with them. Patterning of metal surfaces with sub-wavelength grooves can produce anisotropic metasurfaces that offer robust enhancements in the magnitude of fields near the surface, and control over their direction. Here, we use this control to analyze the Raman tensor for vibrational modes of atomically-thin graphene.

To study metasurface-enhanced Raman scattering from 2D materials, we pattern sub-wavelength groove arrays into a gold surface by means of focused ion beam milling. Optical reflection measurements show clear absorption resonances associated with gap plasmon modes excited in the grooves, with a resonance wavelength that can be controlled by varying the groove depth. Metasurfaces with groove depths ranging from 30-210 nm were prepared, and monolayer graphene was transferred onto the patterned surfaces. Reference Raman spectra taken on a smooth gold film under 532 nm excitation show a weak Raman signal from the graphene G peak and 2D peak. Spectra taken on the nearby patterned region show clear enhancement of the graphene G peak and 2D peak, with maximum Raman enhancement occurring for a groove depth of 70 nm. Notably, a much larger enhancement factor of ~50 was observed for the 2D peak compared to a factor ~25 for the G peak. We show that this vibrational mode-dependent enhancement can be quantitatively understood by considering the anisotropic nature of the metasurface and the specific tensorial nature of the Raman polarizability of the graphene vibrational modes. Numerical simulations of the enhancement as a function of groove depth show remarkable agreement with the measured data. Our findings demonstrate that anisotropic metasurfaces can be used as reliable and tunable surface enhanced Raman scattering substrates for the investigation of the vibrational modes of 2D materials.

We outline future directions in the development of a platform for high-speed integrated quantum photonics, the use of plasmonics to outpace quantum decoherence and the application of machine-learning techniques for photonics designs and quantum optical measurements.

We show both experimentally and theoretically that dielectric nanowires made of Silicon (Si-NW) are efficient waveguides allowing the wavelength-dependent transfer of visible photons from broadband quantum emitters. We first study the photodynamics of single NV centers in nanodiamonds positioned in the vicinity of such high index dielectric nanowires. Then, we demonstrate that the 1D propagation of light can be efficiently controlled by the geometry of the wire. These emitter-nanowire hybrid structures might be good candidates as building blocks for the design of CMOS-compatible optical nanodevices operated in the single photon regime.
Discussion: For a couple of decades, nanoscale optics has mainly been driven by plasmonics since noble metal nanostructures sustain strong resonances that can be used to enhance, confine, propagate or redirect visible light. Such properties have led to numerous actual or potential applications in integrated optics, sensors, nonlinear optics, field-enhanced spectroscopies, or photovoltaics. Recently, an alternative to plasmonics emerged with high refractive index dielectric nanostructures, which offer the same range of applications as plasmonics by manipulating waveguide and Mie optical resonances instead of plasmonic ones [1].
These resonances can be efficiently tuned by modifying the size, shape, and material of those nanostructures (e.g. silicon, optical index n≈4). Furthermore, high index dielectric nanostructures offer several advantages when compared to their metallic counterparts: absorption losses are far weaker for wavelengths greater than the direct band gap, access to semiconductor (CMOS) technology for nanostructure fabrication, and presence of intrinsic strong electric and magnetic resonances [2,3].
In the context of quantum nanophotonics, which aims to combine the confinement and propagation of light at the nanoscale along with quantum properties of light, their appealing properties make Si nanostructures an interesting platform to investigate classical to quantum optics transition in coplanar devices.
In this work, we first discuss the effect of high index dielectric nanowires on the spontaneous emission of NV colored centers in nanodiamonds. The emission rate of punctual emitters is driven by the presence of the nanostructure which tailors the local density of optical states (LDOS) [4]. With time-resolved photoluminescence acquisitions, we show that the photodynamics of the quantum emitters is modified in the vicinity of the Si wires. The experimental data are systematically compared to simulated decay rates and LDOS computed with the Green Dyadic Method (GDM).
In a second stage, we show that visible photons emitted by the broadband single emitters coupled at one extremity of Si nanowires are efficiently guided up to the second extremity, located several micrometers away. By performing image plane acquisitions on a set of emitter-wire hybrid structures, we observed that the geometrical parameters of the wires, and the related waveguide modes, determine the efficiency of the transfer. The experimental results are in good agreement with numerical experiments.
References:
[1] P.R. Wiecha, A. Arbouet, C. Girard, A. Lecestre, G. Larrieu, V. Paillard, Nat. Nanotech. 12: 163–169, 2017.
[2] P.R. Wiecha, A. Cuche, A. Arbouet, C. Girard, G. Colas des Francs, A. Lecestre, G. Larrieu, F. Fournel, V. Larrey, T. Baron, V. Paillard, ACS Photonics 4: 2036–2046, 2017.
[3] P.R. Wiecha, C. Majorel, C. Girard, A. Arbouet, B. Masenelli, O. Boisron, A. Lecestre, G. Larrieu, V. Paillard, A. Cuche, App. Opt., 58: 1682-1690, 2019.
[4] A. Cuche, M. Berthel, U. Kumar, G. Colas des Francs, S. Huant, E. Dujardin, C. Girard, A. Drezet, Phys. Rev. B 95 : 121402(R), 2017.

Many plasmonic processes, from photocatalysis to photothermal therapy, rely on nanoparticle-molecule interactions. Mapping these interactions with high spatial resolution is essential to optimize system efficiency, but can be incredibly challenging due to the molecules’ small size and generally complex microenvironment. Here, we propose a new method capable of identifying and mapping nanoparticle-molecule with sub-10nm spatial resolution. Our method, Electron and Light Stimulated Raman Scattering (ELISR), utilizes in-situ optical spectroscopy within a transmission electron microscope (TEM). Unlike existing correlated optical and electron microscopy techniques, where a sample is first imaged in a fluorescence optical microscope and then sectioned for cryo-electron microscopy, our method enables simultaneous high-resolution microstructure visualization with protein identification through Raman scattering. In particular, our technique uses a laser source as the pump and the electron beam as the broadband Stokes excitation. The electron beam serves as a highly-localized Angstrom-scale source to locally excite the plasmonic resonances of individual nanoparticles whose resonance is red-shifted from the pump laser to resemble Stokes excitation. Accordingly, the Raman scattering is locally enhanced by the electron beam and the spatial resolution is determined by the electron beam spot size and the nanoparticle size. We theoretically model this process using boundary element method (BEM) calculations. Attention is given to gold nanorods (NRs) with plasmonic resonances between 800nm and 900nm; we assume 785nm laser illumination and 80keV electron beam excitation. We investigate the enhancement of the 992cm^-1 benzene line, showing that enhancement in the stimulated over spontaneous Raman can be up to six orders of magnitude under electron beam illumination, even with laser pump intensities less than 10mW/µm². Experimentally, we use aberration corrected environmental transmission electron microscope combined with light excitation. The setup uses two parabolic mirrors coupled to two optical fibers. We use one of the optical fibers to couple the 785nm pump laser into the TEM and the other fiber to couple the light scattered from the sample out of the TEM. The scattered light collected from the sample is then coupled into a spectrometer after filtering out the pump laser. To characterize the e-beam stimulated Raman we use a model system of colloidally synthesized gold and silver NRs functionalized with 4-Mercaptobenzoic acid (4-MBA) as a Raman reporter. The lengths and radii of the rods are 105nm and 12nm, respectively, with transverse and longitudinal plasmon resonances at 510nm and 830nm, as confirmed with cathodoluminescence. Upon electron beam excitation of the NR plasmon modes, we observe a significant increase in the Raman intensity of the1570cm^-1 line of the 4-MBA. We locally map the stimulated Raman enhancement upon electron-beam excitation as a function of electron beam position and nanorod material and arrangement, including nanorod dimers. Our results demonstrate the promise of Raman spectroscopy with electron microscopy to enable single nanometer resolution molecular mapping, with simultaneous imaging of the nano-to-atomic-scale structure of the sample.

In recent years, research at the interface of chemistry, material science, and quantum optics has opened new possibilities to study strong light-matter interactions for nanophotonic chemistry [1,2]. In this new regime of nano-optics, correlated electron, nuclear and photon interactions have to be treated on the same quantized footing. Towards this goal, we have introduced a general time-dependent density-functional theory [3] and variational QED framework [4].
In this talk, we demonstrate how these QED-derived frameworks are used to study strong-light matter coupling to change the chemistry of the system. As a concrete example, the effect of strong-coupling on photochemical reaction is studied for Formaldehyde. We construct the polaritonic excited-state potential-energy surfaces (PES) of a CO bond stretching that are modified under strong-light matter coupling [5]. We show how strong coupling can be exploited to alter photochemical reaction pathways by influencing avoided crossings. For CO2 molecules, we study the Rabi splitting and finite temperature effects [3] in vibrational strong coupling. Additionally, we study how strong vibrational coupling influences chemical reactions, recently observed in experiment. By using the variational QED framework, we demonstrate how we can calculate polaritonic observables for a wide range of coupling strengths [4].
Our work opens the important new avenue in introducing ab initio methods to the nascent field of nanophotonic chemistry with strong light-matter interactions and will enable a systematic search for potential new reactions that can be altered by these interactions.
[1] J. Flick, N. Rivera, P. Narang, Nanophotonics 7(9), 1479 (2018).
[2] F. Benz et al., Science, 354 6313, 726-729 (2016).
[3] J. Flick, P. Narang, Phys. Rev. Lett. 121, 113002 (2018).
[4] N. Rivera, J. Flick, P. Narang, Phys. Rev. Lett. 122, 193603 (2019).
[5] J. Flick, P. Narang, in preparation (2019).

An important building block in integrated optical circuits is an efficient link between the optical and electrical domain. Well-known examples of such links are integrated high-speed modulators to convert electrical signals into optical signals at very high-speed, and low-power tuning elements to compensate for variations in the device operation temperature and for device-to-device variations during fabrication. To enable such electro-optic links, the two most widely used physical effects are the plasma-dispersion effect and Joule heating. Although these effects are attractive to use due to their compatibility with standard photonic fabrication processes, their performance in integrated devices is intrinsically limited by high insertion losses and high-power dissipation.

Over the past decade, we established an alternative electro-optic switching technology by embedding a Pockels material into silicon-based photonic devices. We reached this goal by developing a process to fabricate ferroelectric barium-titanate (BTO) thin films on silicon substrates using advanced epitaxial deposition techniques and by developing a BTO process technology. We correlated the electro-optical properties of the thin films with their structural properties such as porosity and crystalline symmetry to show guidelines for improving the functional properties [1]. By realizing integrated hybrid BTO/silicon devices, we demonstrated record-high, in-device Pockels coefficients of >900 pm/V [2]. The Pockels effect in BTO-based photonic devices indeed enables extremely fast data modulation at rates beyond >40 Gbps and ultra-low-power electro-optic tuning of silicon and silicon-nitride waveguides. We also show ways of how to integrate and use BTO in plasmonic slot waveguide structures for very compact optical devices. With the development of a wafer-level integration scheme of single-crystalline BTO layers to a 200 mm process, we could demonstrate a viable path to combine the BTO-technology with existing fabrication routes [3].

With major breakthroughs in the past years, BTO has emerged as a strong candidate for a novel generation of electro-optic devices. Major achievements of the BTO technology will be covered in the presentation, ranging from important materials aspects, device development, integration concepts, and novel applications in the area of quantum computing, high-speed communication, and neuromorphic optical computing.

Acknowledgements
The work discussed has received funding from the European Commission under grant agreement numbers FP7-ICT-2013-11-619456 (SITOGA), H2020-ICT-2015-25-688579 (PHRESCO), and H2020-ICT-2017-1-780997 (plaCMOS), from the Swiss State Secretariat for Education, Research and Innovation under contract number 15.0285, and from the Swiss National Foundation project no. 200021_159565 (PADOMO).

References
1. K. J. Kormondy, Y. Popoff, M. Sousa, F. Eltes, D. Caimi, M. D. Rossell, M. Fiebig, P. Hoffmann, C. Marchiori, M. Reinke, M. Trassin, A. A. Demkov, J. Fompeyrine, and S. Abel, "Microstructure and ferroelectricity of BaTiO₃ thin films on Si for integrated photonics," Nanotechnology 28, 075706 (2017).
2. S. Abel, F. Eltes, J. E. Ortmann, A. Messner, P. Castera, T. Wagner, D. Urbonas, A. Rosa, A. M. Gutierrez, D. Tulli, P. Ma, B. Baeuerle, A. Josten, W. Heni, D. Caimi, L. Czornomaz, A. A. Demkov, J. Leuthold, P. Sanchis, and J. Fompeyrine, "Large Pockels effect in micro- and nano- structured barium titanate integrated on silicon," Nat. Mater. 18, 42–47 (2019).
3. F. Eltes, C. Mai, D. Caimi, M. Kroh, Y. Popoff, G. Winzer, D. Petousi, S. Lischke, J. E. Ortmann, L. Czornomaz, L. Zimmermann, J. Fompeyrine, and S. Abel, "A BaTiO3-Based Electro-Optic Pockels Modulator Monolithically Integrated on an Advanced Silicon Photonics Platform," J. Light. Technol. 37, 1456–1462 (2019).

Light is a powerful tool to manipulate matter, but existing approaches often necessitate focused, high-intensity light that limits the manipulated object's shape, material, and size. Here, we discuss self-stabilizing optical manipulation of macroscopic objects achieved by controlling the anisotropy of light scattering along the object surface. In a scalable design that features silicon resonators on silica substrate, we identify nanophotonic structures that can self-restore when rotated and/or translated relative to the optical axis. Nanoscale control of scattering across a large area creates restoring behaviour by engineering the scattered phase, without needing to focus incident light or excessively constrain the shape, size or material composition of the object. These findings may lead to platforms for manipulating macroscopic objects, with applications ranging from contactless wafer-scale fabrication and assembly, to trajectory control for ultralight spacecraft, and even laser-propelled lightsails for space exploration.

Many infrared imagers comprise a focal plane array of thermal detectors that convert incident infrared to heat and thence to an electrical signal. The thermometric property of the well-established micro-bolometer is a change in resistance of a semiconducting element with temperature. These require a bias voltage and draw a current that consumes power, and the response of sufficiently sensitive bolometers is fairly slow. An opportunity is nano-scale thermocouples connected to planar antennas, which collect the incident infrared and drive a current that Joule heats the thermocouple junction. The antennas are wavelength specific, which has value for spectral sensing. The heated junction of dissimilar materials generates a thermoelectric voltage, which can be detected at the free ends of the thermocouple. The nano-scale junction can change temperature quickly due to small thermal mass, giving faster response than for traditional bolometers. The devices in principle require fewer processing steps and can use low-cost earth-abundant materials, potentially reduction the cost of infrared sensor systems in comparison to microbolometers. The thermoelectric elements generate their own output voltage without external bias, so that the only power required is that needed to operate the read-out circuit. We present results for a Seebeck nano-antenna pixel optimized for 10.6 micron wavelength radiation. Design and performance calculations are presented, together with experimental results for responsivity, noise, noise-equivalent power, detectivity, and time constant.

Surface plasmon polaritons have rapidly established themselves as a promising concept for emerging integrated ultra-compact photonic circuits rivaling electronics in both speed and critical feature sizes. Aside of effective sources, detectors and modulators, the overall performance of plasmonic networks critically rely on symmetry broken waveguides enabling the routing of signals within a circuit.
We systematically investigated the guiding of plasmonic beams in ultra-thin (<50 nm) monocrystalline Al nanowires enwrapped by a passivating Al₂O₃ shell. The Al heterostructures were synthetized on 40nm thick Si₃N₄ membranes by a thermally induced exchange reaction of single-crystalline Ge nanowires and lithographically defined Al contact pads. Due to limited spatial resolution, optical methods are inappropriate to determine the short propagation length in ultra-thin metallic nanowires. Thus, we explored a well-tuned focused grating coupler to launch plasmons in the Al nanowire waveguides. To optimize the focussed grating coupler with respect to plasmon generation and coupling into the c-Al NW, numerical simulations based on the finite-difference time-domain (FDTD) method were performed.
Further we explored and a highly efficient electrical plasmon detector based on a monolithic quasi 1D metal-semiconductor-metal heterostructure device. Based on this system, we experimentally determined a plasmon propagation length of 140 nm for monocrystalline Al plasmon waveguides with diameters of only 40 nm. Further, our monolithic approach of plasmon generation, guiding and sensing enables us to examine bending losses of kinked Al nanowire waveguides.
This particular device allows a clear separation of the contributions of plasmon-induced and photo-excited carriers enabling to separately examine plasmon-induced hot electron injection and photoexcitation. The architecture features further precise control of the injection barrier at the abrupt metal-semiconductor interface, enabling selective probing the hot electron distribution from surface plasmon decay.
These systematic investigations of ultra-thin monocrystalline Al nanowires provide in general a platform for the evaluation of nanoscale metal based waveguides for transmission lines of next generation high-speed ultra-compact on-chip photonic circuits.

Spatial light modulators (SLMs) are central to numerous applications ranging from high-speed displays to adaptive optics, structured illumination microscopy, and holography. After decades of advances, SLM arrays based on liquid crystals can now reach large pixel counts exceeding 10⁶ with phase-only modulation with a pixel pitch of less than 10 μm and reflectance around 75%. However, the rather slow modulation speed in such SLMs (below hundreds of Hz) presents limitations for many applications. Here we propose an SLM architecture that can achieve high pixel count with high-resolution phase-only modulation at high speed in excess of GHz. The architecture consists of a tunable two-dimensional array of vertically oriented, one-sided microcavities that are tuned through an electro-optic material such as barium titanate (BTO). We calculate that the optimized microcavity design achieves a π phase shift under an applied bias voltage below 10 V, while maintaining nearly constant reflection amplitude. As two model applications, we consider high-speed 2D beam steering as well as beam forming. The outlined design methodology could also benefit future design of spatial light modulators with other specifications (for example amplitude modulators). This high-speed SLM architecture promises a wide range of new applications ranging from fully tunable metasurfaces to optical computing accelerators, high-speed interconnects, true 2D phased array beam steering, and quantum computing with cold atom arrays.

In the past decade, lead halide perovskite material have received considerable attention to be a promising candidate for solar cells due to its excellent optoelectronic properties. Beyond of the remarkable power conversion efficiency. However, the optical dielectric constants (complex refractive index) are far less discussed, especially the dynamic control of refractive index in lead halide perovskite material has never been reported. Understanding and modulating the material properties could accelerate to the progress of applying perovskite in optoelectronic devices comprehensively. Here, we report electric field modulated dielectric permittivity in CH₃NH₃PbBr₃ film by using ellipsometry measurement. The designed device consists of Silver/ Aluminium Oxide / CH₃NH₃PbBr₃ / PMMA / Silver and results in a metal-insulator- semiconductor-insulator-metal (MISIM) heterostructures. Under an applied electric field (~10⁶ V/m) crossing the two silver electrodes, 5% change in the refractive index is observed between 440 - 520nm. We believe the results open new avenues for the application of perovskite based optoelectronic devices such as in electro-optic modulator in the visible and active tunable transistor lasers.

The properties (e.g., wave vector, polarization, spontaneous emission rate, and so on) of generated electromagnetic waves is inherently dictated by optical density of states that well-ordered structures create. For example, a one-dimensional metal/dielectric subwavelength periodic pattern, which serve as an artificial birefringent film, exhibits a hyperbolic (as opposed to ellipsoidal) material dispersion, thus being capable of generating linearly polarized light into specific direction.
More specifically, when an Al/ITO metal/dielectric metamaterial film designed for a quarter wave plate is evanescently coupled to a quantum emitter, the far-field distribution of radiation is vertically localized for one polarization and horizontally spread for its orthogonal polarization. The contrast of the polarization-selective far-field distribution is the most pronounced when a quantum emitter is positioned at which an interference condition is completely met. As a practical example, such artificial, birefringent films can be applied to organic light-emitting diodes (OLEDs) to attain polarized and vertically directed emitted light; according to electromagnetic simulation result, the maximum extinction polarization ratio is up to approximately 30.
Likewise, circular dichroism materials such as helical reactive mesogen (RM) can be chosen to manipulate the chiral states (e.g., circular polarization) of quantum emitters. A RM embedded film synthesized on a glass substrate shows highly selective reflectance for opposite circular polarization over a particular wavelength range; the wavelength range is readily tuned by the pitch of helical morphology within RM material. Such a circular dichroism layer can help quantum emitters inherently yield circular polarized light. We will discuss the polarization and wavelength resolved far-field distribution of quantum-dot films when they are integrated with tailored circularly dichroism materials.

Dielectric nanoantennas are capable of sculpting the amplitude and phase of light through resonant modal coupling. Properly designed, these structures promise to transform modern optics by replacing traditional optical components with miniaturized films with curated phase, amplitude, and dispersion relations to generate arbitrary transfer functions for incoming light. The inclusion of nonlinear optical responses into metasurfaces will be crucial for applications spanning optical and neuromorphic computing, optical communications, sensitive detectors and dynamic sensors, but the low nonlinear susceptibility of dielectric materials significantly challenges these applications.

Here, we introduce a route towards efficient nonlinear wavefront shaping and beam-steering using high quality factor (high-Q) resonances in dielectric phase gradient metasurfaces. Typically, metasurfaces operate based on phase pickup from Fabry-Perot-like or Mie-like modes in the propagation direction of interest. In our case, we exploit coupling to in-plane waveguide modes, which interfere with the far-field Fabry-Perot modes to produce extremely high-Q modes, with Q’s exceeding 1000. We use a silicon on sapphire platform and pattern spatially arrayed silicon beams with widths of 200 to 370 nm as metasurface elements. Our beamsteering structures efficiently diffract light to the +1st order with wavelengths between 1300nm and 1500nm. Including small periodic notches, with depths varying from 30nm to 150nm, into one Si beam provides the extra momentum to efficiently couple to a waveguide mode; the interference between the Fabry-Perot mode and waveguide mode leads to a resonant response in the scattered spectrum, which manifests as a resonant decrease in the first diffracted-order scattered intensity. Our experimental quality factors range from 1400-2500 with perturbation periods ranging from 530 nm to 610 nm along a particular Si bar.

The high, experimentally-observed Q factors indicate that the illuminating electric field can locally be enhanced by more than 50 times. We utilize this enhancement to induce nonlinear shifts in the scattered transmission spectrum, using a nanosecond Q-switched laser centered at 1534nm. Here, we exploit the nonlinear Kerr effect to induce modulations to the refractive index of individual silicon bars dependent on the incident laser power. As the incident power increases, the refractive index, and hence the scattered amplitude and phase, of the metasurface elements are modified. Including Silicon’s nonlinear susceptibility in simulation, we show laser powers of 83 uW/um² can shift the resonant feature by greater than a full width half max. Our approach represents a facile route to achieve nonlinear optical phenomena in metasurfaces, and can be readily extended to other common nonlinear schemes, including harmonic generation and parametric wavemixing. Our high-Q scheme is generalizable to almost arbitrary transfer functions and materials, laying a foundation for a range of nonlinear-based technologies in computing, sensing, and communications.

Ultra-compact, high-resolution, and low energy-consumption tunable phased arrays and displays play a critical role in future’s daily life as they provide an efficient and user-friendly access to information. Nanophotonic devices, made by judiciously engineered optical resonators, are capable of harnessing the amplitude, phase, and spectrum of its scattered light with subwavelength resolution. However, active tuning of these optical resonances is still in its infancy. Active tuning is challenging as the semiconductors and noble metals display a limited tunability due to the generally weak electrorefractive and electroabsorptive effects . Here, we demonstrate the large, fast, and repeatable tuning of optical resonances by tailoring the dielectric environment of silicon (Si) nano-resonators that operate in the visible spectral range. The dramatic tunability results from the strong dispersion and interference of various optical resonances supported by Si nano-resonators when embedded in different refractive-index materials. This working principle is then realized by integrating the nanophotonic device with a microfluidic cavity that can programmably control the refractive index (n=1-1.7) by flowing different liquid in real time.

Active beam steering, dual-wavelength switchable holography, and wavelength-selective focusing and imaging are all demonstrated to show the power and flexibility of the design principle. We also demonstrate the possible use of these elements in display application by showing both a broadband (~100 nm) amplitude tuning and full-color tuning (from blue to red, reflection peak from 480nm to 580 nm) for individual pixels. This large tunability results from the interference between the symmetric (electric dipole mode, toroidal dipole mode, magnetic quadrupole mode) and anti-symmetric (magnetic dipole mode, electric quadrupole mode) modes supported by Si nanodisk arrays, as their resonant frequencies and bandwidths all depend on the refractive index of surrounding materials. The dielectric environment is programmed up to 20 Hz with microfluidic cavities to show practical multi-color display function.

Altogether, these results demonstrate the unprecedented tunability to control the dielectric optical resonators in practical applications, ranging from active beam steering, real-time holography, to fluorescence microscopy and reflective displays with color-tunable pixels. The successful integration with mature microfluidic cavity technologies further paves the way towards next-generation ultra-compact active optical elements.

Structural colors, which can occur by the interaction between visible light and exceptional photonic structures, have rapidly emerged as a key alternative to the traditional dyes or pigments because of their conspicuous advantages as following: 1) outstanding spatial resolution, 2) durability under harsh environments, 3) versatile utilization, 4) compactness, 5) eco-friendly materials, and 6) spectral selectivity. With these strong points, various structural colors including plasmonic nanostructures, metal-dielectric multilayers, and photonic crystals have attempted to substitute with the conventional dyes and pigments. Among them, silicon (Si) nanostructures are considered as the most suitable candidates because of the low-cost/mature fabrication process and excellent optical constants (i.e., high refractive index and low absorption loss compared to metals). Using the aforementioned properties, multicolor generation by Si nanowire arrays (Si NWAs) has been successfully demonstrated for reflective/transmissive structural color filters. However, the restricted color presentation of currently-reported structural colors has hindered the wide spreading of the promising structural color printing.
Here we propose a new class of reflective color filters to enlarge color gamut by attaching a sticker form of Si NWA on ultra-thin one-dimensional resonators which are in nanoscale. To implement the sticker form of Si NWAs, we fabricate transferable Si NWAs (T-SiNAs) embedded in polydimethylsiloxane (PDMS) which is a visibly transparent polymer. The nanoscale structures of each photonic configuration allow an exceptional mechanical softness, hence it enables flexibility and reusability without mechanical failure. First, we design and fabricate the T-SiNAs to possesses optical resonances covering from visible to near-infrared (NIR) ranges (i.e., 400 to 1000 nm). The resonance dip positions are shifted from short to long wavelengths by increasing the diameter of T-SiNAs. The fabricated T-SiNAs have the optimized structural parameters as follows: height = 2 μm, diameter = 50 to 150 nm with 10-nm-step, and period = 1250, 900, and 600 nm. To demonstrate the widening ability in color gamut, we selected metal-insulator-metal (MIM; Ag-SiO₂-Ag) structure as ultra-thin 1D resonator. Generally, MIM structures exhibit subtractive primary colors such as cyan, magenta, and yellow. However, by transferring T-SiNAs on MIMs, we enlarge the pristine color gamut of MIMs and experimentally realize additive primary colors such as red, green, and blue. Furthermore, such intriguing optical and mechanical characteristics facilitate a novel optical anti-counterfeiting sticker.

Reflective asymmetric coloration in a metal−insulator−metal (MIM) structure is proposed to achieve direction selective message encryption through the manipulation of physical chracteristic of the top metal and effective refractive index of the dielectric insulator layer. A semicontinuous top metal film with nanoapertures, adopted as a transreflective layer for MIM resonator, allows to exhibit direction-sensitive optical effect (called Janus effect) as well as to tailor the nanomorphology of a dielectric layer, which plays crucial roles in recognizable color changes when exposed to external liquids. This new concept of direction sensitivie distint colorations, followed by color tuning with respect to liquid exposures were thoroughly analzed by theoretical simulations and experiments. Our liquid permeable approached in color changes indeed provide dramatic color tunablility, a real-time sensing scheme, long-term durability, reproducibility, and most importantly, exhibit direction sensitive message hidings (Optical Camouflage) in a simple and scalable manner.

Integration of plasmonic metal nanostructures with semiconductor nanomaterials has been proposed as a promising strategy to achieve enhanced light absorption and solar energy conversion efficiencies. In the present work, we report the fabrication of platinum coated Ni pillar arrays integrated with CdS nanoparticle continuous layer (Ni/Pt/CdS pillars) for photoelectrochemical water splitting. We use several Ni/Pt pillar array with varying pitch and diameter to achieve a wide wavelength (visible to near-infrared (NIR)) light harvesting. Compared to CdS coated Ni/Pt planar film, the best Ni/Pt/CdS pillar array shows ~200% photocurrent enhancement. The same is confirmed by an incident photon to current conversion efficiency (IPCE) and electromagnetic simulations. We will discuss the effects enabling the photocurrent enhancement, distinguishing the contribution from increased surface area, photonic and plasmonic light concentration.

Metasurfaces have recently gained attention due to their physical compact nature, versatility, and exotic electromagnetic properties in manipulating propagating waves. In this work we present the design, fabrication, and experimental verification of a beam-splitting metasurface reflector with arbitrarily chosen split beam directions. A beam splitter is an integral component in many optical devices such as interferometers and multiplexers. Traditional beam splitters are based on cubes or plates composed of glass and mirrors that are comparatively large and bulky. Being able to fabricate a metasurface beam splitter will prove to be extremely useful for a variety of applications.

The metasurface under investigation was designed to split a normally incident wave into two beams, one directed to 25 degrees and the other directed to 45 degrees, using the Fourier transformation method of array synthesis. This method is used to calculate the necessary metasurface reflection properties and High-Frequency Electromagnetic Solvers (HFSS) simulations are performed to determine the required surface pattern for fabrication. This method can easily be extended to any combination of splitting angles and operating frequency. The beam-splitting metasurface is fabricated using traditional silk printing techniques with conductive ink on selected substrates. For this work conductive nickel ink patches were printed on an aluminum-backed acrylic substrate. Measurements were performed at 10.5 GHz using a single frequency microwave transmitter and receiver, the receiver was scanned through 180 degrees and electric field intensity was measured. These measurements show excellent agreement with the scattering predictions.

The integration of imaging spectrometers with high-sensitivity photodetectors for spectroscopic identification of multi-band infrared (IR) signals is of great potential for a number of applications that range from infrared imaging to environmental monitoring and biochemical detection. In our recent work, we designed, fabricated, and characterize novel diffractive optical elements (DOEs) that integrate imaging and spectroscopic functions on the same silicon chip. In particular, we demonstrate multi-functional focusing gratings and multi-focal lens flat lenses based on the versatile axilens concept with engineered phase modulations over 100µm footprint dimensions. Using electron beam lithography and sequential etching processes we demonstrate a novel four-level multi-functional phase element that combines the phase modulation of a traditional axilens (i.e., a diffractive lens with a large and programmable focusing depth) with the spatial dispersion behavior characteristic of a compact 2D periodic grating. Furthermore, we demonstrate that the operation spectrum of our designed devices doubles the one of a traditional Fresnel lens. A comprehensive analysis of the operation bandwidth and cross-talk ratio on the detection plane between targeted wavelengths across the long-wavelength IR range (LWIR) is performed demonstrating broad band operation with a 15% cross-talk figure. Device-level simulations are performed using the three-dimensional finite element method and show excellent agreement with predicted device performances based on the rigorous Rayleigh-Sommerfeld diffraction theory.

Optical rectification (OR) is an optoelectronic phenomenon where an illuminating optical field is rectified by a nonlinear material to produce a DC voltage and, in a conductive material, a current which can be a combination of thermionic and tunneling current. OR has been difficult to observe in many systems, due to the need for tunneling junctions with exceedingly small capacitance and surface area.^1,2 We report optical rectification in a nanoscopic Metal-Insulator-Metal (MIM) diode’s tunneling junction, which is voltage-reconfigurable via simple resistive switching, thus enabling tunable conductivity. The conductively-tunable resistive switching MIM diode we explore can be tuned, set, and reset via the application of DC electric fields, making it ideal for exploring optical rectification phenomena under different nonlinear conductivity conditions and for dynamically tuning the device’s responsivity. Other nonlinear junctions cannot be so readily set and re-set. The resistive-switching MIM diode that we research also has the advantage of being scalable to large areas, due to its simple fabrication. Using a relatively simple MIM diode consisting of a planar Al electrode (“passive electrode” or PE), a thin Al₂O₃ barrier layer, and an initially planar Ag counter-electrode (“active electrode” or AE), we demonstrate optical rectification. By applying a voltage, a metallic filament, having a cross-section as small as (10 nm)², grows within the dielectric layer, from the PE to the AE, resulting in a nanogap between metals, bridged by a barrier layer, and with tunneling resistance that exponentially depends on the distance between AE and PE, set by a voltage-tunable nanogap. We measure current-voltage curves and the response to a modulated visible laser beam, and compare to our analytical MIM diode model, which was derived for 2D junctions³. The filament’s separation from the AE is controlled precisely by controlling the junction’s voltage and current. The tunability of this platform opens the possibility for adaptive ultrafast photon detectors, wireless power transmission, and energy harvesting systems, potentially useful for reconfigurable antennas useful for flexible electronics applications, and/or for neuromorphic computing.

[1] J. Small et. al., Applied Physics Letters 24, 275–279 (1974).
[2] G. Moddel and S. Grover, Rectenna solar cells, Vol. 4 (Springer, 2013).
[3] R. M. Osgood III et. al. J. Vac. Sci. Tech. A 34 051514 (2016).

The potential of THz waves for applications has become widely recognized in the last few years and the use of phase transition phenomena for reversibly tuning the properties of functional materials in devices is an attractive research area of materials science. We report on various kinds of designed and microfabricated metastructures for actively switching the wavefront of terahertz (THz) radiation based on the well known insulator-to-metal (IMT) phase transition of pure vanadium dioxide (VO₂) and its ternary tungsten (W) doped V_1-xW_xO₂. The IMT is induced by a temperature change of the structured thin films and occurs at about 68 °C for the undoped bulk VO₂ while doping with W strongly influences the phase transition temperature T_C by about -12.6 K / (at.% of W) . The microstructures, based on metasurfaces of C-shaped slot antennas, are etched into the rf-sputtered thin films on c-sapphire substrates using micro- and nanofabrication methods, i.e. photolithography and ion beam etching. The C-shaped slot antennas are active only when the pure VO₂ or the V_1-xW_xO₂ is in its metallic phase, i.e. at temperatures T > T_C . Examples are temperature-switchable THz multi-focus lenses which focus impinging THz radiation into four focal spots or Airy beam generators. Using both materials, VO₂ and V_1-xW_xO₂, allows us to fabricate stacked lenses that can be switched on and of at different temperatures linked to the critical temperatures T_C of the thin films. We characterized the function of the THz wavefront modulators over a broad frequency range from 0.3 to 1.2 THz. Such thermally switchable THz wavefront modulators with a capability of dynamically steering THz fields will be of great significance for the future development of THz active devices.

In conjugated polymers, electronic excitations are coherently spread over the whole conjugated polymer, the individual transition dipole moments interact by dipole-dipole coupling. As this dipole-dipole coupling is a near-field effect, the interaction is limited to a small spatial region, and the participating quantum emitters cannot be addressed and probed individually. With a colloidal approach, we build a larger-scale analogue of a conjugated polymer to study coherent energy transfer. We will couple a small ensemble of quantum emitters with a plasmonic colloidal cavities [1] and a spatially extended hybrid plasmonic lattice mode [2] to study the weak and strong light matter interaction. The fluorescence enhancement (optical gain) is provided by a lattice of silver indium sulphide (AgInS) quantum dots fabricated by confinement assembly. The light annihilation (optical loss) is achieved using a gold particle lattice by soft lithography templates and directed self-assembly.[3] By stacking of those two components, geometrical parameters can be varied, which allows to study the coherent energy transfer systematically by time-correlated scattering and reciprocal space imaging methods. Due to the scalability of both fabrication methods, we can produce substrates with areas larger than 2 cm², which can be expanded even further. In the end, the stacked structures pave the way to a quantum simulator of the underlying conjugated polymer.
[1] Fabian R Goßler et al., J. Phys. Chem. C, 2019, 123, 6745-6752
[2] Kirsten Volk et al., Adv. Opt. Mater, 2017, 5, 1600971
[3] Martin Mayer et al,, Adv. Opt. Mater, 2019, 7, 1800564
Acknowledgement: This project was financially supported by the Volkswagen Foundation through a Freigeist Fellowship to Tobias A.F. König. The authors acknowledge the Deutsche Forschungsgemeinschaft (DFG) within the Cluster of Excellence ‘Center for Advancing Electronics Dresden’ (cfaed) for financial
support.

High temperature stability, chemical stability, low surface energy and mechanical robustness, combined with a zero-crossover wavelength in the visible region make titanium nitride (TiN) a promising material for plasmonic and metamaterial applications in harsh environments. In this work we demonstrate the heteroepitaxial growth of TiN on (0001)-Al₂O₃, (001)-MgO and (0001)- LiNbO₃ substrates as well as incorporation of TiN in metal-dielectric stacks. We discuss the differences in growth and crystalline properties based on substrate material and growth conditions. Additionally, we discuss the relationship between TiN crystalline quality and optical properties. Coupled x-ray diffraction (XRD) of our TiN show high quality epitaxial growth on all three substrates, however, further structural characterization reveals differences in crystal defects, strain and surface morphology based on substrate crystal structure and lattice mismatch. Pendellosung fringes on the (111)-TiN diffraction peak for the coupled XRD of TiN on c-plane sapphire and LiNbO3 indicate uniform d-spacing and a pristine interface. 6-fold symmetry for the TiN grown on sapphire is seen in XRD pole figure scans, indicating the presence of stacking faults. These domains, which are further evident in atomic force microscopy (AFM) of the TiN surface, are attributed to different stacking within the TiN domains. XRD of the TiN on MgO show Pendellosung fringes on the (001)-TiN diffraction peak. Cross-hatching features similar to those on the bare MgO substrate are seen in the AFM of the TiN surface. Variable angle spectroscopic ellipsometry (VASE) shows that TiN behaves metallic on all substrates with a zero crossover wavelength between 470 nm and 490 nm. Differences in the real (ε₁) and imaginary (ε₂) permittivity for TiN on the different substrates are also seen in the VASE measurements.

The emergence of metasurface technology and its accompanying design principles are enabling the development of optical components with multiple functionalities, e.g., polarization discrimination and focusing. In this report, we highlight our computational and experimental results associated with the design, fabrication, and experimental characterization of infrared (IR) metasurface optics that were designed for wavelength and polarization-specific analysis of infrared fields in the near and mid-wave IR region of the spectrum. For the former, we developed an all-dielectric metasurface that enabled polarization analysis of a 1.55 micron laser source. For the mid-wave metasurface, a reflective metasurface architecture was observed to exhibit polarization-selective imaging throughout the 4.5 to 5 microns region.

Light displays a wide range of different possible interactions with naturally-occurring materials. Optical sensing and imaging applications capitalize on this fact to study such materials and structures of interest. Alternatively, it is possible to engineer new, nanostructured materials that only exhibit a strong interaction to light waves with specific properties, e.g. a specific incident angle, wavelengths, or state of polarization. We are interested in using such designer materials to realize new types of specialized photodetectors for angular sensing and spectropolarimetry.
To achieve specific sensing functions, one typically combines a conventional photodetector with a set of bulky optical elements capable of filtering light waves with properties of interest. These can include lenses, gratings, apertures, prisms, and polarization filters. This strategy makes dense integration a significant challenge. To overcome this challenge, we proposed the use of different types of high-index semiconductor nanostructures that exhibit an optically resonant interaction with the light waves of interest. These resonances can boost light-matter interaction and enhance absorption and photocarrier generation. This ultimately facilitates the creation of compact, highly integrated photodetectors that do not require additional external elements.
Here, we illustrate our approach with a nanopatterned silicon layer that displays a high differential absorption for left and right-hand circularly polarized light. It achieves this function by engineering both Mie and guided mode resonances in dislocated silicon nanowire patterns. We will present the fabricated photodetector structures and their experimental characterization. The proposed structures could potentially be used in a wide range of applications, for sensing, imaging, and augmented/virtual reality.

Metamaterials and metasurfaces have enabled myriad applications by breaking the limitations of natural materials, including negative refraction index, subwavelength imaging, near zero epsilon, metalens, and invisibility cloaking. Tunability in metamaterials has generally been achieved through inclusions of condensed matter systems in which a dynamic response is induced by external excitations. The tunability by this method is both the extension and restriction of the metamaterials capability as the limits of the tuning are defined by the dynamic range of the included material. Overcoming this limitation of tunable materials and achieving better performance in hybrid devices is a challenge that can lead to greater advances in these devices. Vanadium dioxide (VO₂) is a correlated electron material, which has been particularly well studied largely because the insulator-to-metal transition (IMT) occurs above room temperature at around 340K. Upon undergoing the IMT, the resistivity changes by four-orders of magnitude and across a broad spectral range making VO2 a powerful dynamic material to be combined with metamaterials to introduce nonlinear responses, state switches, and modulators. The physics of the accompanying crystal transformation from monoclinic to rutile state has historically obscured the exact mechanism of the transition.

In this paper, we patterned a complementary metamaterial on a thin layer of vanadium dioxide films by direct laser writing technique and photolithography to explore the tunable terahertz (THz) response. The metamaterial exhibits a transition in THz transmission which is larger than of a pristine VO₂ film. The modulation enhancement of dynamic materials by metamaterial resonators provides a way to enhance the natural utility of VO₂ through metamaterial inclusions while the sensitivity of the complementary resonator provides a straightforward way to investigate the nature of mesoscopic transition phenomena in correlated electron materials. We characterize the phase transition response of the VO2 film and the tunable metamaterial using THz time-domain spectroscopy. For the complementary metamaterial, the resonance peak occurs at 0.48THz with a peak transmission of 82.8% when the VO2 is in insulator state at the temperature below 315K. As the temperature increases, the VO2 thin film transits to the metallic state, thus the resonance peak vanishes. The modulation amplitude of THz transmission at 0.48THz is 68.3%. We measured the dynamic transition as a function of temperature for heating and cooling processes. The transmission of VO₂ CSRR during the heating process does not change until the temperature reaches 334K. A dramatic reduction of transmission occurs within a 2K span from 335K to 337K during heating. Upon cooling, the reverse transition occurs albeit at a lower temperature. The rapid onset of this transition coupled with the wide hysteresis presents an attractive feature for applications in THz switches and modulation devices.

There is a critical need for compact antenna arrays capable of transmitting and receiving information in the terahertz band (0.1-10 THz). In order to achieve this goal, control of the array response is essential. Here we report design, fabrication, and experimental characterization of a hybrid graphene/metal antenna array consisting of metallic elements fabricated atop graphene. The antenna design was developed through numerical simulations that were informed by experimentally obtained graphene parameters. Experimental characterization revealed a clear enhancement due to the array, and the presence of the underlying graphene affected the reflected intensity. Graphene’s Fermi energy can easily be tuned by electrostatic gating, which makes this structure promising for active, steerable THz reflectarrays.

Photonic crystals are materials composed of mixed dielectric media that result in the reflection of all electromagnetic waves within a range of wavelengths commensurate with the length scale of the crystal. Such complete photonic band gaps allow for light to be controlled through materials design. Since first theorized in 1987, much effort has been made to define and synthesize photonic crystal structures. In the decades since, many photonic structures have been discovered, often by using naturally occurring crystal structures as templates for design. However, these studies have yet to answer the question: what features of a 3D structure will produce a complete photonic band gap? Here, we present data on over 150,000 potential photonic crystals, and show that complete photonic band gaps are possible for many unexpected structures that have yet to be explored. Our simulations suggest that when designing novel photonic materials, the toolbox of structural templates may be larger and richer than previously thought, widening the field of target crystal structures.

Novel van der Waals materials and their heterostructures are especially promising in an ongoing quest for suitable photonic materials that enable miniaturization of optical components. van der Waals materials possess high optical anisotropy as the strong covalent bonding within a specific layer and weak van der Waals forces between adjacent material layers result in highly anisotropic lattice vibrations along the in- and out-of-plane directions of the layered material. It is proposed to use optical antennas made out of natural hyperbolic material hexagonal boron nitride (hBN) as an alternative way of realizing efficient subwavelength scatterers and overcoming limitations of plasmonic and all-dielectric material platforms [1,2]. Transdimensional designed lattices of resonant hBN antennas in the periodic arrays can serve as functional elements in ultra-thin optical components and photonic applications.
In this talk, I will show that the hBN antenna possesses different multipole resonances enabled by the supporting high-k modes and their reflection from the antenna boundaries. An electric quadrupole mode causes a resonant magnetic response of the scatterer because of wave reflections from its boundaries similar to the formation of magnetic resonance in high-refractive-index elements. The full range of the resonances is demonstrated for the hBN cuboid antenna, a decrease of reflection from the array, and highly directional resonant scattering from antennas pairs. Multipole resonances cause the decrease in the reflection from antenna array to zero, which can be ascribed to resonant Kerker effect satisfying generalized zero back-scattering condition for particles in the array [3]. Transdimensional hBN lattices include 3D-engineered nanoantennas supporting multipole resonances in hBN antennas and arranged in the 2D arrays to leverage collective effects in the nanostructure. The effect can be used in designing optical elements and metasurfaces based on hBN scatterers for applications in mid-infrared photonics.
The work is supported by AFOSR grant FA9550-19-1-0032.
[1] V.E. Babicheva, "Multipole resonances and directional scattering by hyperbolic-media antennas," arxiv.org/abs/1706.07259
[2] V.E. Babicheva, "Directional scattering by the hyperbolic-medium antennas and silicon particles," MRS Advances 3, 1913 (2018).
[3] V.E. Babicheva, "Lattice Kerker effect in the array of hexagonal boron nitride antennas," MRS Advances 3, 2783-2788 (2018).

Oxides exhibiting extremely low thermal conductivity (κ) are essential for heat management technologies such as thermoelectrics and thermal barrier coatings. Fabricating artificial superlattices is an effective approach to minimize the κ of oxides because of the Kapitza resistance (interface between two different phases). However, their practical applications are limited by difficulties and cost of the atomic-scale fabrication. Therefore, materials exhibiting natural superlattice structures are of great value. The κ of some natural superlattice InMO₃(ZnO)_m (M = In, Fe, and Ga, m = integer) polycrystals have been reported, but it has been very challenging to examine the effect of the Kapitza resistance due to the randomly-oriented grains. Here we fabricated InGaO₃(ZnO)_m single crystalline films by the reactive solid-phase epitaxy method and measured the κ perpendicular to the natural superlattices. When the superlattice period (d_SL) was longer than 1.93 nm, the thermal resistivity increased proportionally with the interface density (d_SL⁻¹), indicating that the ballistic phonon transport was suppressed at the InO₂⁻/GaO(ZnO)_m⁺ interfaces, which exhibited the Kapitza resistance of 1.64 m² K GW⁻¹. The minimum κ was 1.12 W m⁻¹ K⁻¹ (m = 5), which is approximately 1/3 of the polycrystalline ceramics and lower than the amorphous InGaO₃(ZnO)_m. These results would be useful for advanced heat management technologies.

Two-dimensional transition metal dichalcogenides (TMDCs) have emerged as novel quantum emitters. Achieving control of their radiation properties, such as excitation rate, radiation efficiency, and radiation pattern, is of importance for next-generation photonic and optoelectronic devices. Recently all-dielectric nanophotonics has been identified as a promising way to control such radiation properties at the nanoscale by coupling the emitter to Mie resonators. It has been experimentally and theoretically demonstrated that Mie-resonant silicon nanowires (diameter/length ~ 100 nm/40 um) can provide directionality to the emission of monolayer MoS₂ emitters due to the interaction between the source dipole and the excited electric and magnetic dipole resonances.[1] In order to further reduce the dimensions of the resonator, we propose a spherical Mie resonator for control over the radiation pattern of monolayer TMDCs.
First, we theoretically analyze the optical coupling of a 100-200 nm silicon nanosphere and a dipole emitter by modifying the classical Mie theory for excitation with a point dipole. As for the case of a silicon nanowire, the interaction of the source dipole and the excited electric and magnetic dipole resonances results in directional emission. To demonstrate this experimentally, we dropcast colloidal silicon nanospheres (100-200 nm in diameter) [2] onto monolayer WS₂ coated silica substrates. After Ar⁺ ion etching of WS₂ around the silicon nanospheres to remove uncoupled emitters, we measure the exciton photoluminescence intensity of WS₂ under single silicon nanospheres using objectives in the top and bottom direction. By studying the top-to-bottom PL ratio over many nanospheres, we demonstrate 8-fold directionality in emission for selected nanosphere sizes as expected from our theoretical analysis.

[1] A.F. Cihan et al., Nat Photonics. 12 284 (2018)
[2] H. Sugimoto et al., Adv Opt Mater. 5 1700332 (2017)

Since the development of diffractive optical elements in the 1970s, major research efforts have focused on replacing bulky optical components by thinner, planar counterparts. The more recent advent of nanophotonic metasurfaces has further accelerated the development of flat optical elements through the realization that resonant optical antenna elements can be utilized to facilitate local control over the light scattering amplitude and phase. In this presentation, I will show how metasurfaces can start to impact Augmented and Virtual Reality applications. I will discuss the creation of free space optical beam tapping systems and high-efficiency metasurface-based optical combiners for near-eye displays. The proposed optical elements can be fabricated by scalable fabrication technologies, such as nanoimprint lithography, rolling optical lithography, and direct write optical lithography.

Metamaterials---artificial electromagnetic media that are structured on the subwavelength scale---were initially suggested for the realization of negative index media, and later they became a paradigm for engineering electromagnetic space and controlling propagation of waves. However, applications of metamaterials in optics are limited due to inherent losses in metals employed for the realisation of artificial
optical magnetism. Recently, we observe the emergence of a new field of all-dielectric resonant metaoptics aiming at the manipulation of strong optically-induced electric and magnetic Mie-type resonances in dielectric and semiconductor nanostructures with relatively high refractive index [1]. Unique advantages of dielectric resonant nanostructures over their metallic counterparts are low dissipative losses and the enhancement of both electric and magnetic fields that provide competitive alternatives for plasmonic structures including optical
nanoantennas, efficient biosensors, passive and active metasurfaces, and functional metadevices [2, 3]. Here, we aim to summarize the most recent advances in all-dielectric Mie-resonant meta-optics including active nanophotonics as well as the recently emerged fields of topological photonics and nonlinear metasurfaces.

In addition, we also aim to review the physics of bound states in the continuum and their applications in metaoptics and metasurfaces [4]. First, we discuss strong coupling between the modes of a single subwavelength high-index dielectric resonator and analyse the mode transformation and Fano resonances when resonator’s aspect ratio varies [5]. We demonstrate that strong mode coupling results in resonances with high quality factors, which are related to the physics of bound states in the continuum when the radiative losses are nearly
suppressed due to the Friedrich–Wintgen scenario of destructive interference. Our theoretical findings are confirmed by microwave and optical experiments for the scattering of high-index subwavelength resonators with a tunable aspect ratio. The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high-Q factor helps to extend substantially many functionalities of all-dielectric nanophotonics that opens new horizons for active and passive nanoscale metadevices. Next, we discuss how bound states in the continuum can appear in the physics of metasurfaces. We reveal that metasurfaces created by seemingly different lattices of (dielectric or metallic) meta-atoms with broken in-plane symmetry can support sharp high-Q resonances that originate from the physics of bound states in the continuum [6]. We demonstrate a direct link between the bound states in the continuum and Fano resonances, and discuss a general theory of such metasurfaces, suggesting the way for smart engineering of resonances for many applications in nanophotonics and meta-optics.

[1] A. I. Kuznetsov, A. E. Miroshnichenko, M. L. Brongersma, Y.S. Kivshar, and B. Lukayanchuk, Optically resonant dielectric nanostructures, Science 354, aag2472 (2016).

[2] S. Kruk and Y. Kivshar, Functional meta-optics and nanophotonics governed by Mie resonances, ACS Photonics 4, 2638 (2017).

[3] Y.S. Kivshar, All-dielectric meta-optics and nonlinear nanophotonics, National Science Review 5, 144 (2018).

[4] K. Koshelev, A. Bogdanov, and Y. Kivshar, Meta-optics and bound states in the continuum, Science Bulletin; arXiv: 1810.08698v1 (2018).

[5] M.V. Rybin, K.L. Koshelev, Z.F. Sadrieva, K.B. Samusev, A.A. Bogdanov, M.F. Limonov, and Y.S. Kivshar, High-Q supercavity modes in subwavelength dielectric resonators, Phys. Rev. Lett. 119, 243901 (2017).

[6] K. Koshelev, S. Lepeshov, M. Liu, A. Bogdanov, and Y.S. Kivshar, Asymmetric metasurfaces and high-Q resonances governed by bound states in the continuum, Phys. Rev. Lett. 121 (2018); arXiv: 1809.00330 (2018).

In recent years, research on passive daytime radiative cooling has seen a variety of metamaterial-based innovations, such as structured polymers [1], photonic designs [2-3], and polymer-dielectric composites [4-6]. However, paints, which combine the optical functionalities of pigments, dyes and polymers across the solar to thermal infrared wavelengths, remain a time-tested, versatile and by far the most established radiative cooling 'metamaterial'. With the rapidly rising global demand for cooling necessitating immediate solutions, it is therefore paints on which research advances stand to be most impactful.

In this presentation, we will provide a broad overview of paints as a versatile radiative cooling metamaterial platform with multiple avenues for exploration. We will first discuss how the individual components of paints, e.g. dyes, pigments and polymers, contribute to the solar reflectance and thermal emittance required for radiative cooling. By altering these constituents and their size, as well as the macroscopic morphology of paint coatings, their optical properties can be drastically altered. We will specifically discuss the optimization of reflective pigments to maximize broadband solar reflectance, ways of enhancing the ultraviolet reflectance, tailoring macroscopic morphology to simultaneously achieve color and high near-to-shortwave infrared reflectance, reducing the diffuse glare of white paints, and enabling optical switching. Associated non-optical aspects, like the durability of optical performance, will also be discussed. Commercial state-of-the-art, recent scientific advances, and outstanding questions on paints will be presented as windows for further explorations in this field.

[1] J. Mandal et. al., Science 362, 315 (2018)
[2] L. Zhu et. al., Optica 1(1), pp. 32-38 (2014)
[3] A. Raman et. al., Nature 515 pp. 540-544 (2014)
[4] A. Gentle and G. Smith. Nano Letters 10, 373-379 (2010)
[5] N. Yu, J. Mandal, A. C. Overvig, N. N. Shi, Patent WO2016205717A1 (2016).
[5] Y. Zhai et al., Science 355, 1062–1066 (2017)

Conventional imaging requires long path lengths and bulky, multi-lens components. For applications including on-chip microscopy, lightweight space probes, LIDAR, and point-of-care diagnostics, metasurfaces promise high-fidelity imaging but in a highly reduced footprint. Metasurfaces, two-dimensional arrays of subwavelength nanostructures, precisely control the amplitude, polarization, and phase of waves in an ultrathin system. However, dynamically controlling their optical output in response to an applied bias remains an outstanding challenge. Here, we present ultrathin imaging metasurface which exploits the nonlinear Kerr effect to locally modulate the refractive index, and hence the focal intensity. Our metasurface lens consists of a series of 25 nanoscale silicon bars, 700 nm in height with a width varying from 100 and 400 nm, separated by 700 nm. The width of each individual bar imparts a phase shift, that in series can be designed to form a parabolically-varying phase profile to focus the incoming light between 4 um and 20 um away. Importantly, Silicon also supports inherent nonlinearities via the optical Kerr effect, which describes a change in a materials refractive index dependent on incident intensity. To achieve efficient nonlinearities, we introduce periodic notches symmetrically on both sides of the 219 nm wide Si bars, which introduces a high quality factor (high Q) mode via a guided mode resonance near the lens’s designed operating wavelength 1500 nm. Our full-field simulations indicate Q factors spanning 10000 to over 100000 at 1473nm for symmetric notches spanning in depth from 10 nm to 2 nm with a period of 650 nm in a 100 nm wide Si slab. We then utilize this high-Q lens to modulate the intensity of the focal spot with input power. In the nonlinear regime, as the power increases from 1 kW/cm2 to 50 kW/cm2, the resonance red-shifts from 1499.4 nm to 1499.7 nm and the normalized power at the focal spot decreases 4-fold. Importantly, this power-limiting meta-lens operates at optical frequencies, utilizes Si in a lossless regime, and can be readily extended for multi-wavelength operation by integrating additional notches into the adjacent Si bars. Our presentation will describe both the design and fabrication of this nonlinear imaging metasurface, as well as applications to dynamic imaging.

The ability to comprehensively control the constitutive properties of light including wavelength, amplitude, phase and polarization is crucial to the development of novel technologies for sensing, imaging, flat optics and holography. In recent years, metasurfaces have enabled extraordinary advances in light manipulation through precise design of their subwavelength resonant elements. However, the static nature of passive metasurfaces does not allow for post-fabrication modifications to their optical response, which limit their utility. Active metasurfaces overcome this limitation by enabling real-time control of the properties of scattered light. The active control is achieved by applying external stimuli to tune the intrinsic properties of subwavelength antenna elements. While this greatly enhances the versatility of metasurfaces, active metasurfaces typically do not exhibit ideal phase-amplitude response, i.e. constant reflectance and a phase shift of up to 360°. In addition, experimental non-idealities result in discrepancies between desired and measured performance and often generate scattering in undesired directions.

We report an inverse design approach to achieve optimal beam-steering behavior for active metasurfaces that includes experimentally relevant non-idealities, such as limited phase response, non-unity reflectance and correlation of phase and amplitude. By evaluating several multi-parameter global optimization methods, we develop an approach to enable prediction of the amplitude and direction of the scattered light, enabling access to any arbitrary steering angle. Additionally, our approach successfully overcomes the challenge posed by an inherently high dimensional optimization space. The non-intuitive solutions generated by our algorithm show up to an 86% improvement in the directivity of the steered beam compared to previous ‘forward’ designs, which typically assume constant amplitudes and do not account for experimental artefacts.

In previous experiments we have demonstrated wide phase and reflectance tunability via carrier density and refractive index modulation in gated metasurfaces that employ a semiconducting indium tin oxide (ITO) active layer [1]. Our inverse design approach treats each nanostructured antenna as an individual metal-oxide-semiconductor element, by which phase profiles are generated on a flat surface with the goal of creating arrays suitable for applications in beam-steering devices or focusing metalenses [2]. Furthermore, we use these device geometries for experimental validation of the proposed inverse design approach and discuss the role of near-field coupling between nanoantennas and its effects on the far-field radiation pattern. Finally, we explore the capabilities of our algorithm by applying it to other active metasurface functions such as reconfigurable metalens focusing and simultaneous steering in multiple directions.

[1] Kafaie Shirmanesh, G., Sokhoyan, R., Pala, R. A., & Atwater, H. A. (2018). Nano Letters, 18(5), 2957-2963.
[2] Kafaie Shirmanesh, G. et al., in preparation.

Perfect absorption at desired wavelengths enables efficient operation of optoelectronic switches. We develop and demonstrate an all-dielectric metasurface consisting of a network of electrically connected nanoscale semiconductor resonators with perfect absorption at 800 nm. Our metasurface is designed to support two degenerate magnetic dipole modes with their effective magnetic dipole vectors in and out of the metasurface plane. To enable excitation of the latter mode, which is symmetry-protected from coupling to plane waves at normal incidence, we break the resonator symmetry using simple geometrical perturbations. As a result, two modes can be excited simultaneously, leading to perfect absorption at the wavelength of 800 nm. The broken symmetry design implemented in a thin layer of low-temperature grown GaAs allows us to switch the metasurface between conductive and resistive states with extremely high contrast on the time scale of ~1 ps using an unprecedentedly low level of optical excitation. This capability allows us to make fast and more energy efficient optoelectronic detectors with THz bandwidth.

Recently, exciting new physics of plasmonics has inspired a series of key explorations to manipulate, store and control the flow of information and energy at unprecedented dimensions. In this talk I will report our recent efforts on controlling light absorption and emission process in quasi-two dimensional optical coatings. In the first example, we show how the coherent Coulombic interaction between the molecular dipoles within a molecular aggregate can be tuned by surface plasmons in close proximity. We demonstrate experimentally ultrafast quenching of 2D molecular aggregates at picosecond timescale. Our analysis reveals that the metal-mediated dipole-dipole interaction increases the energy dissipation rate by at least five times faster than that predicted by conventional models. Our results can offer novel design pathways to the light-matter interaction in a variety of photon-exciton systems with applications such as high speed visible light communication.
In a second example, I will introduce versatile 3D shape transformations of nanoscale structures by deliberate engineering of the topography-guided stress equilibrium of gold nanostructures. By using the topography-guided stress equilibrium, rich 3D shape transformation such as buckling, rotation, and twisting of nanostructures is precisely achieved, which can be predicted by our mechanical modeling. Benefiting from the nanoscale 3D twisting features, giant optical chirality is achieved in an intuitively designed 3D pinwheel-like structure, in strong contrast to the achiral 2D precursor without nano-kirigami. The demonstrated nano-kirigami, as well as the exotic 3D nanostructures, could be adopted in broad nanofabrication platforms and could open up new possibilities for the exploration of functional micro-/nanophotonic and mechanical devices.

Recently, extracting hot electrons from plasmonic nanostructures and utilizing them to enhance the optical quantum yield of 2D transition-metal dichalcogenides (TMDs) have been topics of interest in the field of optoelectronic device applications, such as solar cells, light emitting diodes, photodetectors and so on. The coupling of plasmonic nanostructures with nanolayers of TMDs depends on the optical properties of the plasmonic materials, including radiation pattern, resonance strength, and hot electron injection efficiency. Herein, we present three cases of intensifying the light-matter interactions between nano-scale plasmonic structures and a large-scale, transfer-free bilayer MoS₂. These methods include 1. unusual quadrupole gap plasmons (QGPs) in the tailored nanoantennas, 2. four different morphology-controlled plasmonic nanoparticles, and 3. engineering the bandgap of the bilayer MoS₂ with the localized strain from the plasmonic nanostructures. By introducing the plasmonic effects aforementioned into the large-scale, transfer-free bilayer MoS₂, our experimental results demonstrate not only excellent optoelectronic response, but also practical applications in hydrogen evolution reaction, photodetection and others.

The decay dynamics of excited carriers in graphene have attracted wide scientific attention, as the gapless Dirac electronic band structure opens up relaxation channels that are not allowed in conventional materials. We report Fermi-level-dependent mid-infrared emission in graphene originating from a previously unobserved decay channel: hot plasmons generated from optically excited carriers. In infrared emission spectroscopy measurements taken under optical excitation with a Ti:Sapphire laser operating at 850 nm with 100-fs pulse duration, we have measured emission for several sample geometries: planar graphene, and non-resonant and resonant gold nanodisks (NDs)-decorated graphene. The Fermi-level dependence of the observed radiation under pulsed laser excitation rules out Planckian light emission mechanisms, and is distinctly different from that observed under CW laser excitation. Our experimental results are consistent with the calculated plasmon emission spectra in photoinverted graphene, which suggest that quasi-equilibrium ‘hot’ carrier distributions in graphene upon ultrafast optical excitation support bright mid-infrared plasmonic excitation. Evidence for bright hot plasmon emission is further supported by a large emission enhancement observed from graphene decorated with gold NDs, which serve as out-coupling scatterers and promote localized plasmon excitation. The spectral flux of spontaneously generated plasmons is found to be several orders of magnitude higher than blackbody emission at a temperature of several thousand Kelvin. In addition, calculations for our experimental conditions indicate that conditions for plasmon gain exist on the sub-100 fs timescale during which stimulated plasmon emission dominates spontaneous plasmon emission. These observations set a framework for exploration of ultrafast and ultrabright mid-infrared stimulated and spontaneous emission processes and bright infrared light sources.

Nonlinear optics is essential for information processing, sensing, and optical characterization applications. There are well known strong nonlinear homogeneous materials available at visible frequencies and composite structures that can exhibit a strong nonlinear response already engineered at long-wave-IR frequency ranges, but there are few materials with strong second order nonlinearity that exist within the infrared frequency range. Over the past decade, metamaterials, composites with subwavelength (often, plasmonic) inclusions have emerged as a viable platform for engineering linear optical response. Nonlinear metamaterials, where the structure of the composite used to re-shape local fields and enhance the nonlinear response of the components, have also been proposed.

Recently, we have demonstrated the emergence of structural nonlinearity, where the bulk nonlinear response of the composite results from a nonlinear interaction of nano-shaped light and free-electron plasma in nominally non-second harmonic generation (SHG) active gold. For this work we explore the potential of structural nonlinearity in plasmonic metamaterials for the second harmonic response at infrared frequencies. We demonstrate the emergence of structural response in plasmonic nanowire metamaterials and analyze perspectives of structure-induced plasmonic nonlinearities for the infrared frequency range.

Nanolasers generate coherent light at the nanoscale. In the past decade, they have attracted intense interest, because they are more compact, faster and more power-efficient than conventional lasers. The eigenmode of a nanolaser can be engineered in a controllable manner for novel inner laser cavity field and/or emission beam synthesis. Furthermore, ensembles of nanolasers operating in unison can provide a macroscopic response that would not be possible in conventional lasers.

Lead halide perovskites have recently received considerable attention as promising candidates for laser devices due to excellent material properties such as high optical gain coefficient and low non-radiative recombination rate. However, most of the lead halide perovskite lasers were operated in pulsed mode in order to reduce the thermal loss to achieve population inversion, which requires extra efforts to generate the external pulsed excitation source. Here, we demonstrate a continuous wave lasing from single lead halide perovskite (CsPbBr₃) quantum dot integrated with plasmonic nanocavity with undetectable threshold (power density lower than 90 mWcm^-2) and ultra-sharp single mode lasing (linewidth of 1.4 nm) under a cryogenic temperature at 4 K. In our design, a silver nanocube was placed on a gold substrate, with a single CsPbBr₃ quantum dot placed in between. By finite-difference time-domain simulation method to calculate the electric field distribution of the designed structure, we theoretically observed a strong localized optical confinement, plasmonic gap mode, formed between the silver nanocube and the gold substrate. The plasmonic gap mode acts as plasmonic nanocavity and contributes a large Purcell enhancement (Purcell factor ~200) which can be determined via time-resolved photoluminescence measurement. In order to avoid quenching effect, a 6 nm Al₂O₃ spacer was used to separate the quantum dot and the gold substrate, and the silver nanocube was covered by 1 nm Polyvinylpyrrolidone to isolate the emitter. In order to measure the lasing threshold, the temperature dependent light-in-light-out curve was performed and we observed a very small lasing threshold (power density lower than 188 mWcm^-2) at 80 K. This work is the first demonstration of continuous wave lasing from single quantum dot integrated with plasmonic nanocavity. In the end, we will discuss the detail working mechanisms as well as the practical applications.

The integration of plasmonic nanoparticles and optical gain materials has opened a new avenue in developing micro- and nano-lasers. Lead halide perovskite is as a promising semiconductor gain material with high optical gain and solution processability. Here, we describe a technique to integrate plasmonic particles into perovskite-based matrices and demonstrate lasing from various micro-structures. In particular, our results suggest that photonic Anderson localization can be achieved owing to the combination of strong light scattering from the embedded plasmonic particles and high optical gain from perovskite nanocrystals. In this work, we used a spin-cast precipitation of perovskite precursors and metallic particles to produce spherical microshells, which comprises a Ruddlesden-Popper phase CsPb₂Br₅ matrix incorporating CsPbBr₃ nanocrystals and silica-coated silver spheres with a diameter of 100 nm. In a coherent-backscattering experiment, we measured the product of wavenumber and disorder length to be 0.7, satisfying the Ioffe-Regel criterion for strong transverse localization. We observed stable multimode Anderson lasing from the hybrid microparticles under nano-seconds optical pumping at room temperature. The smallest Anderson localized laser we observed has an outer diameter of 1.1 μm and a shell thickness of 210 nm containing about 15 silver particles, with a lasing threshold of 4 mJ/cm².

Optical imaging owns the privilege of high specificity and high sensitivity due to the spectral multiplexing for in vivo applications, but is always hampered by the tissue autofluorescence and scattering. One promising approach to preventing tissue autofluorescence for optical imaging is to use up-conversion fluorophores. However, traditional up-conversion nanoparticles emit in the visible range, where the emission signal is heavily absorbed by the tissue, reducing the signal-to-noise ratio. In this work, we developed an autofluorescence-free imaging probe with plasmon-enhanced up-conversion fluorescence in the short-wave infrared window (850-1700 nm). We find the up-conversion enhancement is more than 125 times compared to the free fluorophores. Thermodynamic studies show that the dependence of up-conversion emission intensity on temperature fits the Arrhenius equation. In addition, the up-conversion emission intensity increases linearly with the excitation power. These results reveal that the observed plasmon-enhanced up-conversion is thermally assisted. Moreover, the tunable width of the working optical window for the up-conversion probe is around 200 nm, providing high flexibility for in vivo applications. Finally, we find that in a tissue phantom the signal-to-noise ratio of the plasmon-enhanced up-conversion fluorescence is more than 4 times better than the corresponding down-conversion fluorescence. The above results demonstrate that our plasmon-enhanced up-conversion fluorescent probe is a promising candidate for high-contrast in vivo imaging at short-wave infrared.

Complex multi-phase nanocomposite designs present a great opportunity for developing next generation integrated photonic and electronic devices. In this presentation, we demonstrate a unique three-phase nanostructure combining a ferroelectric BaTiO₃, wide band-gap semiconductor ZnO and plasmonic Au metal towards multifunctionalities. By a novel two-step templated growth, a highly ordered Au-BaTiO₃-ZnO nanocomposite in a unique “nanoman”-like form, i.e., self-assembled ZnO nanopillars and Au nanopillars in BaTiO₃ matrix, has been realized, and is very different from the random three-phase ones with randomly arranged Au nanoparticles and ZnO nanopillars in BaTiO₃ matrix. The ordered three-phase “nanoman”-like structure provides unique functionalities such as obvious hyperbolic dispersion in the visible and near infrared regime enabled by the highly anisotropic nanostructures compared to other random structures. Such self-assembled and ordered three-phase nanocomposite is obtained through a combination of Vapor-Liquid-Solid (VLS) and two-phase epitaxy growth mechanisms. The study opens up new possibilities in design, growth and application of multi-phase structures and provides a new approach to engineer the growth of complex nanocomposite systems with an increased control over electron-light-matter interaction at the nanoscale. Finally, in-situ Transmission Electron Microscope (TEM) heating studies is presented to determine the thermal stability of the three-phase nanocomposite.

Under illumination, noble metal nanoparticles such as Au, Ag and Cu can act as efficient sources of heat thanks to the decay of localized surface plasmon resonances, which are light-driven oscillations of their free electrons.¹ Such heating effect is confined to the immediate surrounding of the particles and could therefore be used to locally activate chemical reactions with nanoscale spatial resolution.^2,3 Here, we show how we can activate, control, and spectroscopically follow the growth of different semiconducting shells on individual plasmonic nanoparticles under laser irradiation. We first adapted a literature reported temperature-dependent colloidal synthesis of Au@CeO₂ core@shell nanoparticles in the dark⁴ and then perform the same synthesis at the single particle level at room temperature using plasmonic heating. We show that, under plasmon excitation, we can grow conformal metal oxide shells with growth rates that scale with the nanoparticle surface temperature. Furthermore, the shell growth can be monitored in-situ by tracking the photoluminescence spectra of the plasmonic nanoparticle under irradiation. We also demonstrate that such photothermally-driven synthesis of core@shell nanostructures can be extended to grow other semiconductors such as zinc oxide and zinc sulfide, illustrating the universality of our technique. The use of light as a tool to activate and control chemical reactions at the nanoscale can lead to the synthesis of shape- and size-controlled hierarchical nanostructures which are inaccessible with classical colloidal synthetic methods, with potential applications in nanolithography, catalysis, energy conversion, and photonic devices.
¹ M.L. Brongersma, N.J. Halas, and P. Nordlander, Nat. Nanotechnol. 10, 25 (2015).
² G. Baffou and R. Quidant, Laser Photonics Rev. 7, 171 (2013).
³ G. Baffou, P. Berto, E.B. Ureña, S. Monneret, J. Polleux, and H. Rigneault, ACS Nano 7, 6478 (2013).
⁴ B. Li, T. Gu, T. Ming, J. Wang, P. Wang, J. Wang, and J.C. Yu, ACS Nano 8, 8152 (2014).

Controllable integration of plasmonic nanomaterials is a key foundation towards diverse applications in enhanced spectroscopies, sensors and electronic devices. Using top-down lithographical methods, including electron-beam or focused-ion beam lithography, could fabricate metallic nanostructure arrays with prescribed shapes, dimensions, and long-range ordering. However, lithography-defined metal nanostructures are usually polycrystalline in nature, which scatters the electrons and lowers the quality of surface plasmonic resonance. Besides, limited by the lithographic resolution, it is still difficult for scaling the edge-to-edge spacing down to sub-5 nm, further hampering the plasmonic coupling strength.

Alternatively, bottom-up assembly that uses chemical interactions to direct the integration of single-crystalline nanoparticles could provide both atomic-smooth nanoparticle boundaries and sub-1 nm edge-to-edge spacings. Both features promote the surface plasmonic resonance and minimize the energy losses at grain boundaries. Particularly for anisotropic nanoparticles, such as nanorods and cuboids, they exhibit coherent electron oscillations perpendicular and parallel to the long axis (i.e. transverse and longitudinal LSPR). By tuning the chemical recognitions, large-scale assembled nanoparticles display different lattice morphologies, periodicities and compositions. To in silico design and selective prepare prescribed superlattice structures from the anisotropic nanoparticles, it further requires controlling the relative sliding and rotation of the nanoparticles. However, limited by the difficulties for precise calculation and modulation of different assembly modes (i.e. head-to-head or side-by-side), it remains challenging to assemble anisotropic nanoparticles into monolithic superlattices with prescribed plasmonic properties over centimeter scale.

We here report a general framework that could in silico design and selectively prepare monolithic superlattices from anisotropic gold nanoparticles. We discover that precisely designed surface patterns could direct both the surface positions and the assembly modes of anisotropic gold nanoparticles. By designing different surface patterns, anisotropic nanoparticles could be selectively assembled into five prescribed modes, including head-to-head and side-by-side conformations. Epitaxial growth further extends the structural programmability into 3D space. The assembled superlattices exhibit wafer-scale uniform orientations, structural parameters, and assembly morphologies. Complex alphabets, numbers, and polygon shapes could also be constructed using the aligned nanoparticles. Because the minimal edge-to-edge spacing is less than 2 nm, we observed strong plasmonic coupling, as evidenced by the dark-field scattering measurements and FDTD simulations. Additionally, we have revealed that the nanogap induced subwavelength “hot-spots” lead to dramatically enhanced signals for SERS and fluorescence within the superlattices. Using the assembled superlattices, we could further construct sensitive diagnostic platforms that sense multiplex disease markers at single-molecule resolution.

In the United States, 68.5% of energy consumed is lost as waste heat or transmission losses [1]. If we could harness even a small portion of that thermal radiation we could heat and electrify millions of American homes. Rectifying antennas, or rectennas, absorb EM radiation and convert it to a DC voltage using an antenna and a rectifying diode. In a rectenna system with an EM source, the signal is converted into electrical AC signal by the antenna array and AC voltage signal is rectified by the rectifying diode to generate DC. Rectennas with conversion efficiencies greater than 80% are achieved both by using high input power at 915 MHz and 2.45 GHz as in [2] and [3], or by harvesting ambient environment RF from 876 MHz to 2.48 GHz using a multi-band stacked RF rectennas [4]. To move rectennas toward the THz range using infrared radiation sources, we consider the broad-band emission from our sources approximated as a blackbody spectrum. However, a full spectrum IR blackbody source requires high frequency antenna materials. Additionally, using broad-band sources introduces challenges such as needing a broad-band antenna response and a broad-band transmission line. To address these concerns, we add a metamaterial selective-emitter to absorb the radiation from the source and emit a narrow range of photons towards the rectenna.

We designed a metal-insulator-metal (MIM) diode made of Al/Al₂O₃/Au, where the top gold metal consists of our antenna array pattern. The MIM diode was simulated in CST and our Al and Al₂O₃ layers are deposited using the AJA 3-gun Sputtering system at Harvard’s Center for Nanoscale Systems’ Nanofabrication Facility. The antenna pattern is rectangular rows of length 5.4 μm and width 1.2 μm separated by 1.2 μm and connected at one end by a 0.5 μm contact bar. To fabricate the antenna array and selective emitter, we are using the Maskless Aligner MLA150 to expose the pattern directly onto photoresist with a 375 nm laser. Using this tool we can write 1 cm² areas in under 10 minutes, which is significantly faster than writing patterns using electron beam lithography. Although the minimum advertised structure size for the MLA150 is one micron, we have successfully achieved sub-micron resolution by using contrast enhancing material to increase resolution and optical proximity correction to reduce corner rounding [5]. Characterization of the full rectenna device is forthcoming, and samples will be characterized by Vector Network Analyzer in the THz range.

[1] Lawrence Livermore National Lab, Estimated U.S. Energy Consumption in 2018: 101.2 Quads (2018).
[2] K. Niotaki, S. Kim, S. Jeong, A. Collado, A. Georgiadis, and M.M. Tentzeris, IEEE Antennas and Wireless Propagation Letters 12, 1634 (2013).
[3] R. Scheeler, S. Korhummel, and Z. Popovic, IEEE Microwave Magazine 15, 109 (2014).
[4] V. Kuhn, C. Lahuec, F. Seguin, and C. Person, IEEE Transactions on Microwave Theory and Techniques 63, 1768 (2015).
[5] J. Hur and M. Seo, Journal of the Optical Society of Korea 16, 221 (2012).

Structural color phenomena exhibited by several organisms produce compelling and vibrant visual displays. These impressive effects result from interference and diffraction of light incident upon multilayer nanostructures, in which color is broadly tuned based on surface structure and geometry. The wings of the Morpho butterfly are a well-studied example of a biological system exhibiting structural coloration and a high degree of wide-angle iridescence due to a non-negligible degree of disorder in the photonic nanostructure. Recent work has demonstrated fabrication of artificial, Morpho-inspired nanostructures that exhibit coloration via a variety of fabrication techniques, including multi-step deposition, etching, and assembly processes. However, existing work has largely focused only on replication of specific structures found in nature, with few methods incorporating both color replication and the role of disorder on effects like iridescence. Furthermore, many of the processes used to produce these structures are elaborate or time-consuming. Here we explore methods for generating structures to create desired effects, while proposing new approaches for designing structurally-colored surfaces that are physically producible with rapid prototyping.

We propose methodologies for combined design, simulation, and material fabrication to generate color through new structural surfaces that replicate functional properties of natural systems. We depict a design methodology based around computational inverse design for the formulation of nanostructures exhibiting structural coloration. We furthermore depict example design constraints and study convergence with respect to the design trade space. We use simulation to determine optical properties (such as reflectance, and spectral response) of potential photonic surface structures, and compare different simulation methods on the basis of accuracy, as well as computational complexity and speed.

Such intricate biological systems require advanced fabrication techniques, and replication of nanoscale features of this complexity has been difficult. Thus, our designs are constrained for realizable fabrication using a multitude of processing methods, including direct laser writing techniques such as two-photon polymerization. Such techniques provide amenable platforms for rapid prototyping of simulated optimized structures to test for practical fabrication and implementation that provide a cost- and time-effective alternative to traditional lithographic methods. We then evaluate optical properties such as spectral response, diffraction, and reflectance of fabricated photonic structures. Successful samples will passively produce robust, visible colors with wide angle viewability, and surfaces will be iteratively designed towards the goal of specifically tuned color outputs.

When combined with the design and optimization process presented here, these methods allow for the use of the Morpho structure as a baseline for iteration, both to incorporate tailored disorder, and to produce structures with functionality beyond existing systems. In doing so, we present a versatile approach to bio-inspired materials design and provide a platform with applications ranging from light harvesting and steering, to chemical sensing, high performance displays, responsive products and architecture. We aim to compare our simulations to fabricated structures using optical microscopy, scanning electron microscopy, and angular spectrometry to test for quantitative performance. These evaluated and characterized structures could eventually be adapted to roll to roll or imprint-based systems for scale-up and manufacturing on a commercial scale.

Materials that exhibit tunable photonic responses have found widespread application in displays, lasers, sensors, and other optical sources. The combination of inherent material properties and nanostructuring gives fine control of optical transmission, absorption, and reflection in terms of peak wavelengths and intensities. We fabricate thin film and nanostructured designs that apply the transient optical responses of Mg and MgO to produce display pixels that exhibit a color change in response to the chemical reaction between these materials and water [1]. This permanent reaction holds promise for use in data encryption and anticounterfeiting. Color pixels fabricated through both metal-insulator-metal Mg/MgO thin film and Mg nanorod array designs allow for transmission signals corresponding to any color within the sRGB gamut. FDTD simulations are used to verify the accuracy of experimental spectral data and offer new insight into future designs that incorporate both stable and transient elements using spectroscopic ellipsometry data. In order to determine the effects of temperature and deposition method on the transient nature of the optical response, we investigate the etching properties of Mg thin films and nanostructures. Variation of temperature between 10 and 80 ^oC allows for the tunability of distinct hues, besides the modification of the rate at which color changes for additional device control based on the desired application. Mg and MgO are low cost, earth-abundant, and environmentally friendly materials; and, in turn, ideal candidates for optical components in industrial-scale devices. Research into their use in conjunction with other metals (e.g. Al, Ag, and Au) opens up a new opportunity for color pixels than can be tuned between multiple signals set through nanostructured geometry, as will be presented.

[1] T. G. Farinha et al, ACS Photonics 2019 6 (2), 272-278

Probing 3D dielectric photonic crystals in the visible and in the infrared range typically requires fabricating complex 3D architectures with sub-micron features out of low absorption, high refractive index materials. Nano-architecting transparent high refractive index materials in 3D is technically challenging and requires complex experimental procedures, including stacking of individually fabricated 2D layers or double-inversion of a 3D polymer template.
We developed a template-free additive manufacturing (AM) process that is capable of producing complex 3D architectures with sub-micron features out of titanium dioxide (TiO₂). We synthesize a hybrid organic-inorganic precursor and use it to formulate a pre-ceramic photoresist that is shaped into a designed 3D structure using two-photon lithography (TPL). This architecture is then pyrolyzed in air at 900°C, which yields a replica of the structure with 70% smaller dimensions. Energy-Dispersive Spectroscopy (EDS) and Raman spectroscopy reveal such processed material to be predominantly rutile titania.
We fabricated titania woodpile structures with a 1.5 μm lateral period that are comprised of 560 nm x 640 nm beams. Transmission Electron Microscopy (TEM) reveals that the microstructure of individual beams is nanocrystalline, with a 110 nm mean grain size and <1% porosity. Fourier Transform Infrared (FTIR) spectroscopy shows a high reflectance and a low transmittance peaks centered at 2.9 μm, which agrees with the position of a full photonic bandgap predicted by Plane Wave Expansion (PWE) simulations. This titania AM process offers a promising pathway to fabricate complex 3D nano-architectures out of high-index materials for 3D dielectric photonic crystals in the visible and the infrared.

This work presents a hybrid photodetection platform that combines the 2D material graphene, placed on top of well-defined SiO₂/Si nanoscale etched trench structures with simple and cost-effective fabrication processes. The hybrid nanoelectronic devices are consisting of a graphene/Si (Gr/Si) heterojunction in conjunction with graphene/SiO₂/Si (GrOS) field effect structure. Additionally, the suspended graphene naturally placed between two device structure (Graphene/Si and Graphene/SiO₂/Si) enable the nanoscale vacuum electronics formation between two devices, allowing the photoelectron transport and interacting with each other in the quantum regime leading to exceptional characteristics.

In hybrid GrOS based on p-Si substrate, the photo-excited electrons are separated in Si and subsequently transferred to the SiO2/Si interface to combine with the intrinsic two-dimensional electron gas (2DEG) confined in a quantum well. This will significantly increase the amount of 2DEG. Therefore, the localized 2DEG causes two important phenomena: (1) increasing the localized electric field, leading to carrier multiplication generation in confined 2DEG channels and thus amplifying the photocurrent in the devices and (2) reducing the potential barrier height via strong coulombic repulsion at the edge of the Si, enabling electron emission through the air to complete the electrical current circulation as similar to nanoscale vacuum electronics. The hybrid photodetector device exhibits ballistic transport of photoexcited hot carriers with carrier multiplication gain resulting in high quantum efficiency for photodetection characteristic. Two-dimensional (2D) hybrid nanoelectronic devices based on p-Si and n-Si provide fully and finely tunable sensitivity up to 1.2 and 0.45 A/W, respectively, corresponding to external quantum efficiencies (EQE) of 235% ( ~350 % internal quantum efficiencies (IQE)) and 88% EQE (or ~132% EQE), respectively. The multiplication gain in the proposed hybrid device originates from the impact ionization initiated by photoinduced carrier injection into the self-induced localized electric field (up to ∼10⁶ V/cm) distributed in a 2DEG region in Si. The tunable photocurrent was rising with three half-power voltage dependence as a supplied voltage which corresponds to the Child−Langmuir (CL) space charge limited current as electron transport in a vacuum. Thus, the result suggests that the overall carrier transport associated with the electron emission process from the 2DEG is confined in an inversion layer of the GrOS field-effect structure, allowing the ballistic transport of photoelectrons through a nanoscale vacuum (air) channel within the mean free path distance (<100 nm) to be collected by the graphene electrode.

The ON/OFF ratios of proposed device is in the range of ∼10²−10⁵. The hybrid GrOS-based p-Si photodetection has fully tunable responsivity and efficiency that is suitable for imaging applications requiring bright condition adjustments. Therefore, the proposed hybrid photodetection platform is architecturally Si and CMOS-compatible and thus highly promising for ultrafast, low-power, and tunable optoelectronic applications.

III-V nanostructures have shown novel exciting (opto)electronic properties, making them interesting for a wide range of disciplines ranging from bioengineering to quantum information technology and energy conversion. In particular, their optical properties are not only tuned by chemical composition (e.g., in ternary compounds), but also by their size, shape and collective arrangement (e.g., quantum confinement, Mie resonances, collective optical resonances). However, their manufacturing suffers from expensive techniques due extreme operational conditions (e.g., high temperatures and ultra-high vacuum) and has restricted choice on substrate materials.
In this work we explore the combination of electrochemical deposition and soft conformal imprint lithography (SCIL) to grow InAs nanostructure arrays on diverse substrates, including silicon and transparent ITO. We have electrodeposited arsenic from a liquid electrolyte at room temperature onto patterned indium droplets, which spontaneously reacts into InAs. Raman microscopy characterization shows the high crystal quality of the grown InAs nanoparticles by analyzing the lineshape of the longitudinal optical mode. We explain the growth of highly crystalline InAs nanoparticles by a 3D diffusion-reaction model and facile strain release provided by the confined nature of the initial indium particle. We explore the nanophotonic properties of the fabricated InAs nanoparticle arrays, with particular focus on light scattering. By tuning the size and pitch distances we demonstrate the potential of the presented methodology to manipulate the reflection, transmission, absorption and diffraction of light.
While this work represents the first demonstration of crystalline III-V nanostructure growth on a transparent conducting substrate (ITO), combining SCIL and electrochemistry is a promising high-throughput nanofabrication methodology towards inexpensive III-V nanopatterns, with the potential of bringing emerging all dielectric III-V designs to the market.

Whispering gallery mode microcavities stand out due to their unique features, such as narrow spectral linewidth, small modal volume and high power density, making them interesting for several applications in photonics. Microcavities made of polymeric materials offer additional advantages, such as the ease of processing and the flexibility for incorporating dopants that bring functionality to the structure. In this direction, femtosecond laser induced two-photon polymerization has proven to be a powerful tool for the microfabrication of 3D polymeric structures. In this technique, spatial confinement of the polymerization is achieved by the nonlinear nature of the two-photon absorption process.

In this work, whispering gallery mode microcavities containing graphene oxide are fabricated via two-photon polymerization on an acrylic-based resin. The produced structure exhibits good structural quality and smooth sidewall surfaces (undoped cavity Q-factor of 1×10⁵ @ 1550 nm). Raman spectroscopy confirms the presence of graphene oxide in the doped structure. The microresonator modes were characterized by a coupling setup based on evanescent field from a tapered optical fiber, which uses a broadband source centered at 1530 nm as excitation. Light is coupled into the resonator using a 2 µm diameter tapered fiber in the overcoupling regime and the transmitted light is guided to an optical spectral analyzer. The transmitted spectrum exhibits sharp resonances with a free spectral range of 9.3 nm, which is in agreement with the expected value given the structure geometry and index of refraction. Interestingly, we have observed a substantial decrease in the number of modes in the transmission spectrum for the graphene oxide doped microcavity in comparison to the undoped one, which we believe may be related to some thermal processes since saturable absorption of graphene oxide, whose saturation intensity is on the order of 500 MW/cm², is not expected to occur for the excitation level used here. Therefore, we have been able to demonstrate the use of two-photon polymerization in the fabrication of high performance optical microcavities doped with graphene oxide, which can be exploited for further developments in high performance optical microdevices.

A memristor-based crossbar is one of promising candidates for in-memory computing since it features nanosecond switching speed, extremely small cell size, low energy consumption for matrix-vector multiplication and weight update, capability of both storage and computing, three-dimensionality, and many analog weight update steps. Although there have been intensive studies on the development of a large-scale memristor crossbar to implement neuromorphic hardware system for deep neural networks, only limited approaches, such as inference task, were suggested due to device and cycle variations and nonlinear/step-limited weight update properties. To handle those issues, a 1T1R (1-transistor/1-memristor) with a closed-loop or spike-based count methods have been used to possess more control on linear weight update. However, to fully exploit memristor-based crossbar in-memory computing without above temporal solutions, it essentially requires (1) linear/symmetric conductance change, (2) multiple-level conductance steps, (3) less spatial and temporal variations for weight values with more analog components involved.
Here, we developed a simulator which estimates the energy consumption and accuracy of inference/online training in multi-crossbar peripheral system to provide an insight on required specifications of memristor device. By introducing analog components, overheads from ADC/DAC are minimized. Also, we partitioned two types of crossbars for fully-functioning in-memory computing: (1) morphable crossbar for computing, (2) memory crossbar for memorizing values used for backpropagation. With our recommended device parameters as guideline, we achieved 97% and 81% accuracy on MNIST and CIFAR-10 datasets, respectively. Also, it performed approximately X 30 and X 12 better than RTX 6000 and TPU in energy savings.

Transition metal oxides are of great research interest over the past decades because of dramatic changes in optical and electrical properties throughout the phase transition processes, which have potential applications in ultrafast transistors, ultrafast electrical switches, gas sensors and thermochromic smart windows. In particular, vanadium dioxide, VO₂ is one of the scientifically fascinating and technologically promising transition metal oxides depicting a reversible first-order semiconductor to metal phase transition (SMT) at critical temperature Tc = 68°C (341 K) which is much higher than the ambient room temperature. However, carrier transport mechanism in ultrafast timescale and the effects of dopants on it has yet to be clearly understood.

In this talk, we fabricated VO₂ thin films, VO₂ doped with Au, and Pt nanoparticles by pulsed laser deposition (PLD). Integrated VO₂ films with ultrafast photoconductor device structure, resulted in a sub-40 picosecond response time, we studied the carrier photo-physics dynamics such as carrier photogeneration, recombination, transport, trapping, and de-trapping. In addition to investigate the temperature dependent transport dynamics resulted from the phase transition, we revealed the novel effect of Au and Pt nanoparticles doping. These fundamental understanding paves the pathway to functionalize the VO₂, leading to designing nanoelectronics devices.

Proteins consisting of one or more polypeptides play vital functional and structural roles in human body. To be used in research involving early diagnosis, drug delivery and clinic therapy, determining structure and concentration of these proteins are extremely important. To date, conventional analysis tools including enzyme-linked immunosorbent assay (ELISA), X-ray, nuclear magnetic resonance (NMR) and cryogenic electron microscopy (cryo-EM) have contributed significantly to the development of protein research. However, such tools require complex and costly sample preparation procedures as well as skilled experts to analyze the measurement results.
SERS (Surface Enhanced Raman Spectroscopy) is considered as a promising alternative for such measurement methods. Raman spectra directly measured from molecular vibrations can provide crucial information on proteins’ conformation and concentration with minimal efforts compared to conventional analysis. However, obtaining reproducible and distinguishable Raman spectra is challenging due to proteins’ extremely small Raman cross section.
To overcome such limitations, and perform conformational studies and quantitative analysis of proteins using Raman spectroscopy, we present a 3D SERS substrate coated with a carboxylic-acid-functionalized graphitic nano-layer (SSCG). The fact that SSCG can perform both structural analysis and concentration determination in a single measurement is a unique advantage over conventional analysis methods and other SERS research reported thus far. SSCG features stacking of gold nanowire sheets, which form 3D nanostructures. Fine and dense gold nanowire arrays generate strong local E-fields, and enable to obtain characteristic Raman spectra of proteins to analyze conformational features. Furthermore, a carboxylic-acid-functionalized graphitic nano-layer is coated on top of the 3D SERS substrate, and used as an immobilization media by forming a peptide bond with analytes. These functional groups enable the analytes to be dip-coated, completely eliminating the coffee ring effect and obtaining a uniform analytes coating, which leads to the possibility of quantitative analysis.
The proteins used in this study are tau protein and beta-amyloid. Both proteins are associated with Alzheimer’s disease (AD), which is a representative illness causing human dementia. These proteins are present in the body fluids and are well-known candidates for AD biomarkers. Accurate identification and quantification of such biomarkers by SERS will enable early diagnosis of the disease, which is critical in delaying disease progression.

Over the past few years, semiconductor nanowire arrays have been extensively studied for achieving intriguing optical, mechanical, and electrical features. In recent years, the use of nanowires has expanded from the conventional application fields (i.e., electronics and photonics) to medical application such as artificial photoreceptor. This recently-developed nanowire-based artificial photoreceptors interface with blind retinas to restore the vision inability. Their highly ordered structure is more analogous to the architecture of photoreceptors, which allows for proficient photo-absorption and charge separation that is similar in photon-harvesting electronics such as solar cells and photodetectors. Based on these features, the nanowire-based photoreceptors can overcome the representative limitations of predecessors such as requiring an external power supply and low density. They can generate and carry photocurrent to depolarize neurons without an external power supply, and their high/uniform density of photo-responsive units allows an enhanced spatial resolution. However, the reported nanowire-based photoreceptors have not been optically optimized yet, showing a restricted absorption range (i.e., UV range) and low wavelength selectivity and sensitivity.
Here we introduce the approach to the highly selective and sensitive light absorber, like retina, using highly populated III/V nanowire forests fabricated by lithography-free method. Theoretically analyses reveal the diameter-dependent selective photon absorption is allowed even in dense and disordered configuration. Self-catalyzed growth implements the nanowire forest with the high density (e.g., mean nearest inter-distance = 192.4 nm) and high aspect ratio (e.g., mean aspect ratio = 34.3). Also, the averaged diameter of 94 nm with 49 nm-standard deviations sufficiently covers and decomposes visible spectrum. Finally, we observe a selective light-absorbing behavior of the nanowire forests with three monochromatic colors such as red, green, and blue. Weak correlations between each response such as minimally 0.05 and maximally 0.28 demonstrate that the nanowire forest functions like the retina. This work suggests that the nanowire forests could be applied for next-generation optoelectronics and/or bio-compatible light-sensitive material such as retinal prostheses.

Additive manufacturing via two-photon lithography (TPL) has been demonstrated as an effective method to photochemically reduce metal ions to obtain metals structures. Thus far, only a few metals, such as Au and Ag, can be fabricated by TPL, whereas photochemical reduction (PCR) of other metals, such as Ni, Cu, and Ti, remains out of reach. In this work we demonstrate a localized photochemical reduction AM method that is applicable to metallic systems beyond silver and gold using the photoreductant tungsten(0) arylisocyanide, while investigating its reduction mechanism under two-photon excitation. A recent work by Sattler et al has shown that the excited state of tungsten arylisocyanide exhibits a reduction potential of -2.7 V, which can theoretically reduce a wide variety of metal ion species. In this TPL process, we employ an ultra-short pulsed laser that is tightly focused into a voxel, where the tungsten complex absorbs the two photons simultaneously, donates an electron to the metal ion present in the system and reduces it to a metallic atom. We demonstrate a successful reduction of copper in 2D structures, which are characterized by energy dispersive spectroscopy (EDS) to show a uniform distribution of copper. To investigate the reduction mechanism, quenching experiments are performed using time-resolved spectroscopy and an electron transfer rate of 2x10⁹ M^-1 s^-1 is determined, which suggests a diffusion-limited process. This method enables direct reduction of metal ions to metal structures and potentially provides gateways to develop new metal composites and to fabricate rare non-native metals for various applications in microelectronics, photonics, medical implants, and microelectro-mechanical systems.

Transparent Conductive Oxides (TCOs) are appealing plasmonic material candidates due to their transparency, refractory character, CMOS compatibility and their ability for dynamically tuning their properties through the application of an electric field. An important additional asset of TCOs is that their optoelectronic properties depend strongly on the precise fabrication techniques and conditions. Specifically, adjustment of the target doping, deposition and post-growth processing conditions causes variations in the stoichiometry, crystallinity and donor states. These result in changes to the carrier transport properties and thus enable one to tailor the optoelectronic properties of TCOs towards specific device requirements. Such modifications of TCOs have been achieved through thermal annealing in controlled ambient compositions to increase the carrier mobility and adjust the carrier concentration by inducing activation of donor states and modulation of the oxygen vacancy concentration. However, thermal annealing of thin films suffers from long dwell times and high thermal budget, making the process expensive, cumbersome and unable to be utilized for cases where the characteristics of the substrate must be preserved (i.e. flexible displays or manufactured chips with heat-sensitive components). Excimer Laser Annealing (ELA) has been demonstrated to be able to overcome these limitations and offer an ultra-fast, scalable and low thermal budget post-growth processing technique to enhance the crystallinity of TCOs. ELA operates through the application of a highly localised heating (in space and time) and offers a significantly increased level of control over the processes by varying the pulse length, pulse frequency, pulse number, fluence, wavelength and environmental temperature, pressure and composition.
In this work, we couple the ability of the local environmental to probe the defect composition with the advantages of laser processing, via ELA of room-temperature sputtered Indium Tin Oxide (ITO), Aluminium-doped Zinc Oxide (AZO), Gallium-doped Oxide (GZO) and Fluorine-doped Tin Oxide (FTO) thin films within a UV-transparent pressurised cell where we can finely control the ambient composition and pressure of the contained gasses. Through the use of a range of reactive gasses which are: oxidising (O₂), reducing (5% H₂ in N₂) or intermediate (0-100% O₂ in N₂ or Ar), we alter the concentration of oxygen vacancies and activated donors within the lattice after processing and relate these laser-induced compositional modifications to changes in the morphological, structural and optoelectronic properties. Ellipsometry in the IR (1.6-40 microns) enables us to directly probe the free-carrier contributions to the permittivity and thus precisely determine the optoelectronic properties of the as-grown and laser processed films.
We report an ambient-dependent modulation of the plasma energy and damping coefficient with laser fluence and ambient composition after laser processing while preserving the amorphous nature of room temperature sputtered ITO and improving the crystallinity of AZO and GZO. Our results present ‘Reactive Ambient - Excimer Laser Annealing (RAELA)’ as a novel and powerful technique to control the optoelectronic properties of key TCO materials. This allows for the creation of recipes to improve the quality of these alternative material candidates as plasmonic components. Due to the fast and localised nature of RAELA, and the utility of IR ellipsometry, we also present the combination of these two techniques as a useful tool in further material science studies into the, still not fully understood, conduction mechanisms of TCOs.

A number of powerful photonic strategies to study the influence of the local density of optical states (LDOS) on the dynamics of light emitters have been developed over the years [1-3]. One of the simplest approaches involves placing the emitters between two reflecting surfaces forming a Fabry-Perot cavity. To vary the LDOS in such a system, the key tuning knob is the cavity length.
Typical Fabry-Perot cavities are fabricated as vertical stacks using thin film deposition. However, from a practical point of view, tuning the LDOS in such a vertical Fabry-Perot cavity requires the fabrication of many different geometries [4], thus limiting the ability to efficiently optimize the performance. Obtaining a large range of cavity lengths in a single structure is challenging, though some reports using more complex scanning probe methods have been reported [5-6].
Here, we present a method to produce a wedge-shaped cavity in which the full spectrum of cavity lengths can be obtained [7]. It is fabricated by mechanically clamping together two silver-coated microscope slides using a small spacer on one side. This allows for accurate control over the angle (< 1 degrees) between the two silver surfaces and generates a continuously tunable cavity length along the wedge. The simplicity of this technique provides the quick development of samples which can be analysed with single measurements including the thickness of the cavity as free parameter. This platform allows us to precisely align the cavity mode spectrum with the energy spectrum of the active layer. As a first example, we will present results on the fine-tuning of cavity resonances with excitonic excited states in quantum dot solids, and discuss the influence of the LDOS in modifying the excited state and spatial dynamics of these systems. Second, we will discuss the fine-tuning of strong light-matter coupling between cavity modes and molecular vibrational states [4].
Our simple architecture provides a powerful yet easily implementable strategy for the detailed mapping of LDOS in a variety of materials. This will aid the design of optimized cavity induced photonics.

References:
[1] K. H. Drexhage, et al, Ber. Bunsenges. Phys. Chem., 70, 1179 (1966)
[2] J. Johansen, et al, Phys. Rev. B, 77, 073303 (2008)
[3] A. Kwadrin et al, J. Phys. Chem. C, 116, 16666-16673 (2012)
[4] A. Shalabney, et al, Nature Comm, 6, 5981-5987 (2015)
[5] H. Kelkar, et al, Phys. Rev. Applied, 4, 054010 (2015)
[6] D. Wang, et al, Phys. Rev. X, 7, 021014 (2017)
[7] A. J. Magdaleno, et al, (2019), in preparation

In this work, we demonstrate the fabrication and characterization of active, switchable metasurfaces comprised of ordered magnetic nanoparticles (MNP). Metasurfaces can be comprised of periodic scattering structures with dimensions smaller than the operating wavelength. The magnetic and optical properties of MNP arrays are dictated by several factors, including composition, particle size, and spacing. Ferromagnetic particles have high permeability (µ>1) and net magnetic dipole moments in the absence of a magnetic field. Nanoparticles below a critical dimension, can exhibit paramagnetic behavior, i.e., a lack of magnetic moment in the absence of an applied field. Superparamagnetic nanoparticles are similar to paramagnetic nanoparticles, but exhibit a rapid increase in magnetic moment in the presence of an applied field. MNP arrays that exhibit ferromagnetic ordering have novel applications, for example, switchable metasurfaces and magnetically enhanced antenna arrays. In addition, large-area 2-D MNP arrays on flexible substrates can be integrated onto lightweight platforms that enable multifunctional electronics and optics. Finally, large-area fabrication strategies present a path toward technological development or prototyping.
Traditional fabrication approaches for nanoparticle (NP) arrays typically require costly equipment and highly trained personnel. Herein, we employ a scalable fabrication approach for MNPs—nanosphere lithography (NSL). First, a hexagonally-packed monolayer of polystyrene spheres (PS; d≤500 nm) is assembled on a flexible substrate, for example, Willow® Glass. Next, e-beam evaporation or sputter deposition is used to coat the PS layer with Co, Ni, or co-deposited Co and Au. The PS layer template is degraded by exposing the sample to high temperature (for example, 600 °C), leaving behind ordered NPs. The size and spacing of the NPs can be controlled by the size of the PS and the thickness of the deposited metallic film. Another approach is to remove the PS layer using solvent, which produces an ordered, quasi-triangular nanoarray. Total integrated diffuse and spectral scattering are measured using a spectrophotometer with an integrated sphere accessory to account for visible and near-IR scattering characteristics of the nanostructured arrays. The samples are also subjected to sensitive magnetometer measurements, (superconducting quantum Interference device (SQUID)) to establish if the MNP arrays exhibit magnetic properties, and if so, the type of magnetism, e.g., ferromagnetic or superparamagnetic. The optical and magnetic properties of MNP arrays with various sizes and shapes, and materials compositions, are compared. Establishing relationships between materials functionalities and MNP structure and composition can present a path toward developing flexible, lightweight electromagnetic devices, such as textile-based antennas, electromagnetic switches, and optical rectification.

Certain metallic nanoparticles display localized surface plasmon resonance (LSPR) when interacting with light. In this study, we synthesize gold nanoparticles with a variety of shapes and sizes, such as spheres, stars and other shapes that display strong LSPR. These particles are then functionalized with self-assembled monolayers for orientation-controlled surface enhanced Raman spectroscopy (SERS). Various SAM molecules with different end groups are used in order to tailor the interaction with molecules of interest for detection using SERS. End groups with partial positive, negative and zwitterionic charges have been attached to the gold nanoparticles and characterized with zeta potential and scanning probe microscopy. Improved proximity and orientation control between the nanoparticle and the molecules being detected could lead to improved SERS response.

Many researches about plasmonic materials with chirality has been carried out due to their intensive light-manipulating ability¹. However, synthesis of plasmonic nanoparticles have difficulties in delicate structure-making. In an effort to solve the problems, we focus on nature materials which have delicate chiral nanostructure and their ability to transfer properties from starting organic material to inorganic material, and vice versa². Previously, we presented a novel fabrication method for gold helicoid nanoparticles by peptide-assisted method^3,4,5 which transfer intrinsic chirality of organic material (peptide) to inorganic achiral gold nanoparticle to manage their morphology and chirality. The synthesized gold helicoid nanoparticles had fine chiral nanostructure in nanoscale level. Based on this structure, they exhibit strong optical activity with Kuhn’s dissymmetry factor 0.2 in visible range under CPL(circularly polarized light). This intense optical activity also can be seen in polarization-sensitive color modulation in far-field transmission of randomly dispersed solution. In an extension of our newly developed synthesis strategy, now, we are doing research about coupling behavior from periodically arrayed Helicoid gold nanoparticles to see enhanced optical properties and apply our nanoparticles to appropriate field. Assembled chiral nanoparticles show collective resonance behavior and strong CD signal which called chiral lattice plasmon resonance. Also, We confirmed their resonance is tunable upon incidence angle of light and periodicity of array. We expect this strategy and template can be new break-through to colloidal chiral nanoparticles for being applied in various field such as, chiral sensing, wave plate, etc.

References
[1] Meinzer, N. et al., Nat. Photon. 2014, 8, 889.
[2] Orme, C. A. et al., Nature 2001, 411, 775.
[3] H.-E. Lee et al., ACS Nano 2015, 9, 8384.
[4] H.-Y. Ahn et al., J. Mater. Chem. C 2013, 1, 6861.
[5] H.-E. Lee et al., Nature 556, 360–365 (2018)

We present a film-coupled colloidal building-block, comprising of a plasmonic core surrounded by a dielectric shell containing a fluorophore emitter. Due to the small mode volume and the strong loss rate, fluorescent lifetime of the emitter is significantly reduced and the emission rate is enhanced while the energy of the emitted photons remains unaffected.[1] We systematically study the energy transfer mechanism on the single particle level by employing electron microscopy, scattering spectroscopy, fluorescence life-time imaging (FLIM) and time-correlated single photon counting on the same cavity.[2] Moving from single cavities towards periodically arranged gain and loss materials with periodicities close the optical wavelength range, unique properties result from the energetic coupling of those building blocks. In order to investigate the arising properties on a large scale, we fabricated novel quantum dot lattices and coupled them to periodic plasmonic nanostructures using self-assembly of colloidal particles as fabrication method.[2] By combination of the two lattices with different overlap and at different angles, energy transfer of the coupled structures can be studied comprehensively. This will help to fabricate large scale structures featuring optical band gap structures for unique light matter interactions.

[1] Max J Schnepf, et al., Z. Phys. Chem., 2018, 232 (9-11), 1593-1606
[2] Oleksandr Stroyuk, et al, J. Phys. Chem. C, 2018, 12, 25, 13648-13658
[3] Martin Mayer, et al., Adv. Opt. Mater, 2019, 7, 1800564

Acknowledgement: This project was financially supported by the Volkswagen Foundation through a Freigeist Fellowship to Tobias A.F. König. The authors acknowledge the Deutsche Forschungsgemeinschaft (DFG) within the Cluster of Excellence ‘Center for Advancing Electronics Dresden’ (cfaed) for financial
support.

For large-scale optical metasurfaces, tailored active and passive subwavelength building blocks are required to modulate the refractive index.[1] Here, we introduce a colloid-to-film-coupled nanocavity whose refractive index can be tailored by various materials, shapes, and cavity volumes. With this colloidal nanocavity setup, the refractive index can be adjusted over a wide visible wavelength range. For many nanophotonic applications, specific values for the extinction coefficient are crucial to achieve optical loss and gain. Recently, we employed bottom-up self-assembly techniques to sandwich optically active ternary metalchalcogenides between a metallic mirror and plasmonic colloids. The spectral overlap between the cavity resonance and the broadband emitter makes it possible to study the tunable radiative properties statistically. For flat cavity geometries of silver nanocubes with sub-10 nm metallic gap, we found a fluorescence enhancement factor beyond 1000 for 100 cavities and a 112 meV Rabi splitting.[2] In addition, we used gold spheres to extend the refractive index range.

[1] Adv. Optical Mater. 2018, 1800564
[2] J. Phys. Chem. C 2019, 123, 6745−6752

Vanadium oxides are the most interesting materials because of its varying oxidation states between V²⁺ and V⁵⁺, out of which, the most studied compounds being V₂O₅ and VO₂. VO₂ has an interesting semiconducting to metal transition (SMT) property where it changes its phase from low-temperature insulating state to high-temperature (T>68°C) metallic state [1]. Because of this, VO₂ is explored in the field of the metamaterial. Earlier in our group, tuneable metamaterial has been demonstrated on the VO₂ thin films fabricated by UNSPACM [2]. The roughness of the film was found to be ~2.1 mm, thereby making the film inappropriate for metamaterial applications. Thus, the aim was to reduce the roughness of the thin film with a single step synthesis process so that it can be employed as an efficient metamaterial. Pulsed laser deposition (PLD) has been used to fabricate vanadium oxide thin films for several applications. This method has several advantages including easily controllable film composition by deposition parameters, controlled stoichiometry of the target material in the films deposited on the substrate and epitaxial growth of thin films.

We report the synthesis of vanadium oxide thin films by PLD under different parameter conditions on Si/SiO₂ substrate using V₂O₅ target. The microstructure and crystal symmetry of the deposited films were studied with X-ray diffraction, scanning electron microscopy (SEM), and Raman spectroscopy. The film obtained were phase pure as determined from XRD and Raman data. The film roughness and surface morphology were examined by atomic force microscopy. Roughness was reduced to about 4 nm as compared to other synthesis processes used earlier in the group [2, 3]. SMT was studied by I-V measurements on different films where 3-4 orders of resistance change were observed for the best condition. Thus, we were able to successfully synthesize VO₂ thin films with reduced roughness making it suitable for metamaterial applications.

References:
[1] Huotari, J., et al., "Pulsed Laser Deposited Nanostructured Vanadium Oxide Thin Films Characterized as Ammonia Sensors.", Sensors and Actuators B: Chemical, 217, 22-29 (2015).
[2] [4] R. Bharathi, N.R., A. M. Umarji, "Metal-insulator transition characteristics of vanadium dioxide thin films synthesized by ultrasonic nebulized spray pyrolysis of an aqueous combustion mixture.", J. Phys. D, 48(30), 305103(2015).
[3] Rajeswaran, B., et al., "Thermochromic VO₂ thin films on ITO-coated glass substrates for broadband high absorption at infra-red frequencies." J. Appl. Phys., 122(16), 163107 (2017).

There is a growing demand for IR detectors in the application fields such as building, thermal management or night assistance car driving. To overcome the limitations in sensitivity and cost of existing systems, alternative device architectures and technologies are needed which better match the market requirements in terms of cost/performance ratio in particular. With that respect, graphene has already been established as an interesting 2D material which can operate as a photodetector in a vast range of wavelength covering from ultraviolet to far-infrared and THz regime. However, low light absorption and absence of gain mechanism limit the responsivity of graphene photodetector. Introducing hybrid phototransistor with highly efficient light absorbing 0D quantum dots and 2D materials, opens up the possibility to transfer the photogenerated carriers from 0D materials into the high mobility 2D channel, which dramatically increases the responsivity and gain of the photodetector. In the IR range, this type of low dimensional phototransistor based on graphene/PbS QD hybrid was first proposed in literature in 2012 ^{[1, 2]} till the demonstration in 2017 ^[3] of a high-resolution broadband image sensor sensitive to ultraviolet, visible and infrared light (300–2000 nm).

In this study, the potential of similar phototransistors has been revisited and their underlying photodetection mechanisms were gradually investigated. We initially studied CVD-grown single layer graphene as a phototransistor device. In the NIR region, our device based on a low-doped Si substrate showed a responsivity of about 100 A/W, which is 10⁵ orders of magnitude higher ^[4] than graphene phototransistor on conventional SiO₂/highly-doped Si substrate ^{[5, 6]}. We also observed an unconventional hysteresis in the transfer characteristics of graphene associated with the induced electric field at the interface of SiO₂ and low-doped Si substrate. In a second step, we synthesized the colloidal QDs absorbing in the NIR region and developed a layer-by-layer dip coating with simultaneous ligand exchange procedure in order to deposit homogeneous PbS QD layers on graphene sheet leading to a well fabricated hybrid phototransistor. The above mentioned hysteresis persists even after the dip coating of graphene with PbS QD layers. However, in the high absorbing IR range of the QDs, the effect of low-doped Si substrate on photoresponse is somehow screened and the measured photoresponsivity of our hybrid devices matches well with the reported one on graphene/PbS QD hybrids with highly doped Si substrate ^[2]. We achieved a significantly high responsivity of 10⁵ A/W and a photoconductive gain of about 10⁸ at 940 nm with irradiation power of 6.4 µW/cm².


[1] Konstantatos, Gerasimos, et al. "Hybrid graphene–quantum dot phototransistors with ultrahigh gain." Nature nanotechnology 7.6, 363 (2012).
[2] Sun, Zhenhua, et al. "Infrared photodetectors based on CVD-grown graphene and PbS quantum dots with ultrahigh responsivity." Advanced materials 24.43, 5878-5883 (2012).
[3] S. Goossens et al., “Image sensor array based on graphene-CMOS integration”, Nature Photonics, 11, 366-371 (2017).
[4] Guo, Xitao, et al. "High-performance graphene photodetector using interfacial gating." Optica 3.10, 1066-1070 (2016).
[5] Mueller, Thomas, Fengnian Xia, and Phaedon Avouris. "Graphene photodetectors for high-speed optical communications." Nature photonics 4.5, 297 (2010).
[6] Xia, Fengnian, et al. "Ultrafast graphene photodetector." Nature nanotechnology 4.12, 839 (2009).

Visible light communication is an emerging area for use in computer optical buses, light fidelity, etc., due to its low cost and high directionality. Recently, phototransistors have attracted attention because they connect objects and users through light. Amorphous oxide semiconductors (AOSs) based thin film transistors (TFTs) are considered as one of many types of promising electrical devices due to outstanding characteristics, such as high mobility, low-off current, high transparency compared to amorphous silicon-based devices. Despite these versatile advantages, AOSs have a serious limitation as visible light sensors because they cannot absorb visible light due to the wide band gap of around or more than 3 eV. Therefore, AOSs cannot be used for an active channel layer of phototransistors for visible light detection. In order to resolve this issue, researchers have tried to apply the additional absorption layer such as quantum dots, 2D materials, or nanowires on the device. However, these studies have a drawback that requires complicated processes.
In this work, we investigated the defect generation method that improves the light absorption characteristics of indium gallium zinc oxide (IGZO) phototransistors by forming polydimethylsiloxane (PDMS) residues on the gate insulator (GI). To fabricate IGZO phototransistors with PDMS residues, we deposited IGZO channel by radiofrequency (RF) magnetron sputtering after forming PDMS residues on silicon dioxide, used as the GI. Thereafter, IGZO channel was annealed at 300^oC in the air for 1 h, and aluminum electrode was deposited by RF magnetron sputtering. In this process, we controlled the amount of PDMS residues by optimizing the degree of hardening of PDMS through thermal curing time. PDMS residues could be easily formed by exfoliating cured-PDMS made by heated PDMS on the GI. PDMS residues increase the surface roughness of the GI. Hence, PDMS residues form interface trap sites between the GI and IGZO channel. Additionally, since PDMS has a boiling point at 200^oC and consists of (C₂H₆OSi)_n chains, it becomes decomposed during the 300^oC annealing of IGZO channel. Therefore, deep states could be formed by hydrogen diffused into IGZO channel. During this process, deep states were formed by diffused hydrogen into IGZO channel. These trap sites could reduce the energy needed for electrons excitation into the conduction band. Also, there were previous researches that showed these trap sites cause formation of subgap states within the band gap of IGZO, and thus light with wavelengths wider than 420 nm could be absorbed by IGZO. We confirmed formation of trap sites through the transfer characteristics that the subthreshold swing increases from 0.37 to 0.50 V/dec and the turn-on voltage was negatively shifted from -1.02 to -6.81 V by forming interface trap sites and deep states, respectively. IGZO phototransistors with PDMS residues showed the light absorption characteristics with the turn-on voltage change from -6.81 to -23.40 V and I_photo/I_dark of 1.79x10⁶ under red light irradiation (wavelength of 635 nm, intensity of 5 mW/mm²). On the other hand, there was almost no change in IGZO phototransistors without PDMS residues at the same condition. Also, photoresponsivity, photosensitivity, and detectivity were improved compared with IGZO phototransistors without PDMS residues, from 34.78 to 359.08 A/W, from 7.64x10³ to 4.63x10⁷, from 1.58x10⁸ to 6.26x10¹¹ Jones, respectively, under red light irradiation with intensity of 5 mW/mm². As a result, this study achieved improved photosensing characteristics of IGZO phototransistors with PDMS residues for visible light above the wavelengths of 420 nm.

BaTiO₃ (BTO), a perovskite ferroelectric material, has historically been used in capacitors, memory storage devices, and optical devices. At AFRL, we have investigated BTO for use in waveguides for electro-optic modulators. It has been shown that growth conditions used in epitaxial growth have influenced lattice strain in heteroepitaxial thin films resulting in defects. This study investigated how changes in growth parameters affect the types of defects and oxygen vacancies present in BTO films. Tetragonal BTO thin films (a = 3.9945Å, c = 4.0335Å) were grown on cubic SrTiO₃ (STO) substrates (a = 3.9046Å) by pulsed laser deposition (PLD) with an excimer laser of wavelength 248nm. The chamber was evacuated to a base pressure of 10^-9 Torr and a 99.99% pure pressed BTO target was used as the source. STO substrates were chosen because it has been demonstrated that STO can be grown as a buffer layer on Si to help with the integration of BTO into current silicon electronic systems. BTO films were grown at oxygen pressures ranging from 10mTorr – 300mTorr, laser energies from 200mJ – 350mJ, and substrate temperatures ranging from 500 - 800°C. Films were characterized by high resolution X-ray diffraction (HRXRD) coupled 2θ-ω scans and rocking curves to examine film quality and lattice spacing. Results from HRXRD show c-axis interplanar spacings around 4.16Å for strained films and 4.08Å for relaxed films. HRXRD showed that decreasing laser energy during growth increased lattice parameter and improved film quality, as evidenced by higher peak intensities and smaller full width at half maximum. Atomic force microscopy showed root-mean-square surface roughness around 330pm over an area of 2x2µm. X-ray photoelectron spectroscopy was used to verify the stoichiometry of the films. Spectroscopic ellipsometry from 1.5 - 4.5eV was used to determine the relevant optical constants and look for defect bands which give evidence of oxygen vacancies. Lastly, Raman spectroscopy from 100-900cm^-1 was used to identify the types of defects present in the films.

Camouflage patterns are focused on the reduction of visibility by confusing the edge of target and background. The development of thermal observation device (TOD) is greatly threatening the survivability of the weapon systems which emit the infrared radiation at their hot parts where the radiation energy is much higher than surroundings such as tank engines or aircraft nozzles. Therefore, infrared thermal camouflage technology has to be applied to these surfaces for military operations. In addition, these surfaces should have visible camouflage performance, thus requiring a surface with multispectral camouflage capabilities of the infrared and visible wavelength range. Silicon nanowires arrays are well-known metamaterials by their high absorption performance in the visible to infrared range. The nanowire arrays made by metal-assisted etching method have highly disordered structures and the nanowire length is controlled by etching time. In this study, we realize the multispectral camouflage by applying the infrared camouflage pattern with the visible camouflage so as to obscure the interface between the energy emitted from the surrounding background and that from the target by patterning the nanowire array structures. Using the CCD Camera and IR camera, the silicon nanowire surface are captured and it shows that the shorter length nanowire array is dark color in visible range and low apparent temperature (low emissive energy) in infrared range, however, the longer length nanowire array is dark yellow color in visible range and high apparent temperature (high emissive energy) in infrared range. We also measure the optical emittance and reflectance of visible and infrared range to analyze the change of color in the visible range and emissive energy of infrared range. It shows that the reflectance in visible range and emittance in the infrared range goes higher with nanowire length longer.

Surface plasmon polaritons (SPP) with high effective refractive indices offer a new avenue to build sub-wavelength lasers. Here, we present room-temperature plasmonic lasing using CsPbBr₃ perovskite crystals on a gold substrate with a 5-nm-thick poly-norepinephine (pNE) dielectric layer. We colloidally synthesized high-quality CsPbBr₃ microcrystals using our recently-developed sonochemical synthesis. The pNE layer was prepared by solution dipping of the gold substrate. The photoluminescence spectra from such structures show distinct double peaks, which indicate strong coupling between electron-hole pairs (excitons) and SPP modes with a vacuum Rabi splitting energy of about 40 meV. Finite-difference time-domain simulation suggests that the SPP hybrid mode is a whispering gallery mode with a Q-factor of 44. The smallest plexciton laser we have observed is 580 nm in width and 270 nm in height, emitting a linewidth of 0.1 nm at 535 nm above the pump threshold at 2 mJ/cm².

The preparation of new polymers and plasmonic nanomaterials require hierarchical levels of ordering and structuring: from molecular to macroscopic. Au nanoparticle plasmonics phenomena enable demonstration in both localized and propagating plasmons which can be quantified spectroscopically in particles, nanostructures, and thin films. Dendrimers and other hyperbranched polymer systems are of interest for their functionality in catalysis, drug delivery, reactivity, etc. Of high interest are hybrid metal nanoparticle dendrimer functionality in electro-optical applications and photonics. This includes Hybrid NP-dendrimers capable of hierarchical ordering and self-assembly. We highlight the convergent synthesis of a variety of polymer-hybrid dendrimers and their electropolymerizability with electroactive monomer groups. The synthesis of precise dendrons is in the form of modular building blocks for functional dendrimers. Our group has reported a number of these hybrid nanoparticle systems and have reported the step-by-step routes towards structure-property relationships. Energy transfer and charge transfer properties can be observed as well as quenching phenomena. What is also important is the use of surface sensitive spectroscopic and microscopic analytical tools applied rationally to highlight evidence of order and function.

Propagating electromagnetic surface waves called surface plasmon polaritons (SPPs) can be excited by free-space beams on corrugated conducting surfaces at resonance angles determined by corrugation period, permittivity, and optical frequency. SPPs are coupled to and co-propagate with surface charges. Electrical isolation of the conducting corrugations blocks charge propagation between them, and excitation resonances of traveling SPPs are absent. However, SPPs can be excited via electric induction if a smooth conducting surface exists below and nearby the insulated corrugations. The dependence of SPP excitation resonances on that separation is investigated here. We find that excitation resonances for traveling SPPs broaden and disappear as the dielectric thickness is increased beyond ~1% of the free-space wavelength. The resonance line shape depends on whether the dielectric SiO₂ or TiO₂ is used as the separating layer, but this dependence appears uncorrelated with differences in optical constants for SPPs at long-wave infrared frequencies where dispersion is strong.

Recently, tunable nanophotonic devices have drawn intense attention with great promise for practical applications. In this work, we have experimentally demonstrated several dynamically-tunable plasmonic devices based on phase transition of vanadium dioxide, which include dynamic plasmonic color generators [1], dynamically switchable polarizers [2], and dynamically tunable bowtie nanoantennas [3]. We have fabricated periodic arrays of silver-nanodisks on a vanadium dioxide film to realize different colors, relying on the excitation of localized and propagating surface plasmons. Based on insulator-metal transition of vanadium dioxide, the plasmonic colors can be actively tuned by varying temperature. This approach of dynamic color generation can easily realize diverse color patterns, which makes it beneficial for display and imaging technology. We have also designed a system consisting of anisotropic plasmonic nanostructures with vanadium dioxide that exhibits distinct reflections subjected to different linearly polarized incidence at room temperature and in the heated state. The composite structure can thus be used to realize a dynamically switchable infrared image, wherein a pattern can be visualized at room temperature, while it disappears above the phase transition temperature. Besides, we have made the dynamically tunable bowtie nanoantennas integrated on a vanadium dioxide thin film. The investigations here can be applied in dynamic digital displays, optical data storage, and imaging sensors.

References:
[1] Fang-Zhou Shu, Fang-Fang Yu, Ru-Wen Peng, Ying-Ying Zhu, Bo Xiong, Ren-Hao Fan, Zheng-Han Wang, Yongmin Liu, and Mu Wang, “Dynamically plasmonic color generation based on phase transition of vanadium dioxide", Advanced Optical Materials (2018) 6, 1700939.
[2] Zhi-Yong Jia, Fang-Zhou Shu, Ya-Jun Gao, Feng Cheng, Ru-Wen Peng, Ren-Hao Fan, Yongmin Liu, and Mu Wang, “Dynamically switching the polarization state of light based on the phase transition of vanadium dioxide", Physical Review Applied (2018) 9, 034009.
[3] Fang-Zhou Shu, Li-Heng Zhang, Jia-Nan Wang, Ru-Wen Peng, Ren-Hao Fan, Dong-Xiang Qi, Mu Wang, “Dynamically tunable bowtie nanoantennas based on the phase transition of vanadium dioxide", Optics Letters (2019) 44, 2752.

In 2D lattices composed of metal nanoparticles (NPs), the localized surface plasmons (LSPs) of metal NPs couple to the Bragg modes to produce surface lattice resonances (SLRs). By exploiting the intrinsic size and material tunability of these LSP modes, we achieved dipole and quadrupole SLRs from visible to near infrared wavelength range. Our work shows that there is a new mode-mixing scheme, which we call hierarchical hybridization occurring in honeycomb lattices of plasmonic NPs. The non-Bravais nature of honeycomb lattice introduces in-plane quadrupole LSPs mixing with the dipole LSPs to create a sharp and strong SLR. Further, by changing the plasmonic material and engineering the plasmonic NP size, we leveraged the out-of-plane quadrupole and in-plane quadrupole LSP response of plasmonic NPs. Two distinct and simultaneously optimized band-edge states were achieved in a single honeycomb lattice. This work highlights the ubiquity of multipolar LSP coupling in non-Bravais plasmonic lattice systems with different plasmonic materials, which has important implications for the band structure engineering of 2D metamaterials.

The current state-of-the-art in materials used for infrared (IR) optical components (e.g. lenses, waveplates or prisms) suffers from significant material limitations, exacerbated by the long free-space wavelengths of light in the IR. Leveraging polariton modes such as surface plasmon polaritons (SPPs), surface phonon polaritons (SPhPs) or hyperbolic phonon polaritons (HPhPs) is one route to overcome these limitations. The class of HPhP modes occurring in anisotropic crystalline materials such as hexagonal boron nitride (hBN), are of particular interest. They can be supported at extremely high wavectors, and can shown exhibit phenomena such as hyperlensing. Recent studies show that the substrate plays a critical role in controlling the properties of hyperbolic polaritons, and can designed to produce in-plane refraction within hBN heterostructures on phase-changing materials.
In our work we systematically explore the role that the substrate plays in controlling hyperbolic polariton propagation, using a range of scattering-type scanning near-field optical microscopy (s-SNOM) and nano-Fourier transform infrared (nano-FTIR) spectroscopies. We consider propagation on suspended, dielectric and metallic substrates, reducing the thickness-normalized wavevector by up to a factor of 25 simply by changing the substrate. Moreover, by incorporating the imaginary contribution to the dielectric function in lossy materials, the wavevector can be dynamically controlled by small local variations in loss or charge carrier density, while higher-order modes show reduced sensitivity to substrate-induced losses. Finally, we discuss the potential implications for devices including sensors and refractive optics. Sensing HPhP modes out-perform SPhP modes under certain circumstances, and that the choice of substrate is critical for sensing applications. Furthermore, an optimized substrate produces phenomena such as mode sorting within hBN devices. This demonstrates that the choice of substrate choice is critical in determining the properties of propagating HPhP’s, unlike earlier results on localized HPhP resonators.

Two-dimensional layered materials have recently garnered burgeoning amount of interest due to their unique electronic, optical, thermal, mechanical properties emerging at the mono-to-few layer thicknesses. Over the past decade, we have witnessed significant research activity on the area of two-dimensional materials. Most of research has focused on conventional 2D materials such as graphene and 2D TMDCs, having isotropic electronic and optical properties due to their crystal symmetry. Recently, layered materials such as black phosphorus and hexagonal boron nitride investigated for anisotropic crystal structure. In this talk, I will present theoretical investigations on mono-layer borophene plasmons at optical frequencies. Borophene has recently expanded the 2D materials family after its successful deposition on silver substrates. We theoretically investigate the plasmonic properties of nanostructured monolayer borophene using full-field electromagnetic simulations and demonstrate that borophene nanoribbon and nanopatch arrays can support localized plasmon resonances at visible and near-infrared wavelengths. Due to its puckered crystal structure and vacancy distribution in hexagonal lattice, borophene exhibits strong anisotropic in-plane properties which makes it an unconventional plasmonic material. In the second part of my talk, I will introduce α-MoO₃ as an anisotropic photonic and polaritonic material. α-MoO₃ is a layered material that exhibits both in and out-of-the-plane anisotropic polaritonic response at mid-IR wavelengths. We designed and experimentally demonstrated an anisotropic polaritonic absorber and showed that one can couple to all phonon modes and address them individually either using structural tunability or polarization control of incident infrared radiation. I will also discuss our experimental investigations of the birefringent optical properties of α-MoO₃ in visible frequencies. By constructing α-MoO₃ based Fabry-Perot resonator, we observed strong polarization-dependent tunabiltiy of the Fabry-Perot resonance due to different refractive index of MoO₃ for different crystal directions. Anisotropic plasmonic, photonic and polaritonic materials provide additional freedom in controlling polarization dependent properties and could find applications in wide range of fields where polarization control plays important role.

Harmonic generation in plasmonic systems has gained significant interest over the last few years. While the plasmonic near-field enhancement has been studied in great detail, little attention was paid to the origin of the nonlinear signals which in fact lie in the microscopic nonlinearity of the involved plasmonic metals. In order to predict wavelength-dependent nonlinear processes it is therefore crucial to understand these microscopic nonlinearities. Utilizing an optical parametric oscillator as a tunable broadband light source, we study wavelength-dependent third harmonic generation from metallic thin films made of gold, copper, and magnesium.

We find that the linear properties of the metallic films, that is their absorption, strongly influence the third harmonic generation efficiency. Optical transitions between the different bands lead to a resonant enhancement of the third order susceptibilities. Utilizing hydrogen to switch metallic magnesium to dielectric magnesium hydride [2] we can tune the nonlinearity of thin films and observe the metallic-dielectric phase transition in the linear and nonlinear regime.
We find complex relations between the linear transmittance and the radiated third harmonic, which provide new insights into the phase transition. Particularly, the first few seconds after switching this metal-to-insulator phase transition show remarkable nonlinear responses. We suspect that the surface roughness could play a substantial role.

Complimentary measurements of magnesium films that are being hydrogenized and investigated with an in-situ s-SNOM setup [3] and a mid-IR laser that is tuned to the magnesium hydride phonons gives additional information about the relationships between local microscopic structure and optical properties.

Our results might be of great relevance for nonlinear sensing [4] and nonlinear hydrogen detection in the future.

References
[1] J. Krauth, H. Giessen, and M. Hentschel: Wavelength-dependent Third Harmonic Generation in Plasmonic Gold Nanoantennas: Quantitative Determination of the d-band Influence, ACS Photonics 5, 1863 (2018).
[2] F. Sterl, N. Strohfeldt, R. Walter, R. Griessen, A. Tittl, and H. Giessen, Magnesium as Novel Material for Active Plasmonics in the Visible Wavelength Range, Nano Lett. 15, 7949 (2015).
[3] F. Sterl, H. Linnenbank, T. Steinle, F. Mörz, N. Strohfeldt, and H. Giessen, Nanoscale Hydrogenography on Single Magnesium Nanoparticles, Nano Lett. 18, 4293 (2018).
[4] M. Mesch, B. Metzger, M. Hentschel, and H. Giessen, Nonlinear Plasmonic Sensing, Nano Lett. 16, 3155 (2016).

Metasurfaces created from nonlinear III-V semiconductors and heterostructures used to create resonant optical nonlinearities enable nonlinear optics in a new regime, where many simultaneous nonlinearities lead to harmonics and mixing products of laser beams without the need of phase matching. I will present our latest work on optical mixing and harmonic generation in dielectric metasurfaces made from III-V semiconductors and semiconductor heterostructures.

Similar behavior can be obtained in epsilon near zero materials provided that the mobility of the carriers in the material is high enough; these conditions are met in highly doped CdO where we observe high harmonic generation from the infrared to the ultraviolet, and without the need of any patterning.

Second-order optical effects are essential to the on-demand generation of spectral components as well as active control of light via nonlinear processes such as second-harmonic generation (SHG), sum/difference frequency generation, optical rectification, optical parametric amplification, and Pockels effect. These optical processes rely on the interaction of two optical fields through the second-order dielectric susceptibility,χ⁽²⁾ , of optical materials. The portfolio of χ⁽²⁾ media, however, is rather limited as the inversion symmetry in most optical materials inhibits achieving a nonzero χ⁽²⁾ response, under the electric dipole approximation. In such materials, the atomic-scale disordered sites at surfaces and interfaces are limited domains where the termination of the bulk-like crystal structure allows for nonlinear light-matter interactions of second-order type, yet in an inefficient manner. Although the enhancement of the weak surface nonlinearity through the utilization of resonant structures has revealed promising potentials, developing symmetry breaking methods are still necessary to activate χ⁽²⁾ - based nonlinear effects in the bulk of centrosymmetric media. Towards this goal, exerting external strain and applying direct-current electric fields are among the limited proposed approaches for breaking the inversion symmetry and inducing a notable bulk-like χ⁽²⁾ susceptibility. However, the current span of the literature lacks optical symmetry-breaking schemes and initial developments in this area are required. In this work, we present a fundamentally new scheme for breaking the crystal inversion symmetry and enabling χ⁽²⁾ processes via the generation and transport of hot electrons. The sub-picosecond kinetics of carriers enables the ultrafast conversion of statically passive dielectrics into transient second-order nonlinear media, immediately expanding the portfolio of χ⁽²⁾ media beyond the conventional crystals. The induced nonlinearity is accurately tunable using an optical switch that controls the density of generated hot electrons. As an example, we demonstrate that the transient nature of the induced χ⁽²⁾ response enables all optical control of the SHG process, proving the feasibility of dynamically tuning second-order light matter interactions through the optical breaking of the inversion symmetry.

The pursuit of chip-scale and compact data processing capacity in a CMOS-compatible fashion has promoted the investigation of silicon-based photonic platforms for active optical functionalities via the nonlinear light-matter interactions. The use of silicon in integrated photonics, however, is primarily focused on passive photonic components such as waveguides, gratings, and resonators, while the investigation of active silicon photonics is largely limited to the exploration of several specific processes like the free-carrier dispersion and the Raman effect. This restriction mainly stems from the absence of the bulk second-order nonlinear susceptibility, χ⁽²⁾, under the electric dipole approximation due to the centrosymmetric nature of the crystal structure of silicon. To address such a limitation, here we demonstrate the enhancement and tunability of field-induced optical nonlinearity of the second-order type in silicon metasurfaces, where strong magnetic Mie resonances are leveraged to intensify the nonlinear interaction of light with silicon at a prescribed spectral range. On top of the strengthened surface SHG from the silicon metasurface, as facilitated by the magnetic Mie effect, introducing an electrical signal to our silicon structure provides an additional route to the frequency-doubling of light via the EFISH process. Our experimental characterizations and numerical modeling reveal that the efficiency of the field-induced frequency doubling peaks in the spectral vicinity of magnetic behavior, substantiating the synergic role of Mie resonances on the nonlinear optical generation from the silicon platform. Our finding reveals a generic route towards the dynamic control of second-order nonlinear processes, such as sum/difference frequency generation, optical rectification, and Pockels effect, in electrically active silicon metasurfaces.

Strongly enhanced third-order nonlinear optical properties of transparent conducting oxide (TCO) thin films in their epsilon-near-zero (ENZ) region (i.e. the region where the real part of their dielectric permittivity approaches zero), have been reported recently [1-3]. Such giant enhancement can be attributed to the hot electron dynamics resulting from ultrafast laser assisted heating. Upon excitation of leaky ENZ modes, effective nonlinear optical properties of these meta-films can be further boosted. Since the response time of these nonlinear processes can be ~100 fs, the ENZ materials could open distinct functionalities to the path to revolutionary nanoscale nonlinear optics and ultrafast on-chip optical applications.

In this work, we present a method to engineer the nonlinear refraction coefficients (n₂) and the nonlinear absorption coefficients (β) of Al-doped zinc oxide (AZO) ENZ thin films synthesized by atomic layer deposition (ALD) technique. Nonlinear optical properties of TCO films can be attributed to the free carriers which also contribute to linear absorption of the thin films. Furthermore, third order nonlinear optical signals have cubic dependence on the electric field confined inside the TCO films. Hence, to engineer the nonlinear optical properties, we design the linear absorption and electric field intensity enhancement (FIE) associated with leaky ENZ modes in these AZO nanolayers via control over the ENZ wavelengths and optical losses by ALD deposition parameters such as dopant ratio and the number of macrocycles. The dopant ratio determines the material dispersion and ENZ wavelength whereas the macrocycle regulates the film thickness.

We fabricate AZO films with ENZ wavelengths varying between 1400-1600 nm and their thicknesses ranging from 55 to 216 nm [4]. Nonlinear optical properties of the fabricated films are measured using standard open and close-aperture Z-scan technique using an ultrafast femto-second laser (pulse duration ~70 fs). The peak wavelength (λ_p) of the fs pulses can be tuned from 1200 to 1600 nm with an optical parametric amplifier unit. Measured n₂ and β values of the films are evaluated upon fitting the experimental data. Experimental results suggest that nonlinear refraction and nonlinear absorption strengths of these ENZ AZO films can be engineered by controlling the ZnO to Al dopant layer ratios in the deposited AZO films, as well as the film thickness. We also observe an enhancement of the effective nonlinear properties due to excitation of leaky ENZ modes at oblique incidence while illuminating with TM polarized light. Measured values as large as n₂^(eff)≈10^-8 mm²/Watt and β₂^(eff)≈-10^-4 mm/Watt are obtained along with approximately an order of magnitude tunability via ALD parameters. Lastly, we present a way to achieve ultra-strong, dispersion-free ENZ nonlinearity which relies on engineering of material dispersion with a multi-layer structure and excitation of a broadband leaky ENZ mode. The giant n₂ and β values of ALD AZO films in their ENZ region and their controllability will be important for efficient all-optical signal processing.


1. Alam, M. Z.et al.,” Science 352, 795–797 (2016).
2. L. Caspani et. al., Phys. Rev. Lett. 116, 233901, 2016.
3. A. Capretti et. al., Opt. Lett. 40, 1500–1503 (2015).
4. Anopchenko A et. al., Mater. Res. Exp. 5, 014012 (2018).


(This work is supported in part by the Young Faculty Award Program from Defense Advanced Research Projects Agency (grant number N66001-17-1-4047), Robert A. Welch Foundation (Award number: AA-1956-20180324) and the Office of Vice Provost for Research at Baylor University.)

The development of plasmonics and metasurface-based optical structures requires alternative, high-performance plasmonic materials, in replacement of commonly used noble metals. Ideally, plasmonic materials should have the properties of low-cost, low-loss, high chemical, mechanical, and thermal stabilities, biocompatibility, spectral tunability, as well as integrability with existing semiconductor technologies. Recently, we have developed epitaxial growth techniques for forming smooth, single-crystalline aluminum and titanium nitride films on transparent sapphire substrates using molecular-beam epitaxy. In comparison to silver and gold, aluminum- and titanium-nitride-based plasmonics have better stabilities and spectral responses in the ultraviolet and visible spectral regions, making them particular suitable for ultraviolet surface-enhanced surface Raman spectroscopy (UV-SERS), optical energy harvesting, and metasurface-based linear and nonlinear optics. In this talk, I will present our recent experimental results in these areas.

Polymer-based metamaterials provide a lightweight, flexible and low-cost platform exhibiting new optical, thermal, and electrical multi-functionalities that provide structural coloration, visual and/or thermal camouflage, solar and thermal energy harvesting, and personal cooling/heating. We will discuss several examples of new metamaterials with tunable photon, electron and phonon transport properties achieved by either sculpting the internal structure of polymers or using polymers to sculpt the internal structure of inorganic solids or nanoparticle arrays. These include: (i) composite organic-inorganic films with a varying degree of crystallinity and mesoscale internal structure fabricated via extrusion and solvent casting followed by polymer chain alignment, which exhibit new tunable transparency, haze, thermal conductivity, and dynamical tunability properties, (ii) new types of composite organic-inorganic polymer-based network metamaterials grown on low-density porous scaffolds to sculpt their internal structure, and (iii) nanoparticle arrays on stretchable polymer substrates, designed for mechanical tuning and switching functionalities.
This work is supported by the US Army Research Office (via the CCDC Soldier Center and the MIT Institute for Soldier Nanotechnologies), Advanced Functional Fabrics of America (AFFOA), MIT International Science and Technology Initiatives (MISTI), and the UNSW-USA Networks of Excellence. The authors thank Dr. Richard M. Osgood III for useful discussions.

Metasurfaces are ultra-thin, planar optical elements composed of sub-wavelength nanostructures, which can be engineered to modify the local characterizations of light. Recently, it has been demonstrated that dielectric metasurfaces can be implemented to simultaneously and independently manipulate of the phase and amplitude for a near-infrared femtosecond pulse having over 200 nm ultra-wide bandwidth while maintaining high spectral resolution of 0.3 nm. Extending the applications of dielectric metasurfaces in ultrafast optics holds great potential for applications ranging from fundamental light-matter interactions to ultrafast communications. In general, the electric field of a femtosecond pulse is a vectoral quantity defined by its phase, amplitude, and polarization. Further controlling the instantaneous polarization state of a single optical pulse as a function of time would largely expand the capability and impact of ultrafast optics.

Here, we offer the experimental demonstration of polarization shaping using dielectric metasurfaces to control the temporal polarization state of femtosecond optical pulses. The pulse shaper consists of a Fourier-transform setup with a dielectric metasurface positioned in the focal plane. Rectangular silicon nanopillars are fabricated on a fused-silica substrate, with the dimensions carefully designed to provide the targeted spectral phase for two orthogonal polarizations – for example, a quadratic phase shift for p-polarized component and a constant phase for s-polarized component. The input pulse is linearly polarized and orientated 45° with respect to the nanopillars, providing two equal polarization components. After passage through the metasurface, the shaped output pulse is characterized by direct electric-field reconstruction using spectral phase interferometry. The measured spectral phase matches with the targeted phase function for both polarizations: the time-domain signal reveals a stretched pulse for p-polarization due to the quadratic dispersion and a narrow pulse for s-polarization due to the constant phase shift. By stacking the polarization ellipses along the time axis, a three-dimensional representation of the femtosecond pulse can be reconstructed. Within a single femtosecond pulse duration, the polarization state evolves between different linear and elliptical polarizations with varying degrees of ellipticity.

In summary, we have demonstrated femtosecond polarization pulse shaping by controlling the spectral phase for two orthogonal polarizations using dielectric metasurfaces. With the large bandwidth, high resolution, and easy compatibility offered by the metasurfaces, such an approach opens up new possibilities in the field of ultrafast science and technology.

In the past decade we witnessed the development of optical metasurfaces that enabled diffractive optical elements with new functionalities. However, the number of optical modes in a metasurface is limited, which directly affects the functionality and performance that can be achieved with a single layer metasurface device. In this talk I will present how systems of stacked metasurfaces and fully three-dimensional dielectric structures with more optical modes open up a new optical design space with exciting applications in various imaging modalities, spectrum splitting and other applications.

The concept of phase-cancellation metasurfaces is introduced which enables the attenuation of specific optical modes using destructive interference. This concept has been utilized to implement a broadband circular polarizer using dielectric metasurfaces. We design and implement a dielectric metasurface which induces a hybrid phase-shift composed of an accumulation phase component and a geometric phase component. The metasurface is composed of anisotropic Silicon based nanorods in which accumulation phase is controlled by the nanorod thicknesses, and geometric phase is controlled by their orientations. Each unit cell is composed of two sub-units which transmit in-phase left-circularly polarized (LCP) optical component, and out-of-phase right-circularly-polarized (RCP) component. RCP is attenuated a result of destructive interference, and a broadband circular polarizer with high extinction ratio (>13dB) is achieved using a single metasurface layer. The concept of destructive interference using phase-cancellation metasurfaces can be extended to other applications. It can be utilized in dynamic metasurfaces in which high contrast optical switches is achieved by controlling the condition of phase-cancellation to modulate the action of attenuation between ON and OFF.

Chiral light-matter interactions are a potentially efficient and versatile method to resolve enantiomers, improving the efficacy of chiral pharmaceuticals and agrochemicals. However, the chiroptical response of small molecules is weak, thus limiting light based detection and separation schemes. Emerging nanophotonic platforms have been shown to increase the interaction between circularly polarized light (CPL) and chiral molecules through a concentration of the local density of optical chirality, C. We have recently shown that tailoring electric and magnetic Mie resonances in dielectric metasurfaces can locally enhance the magnitude of C over 100-fold compared to CPL in free space. However, both theoretical and experimental demonstrations of enhanced optical chirality have been limited to the infrared and visible, while the chiral absorption features of most industrially relevant small molecules are in the ultraviolet.
In this presentation, we design ultraviolet metasurfaces that overcome this band mismatch, enabling high optical chirality enhancements for chiral sensing and separation. Our metasurfaces consist of diamond biperiodic dielectric disk arrays, where a diameter offset between neighboring disks allows for the excitation of high quality (Q) factor electric and magnetic resonances. Engineering the geometry of the disks allows for independent tailoring of the electric and magnetic near fields, which is optimized to produce resonantly enhanced chiral fields. Using full-field electromagnetic simulations we first show that in a diamond metasurface of identical disks, tuning the disk aspect ratio allows for spectral overlap of low-Q electric and magnetic dipole resonances, producing a maximized C enhancement of 50-fold. A diameter offset is then introduced which produces high-Q antisymmetric electric and magnetic dipole resonances. Similar to the case of a symmetric metasurface, for the biperiodic lattice we can fix the lattice parameter a=200 nm and disk height h=60 nm, then sweep the center disk diameter from 100 nm to 120 nm to spectrally shift the electric and magnetic resonances in relation to each other. For a metasurface with a 10% offset between neighboring disks, the high Q modes overlap at a center diameter of 107 nm. At this diameter, we calculate a local C enhancement of 1000-fold at an excitation wavelength of 263 nm. Furthermore, the circularly polarized phase of the incident CPL is preserved in the near fields resulting in a C enhancement of a single handedness. Thus, regions of large optical chirality persist above the metasurface, yielding averaged C enhancements exceeding 100-fold in a volume extending 200 nm from the disks. We also show that adjusting the structural asymmetry through the diameter offset allows for tuning of the radiative Q factors and consequently the local C enhancements across four orders of magnitude. Our results demonstrate the first nanophotonic platform that increases C more than three orders of magnitude in the ultraviolet, paving the way for efficient all optical chiral resolution of small molecules.

Integration of thermally-sensitive materials with highly-engineered metamaterial absorbers has allowed for the creation spectrally-selective thermal detectors ranging across the MHz-THz frequency bands. However, the fastest detectors possess a millisecond-scale response time limiting their applicability for real-time, high-resolution imaging or time-resolved sensing techniques. Here we demonstrate a high-speed, spectrally-selective thermal photodetector operating at room temperature by integrating a colloidally-fabricated plasmonic metasurface with an aluminum nitride (AlN) pyroelectric film. The subwavelength, absorbing metasurface possesses a picosecond-scale thermal diffusion time, which produces a voltage in the underlying pyroelectric film proportional to its’ temperature change. Impulse response measurements of the metasurface-pyroelectric detectors show instrument-limited responses down to 500 ps, where finite-element simulations reveal the possibility of achieving 25 ps response times rivaling the carrier-limited response times of photodiodes. These scalable, in-expensive, and large-area devices show potential for realizing uncooled thermal photodetectors with high responsivities and GHz speeds without the spectral limitations of bandgap-reliant detectors.

To spectrally, spatially, and/or even temporally manipulating optical wavefronts in the subwavelengthscale, plasmonic metasurfaces consisting of a planar array of patterned metallic nanostructures havegained extensive attention [1]. However, their low coupling efficiency as well as inherent ohmic lossescoming with significant heat generation hinder many practical on-demand applications. The advent ofall-dielectric metasurfaces, which employ optically induced electric and magnetic Mie resonances ofsubwavelength high-index nanoparticles, expedited the realization of miniaturized CMOS-compatiblemetadevices addressing the challenges associated with plasmonic counterparts [2, 3]. Nevertheless, thefunctionality of the implemented metadevices cannot be tuned postfabrication. To dynamically engineerthe amplitude, phase, polarization, and/or dispersion of light for a wider range of applications, exploit-ing active functional materials is indispensable [4]. Here, we present a non-volatile active platform byhybridizing a high-index metasurface with phase-change alloy Ge2Sb2Te5(GST). The intrinsic high in-dex and drastic optical contrast of GST (upon conversion from amorphous to the crystalline in multiplestates) make multipolar Mie resonances of all-dielectric nanoresonators optically tunable. As a proof-of-concept, we demonstrate a small footprint, multi-wavelength, and multi-level optical modulator capableof modulating the light with high modulation depth in extreme states of GST. We also experimentallyshow how the structure takes advantage of the interplay of electric and magnetic resonance modes, dueto the induced intermediate states of GST, leading to a considerable phase shift of the transmitted lightnecessary for beaming applications. We leverage a new deep learning architecture to effectively de-sign optimized metadevices considering the fabrication imperfections while the underlying physics oflight-matter interactions is explained through a sheer mathematical platform [5, 6]. Our findings furthersubstantiate active dielectric metasurfaces as promising candidates for the development of miniaturizedenergy harvesting modules, optical sensors, phased array antennas, and holograms.

Refrences:
[1] Yu, Nanfang, Patrice Genevet, Mikhail A. Kats, Francesco Aieta, Jean-Philippe Tetienne, Federico
Capasso, and Zeno Gaburro. ”Light propagation with phase discontinuities: generalized laws of reflec-
tion and refraction.” Science 334, no. 6054 (2011): 333-337.
[2] Jahani, Saman, and Zubin Jacob. ”All-dielectric metamaterials.” Nature Nanotechnology 11, no. 1
(2016): 23.
[3] Staude, Isabelle, Andrey E. Miroshnichenko, Manuel Decker, Nche T. Fofang, Sheng Liu, Edward
Gonzales, Jason Dominguez et al. ”Tailoring directional scattering through magnetic and electric reso-
nances in subwavelength silicon nanodisks.” ACS nano 7, no. 9 (2013): 7824-7832.
[4] Abdollahramezani, Sajjad, Hossein Taghinejad, Tianren Fan, Yashar Kiarashinejad, Ali A. Eftekhar,
and Ali Adibi. ”Reconfigurable multifunctional metasurfaces employing hybrid phase-change plasmonic
architecture.” arXiv preprint arXiv:1809.08907 (2018).
[5] Kiarashinejad, Yashar, Sajjad Abdollahramezani, and Ali Adibi. ”Deep learning approach
based on dimensionality reduction for designing electromagnetic nanostructures.” arXiv preprint
arXiv:1902.03865 (2019).
[6] Kiarashinejad, Yashar, Sajjad Abdollahramezani, Mohammadreza Zandehshahvar, Omid Hem-
matyar, and Ali Adibi. ”Deep Learning Reveals Underlying Physics of Light-matter Interactions in
Nanophotonic Devices.” arXiv preprint arXiv:1905.06889 (2019).

Nanometer-resolution optical spectroscopic imaging offers fundamental insights of light-matter interactions that has significant bearing on understanding optoelectronic properties of molecules and inorganic nanostructures, surface chemical reactions, and defect engineering.
Optical spectroscopy techniques like Raman, IR spectroscopy and UV-vis-NIR spectroscopy, are widely used for materials characterization. However, their spatial resolution is limited by the wavelength of the light used, which is on the order of hundreds of nanometers to microns. Past attempts to break this diffraction limit include using a scanning probe to locally enhance the interaction between the sample and light, using near field optics to reduce the effective optical spot size, or using super resolution techniques such as PALM, STORM, STED. Electron microscopy, on the other hand, can readily achieve nanometer or even atomic resolution, but surface-sensitive electron spectroscopy of molecules and inorganic nanostructure is difficult due high electron doses and due to the energy mismatch between the high energy (5-300 keV) electron beam and the low energy (5 eV or less) electronic or vibrational states.
We introduce a new imaging technique named PhotoAbsorption Microscopy using ELectron Assays (PAMELA), which combines the high spectroscopic selectivity of photo-excitation with nanometer-scale spatial resolution of electron beams. PAMELA relies on the largely unexplored coupling of light and electron beams within matter that produces unique signatures, which can be utilized for the purpose of nanoscale imaging. This new technique is demonstrated by obtaining optical images of inorganic nanostructures at <10 nm spatial resolution that exhibit spectroscopic selectivity based on photoabsorption. PAMELA provides a more general approach for characterizing materials as it relies on the inherent light absorption, without requiring subsequent fluorescence, luminescence or any specific modification of the sample. This will likely open new opportunities in fields such as surface chemistry, biomolecular imaging, and quantum materials.

Inspired by previous theoretical¹ and experimental work² we will show experimental data and simulations of plasmonic nanopore antennas designed to enhance the Raman-scattered signal of molecules.
Spectroscopy of single molecules can reveal details about the molecular structure that are inaccessible in ensemble measurements. The most prominent example is the DNA molecule. The knowledge of the exact sequence of the nucleotides in the DNA molecule contains the complete genetic information of an organism. However, the individual base pairs are spaced by only a few hundred picometers, requiring extremely high resolution to distinguish adjacent nucleotides. Recent studies using tip-enhanced Raman scattering (TERS) on single molecules have shown that sub-nanometer resolution can be achieved^3–5. While in these studies the probe is brought to the sample, we are going to bring the sample to the probe to streamline the analysis while maintaining the resolution of the TERS technique. In particular, we are going to present the Raman-scattered signal from different molecules in the hotspot of the plasmonic antennas specifically designed to enhance the intensity of the Raman-scattered signal which can be used to identify different molecules. Further, the plasmonic antennas can be used at the same time to influence the spatial dynamics of the molecules present in the hotspot^7,8, which could be exploited to manipulate the molecule. Such a technique this would pave the way for a lab-on-a-chip system to determine sequences of molecules⁶.

References
1. Belkin, M., Chao, S.-H., Jonsson, M. P., Dekker, C. & Aksimentiev, A. Plasmonic Nanopores for Trapping, Controlling Displacement, and Sequencing of DNA. ACS Nano 9, 10598–10611 (2015).
2. Kneipp, J., Kneipp, H. & Kneipp, K. SERS—a single-molecule and nanoscale tool for bioanalytics. Chemical Society Reviews 37, 1052 (2008).
3. Zhang, R. et al. Chemical mapping of a single molecule by plasmon-enhanced Raman scattering. Nature 498, 82–86 (2013).
4. Jiang, S. et al. Distinguishing adjacent molecules on a surface using plasmon-enhanced Raman scattering. Nature Nanotechnology 10, 865–869 (2015).
5. Bailo, E. & Deckert, V. Tip-Enhanced Raman Spectroscopy of Single RNA Strands: Towards a Novel Direct-Sequencing Method. Angewandte Chemie International Edition 47, 1658–1661 (2008).
6. Nicoli, F., Verschueren, D., Klein, M., Dekker, C. & Jonsson, M. P. DNA Translocations through Solid-State Plasmonic Nanopores. Nano Letters 14, 6917–6925 (2014).
7. Gordon, R. Biosensing with nanoaperture optical tweezers. Optics & Laser Technology 109, 328–335 (2019).
8. Huang, J.-A., Zhang, Y.-L., Ding, H. & Sun, H.-B. SERS-Enabled Lab-on-a-Chip Systems. Advanced Optical Materials 3, 618–633 (2015).

The hot-carrier generation in the near-infrared (NIR) in Au nanostructures has recently been explored for applications in photodetection and photocatalysis. While the nonradiative decay of surface plasmons via Landau damping is a pathway to generating hot carriers near the Fermi level, a more efficient mechanism involves direct transitions (interband). However, for Au (and all noble metals) these transitions require at least 2 eV to take place. Therefore, noble metals have a band structure that is not ideal for the generation of hot carriers in the NIR. One possible route to improving the efficiency of hot carrier generation in noble metals at NIR wavelengths, is to shift their electronic density of states closer to the Fermi level via alloying with a transition metal. In this work, we show how alloying Au with Pd couple the individual metal benefits: an oxidation resistance, a longer carrier lifetime and a favorable band structure for NIR transitions. Grazing incidence x-ray diffraction (GI-XRD) and x-ray photoelectron spectroscopy (XPS) measurements support the formation of single-phase alloy and oxidation resistance in the thin films. While, ultraviolet photoelectron spectroscopy (UPS) data shows how the electron density of states (EDOS) increase near the Fermi level when adding Pd to Au with respect to Au. Ellipsometry measurements show that adding Au to Pd improves the plasmonic properties in the NIR with respect to pure Pd. DFT calculations show that Au-Pd alloys are expected to generate more hot carriers than pure Au under NIR. Using NIR-pump, THz probe transient absorption spectroscopy, we also provide evidence suggesting that alloying Au with Pd results in an increased distribution of hot carriers with respect to Au in the NIR.

Plasmonic and photonic structures have dimensions that are typically smaller than the diffraction limit. Here, we use super-resolution microscopy to map the coupling between fluorescent molecules and metal nanoparticle arrays and to study catalysis on individual Au nanostructures.

First, we investigate the coupling between fluorescent molecules and a periodic array of metallic nanoparticles. Due to their ability to support surface lattice resonances, these arrays can enhance the emissive properties of fluorescent molecules across the whole unit cell of the array, instead of just in the vicinity of a nanostructure. By combining super-resolution microscopy with finite-difference time-domain simulations, we find that collective resonances have minimal influence on the spontaneous decay rate of an emitter, but instead can be exploited to enhance the directivity of the emission. Our approach of experimentally mapping emission enhancement with sub-diffraction resolution and numerically disentangling the underlying contributions can inform the rational design of optical devices based on plasmonic particle arrays [1].

Second, we study the effect of the decay of localized surface plasmon resonances on the catalytic properties of metal nanoparticles. As a test reaction we use the nanoparticle-catalyzed reduction of the weakly fluorescent molecule resazurin to the strongly fluorescent molecule resorufin. By using catalysts with a plasmon resonance spectrally separated from the absorption and emission of the reaction products and by controlling the polarization of the incident fields, we can study the different contributions to the total catalytic rate of plasmonic hot electrons, electromagnetic hot spots, and photo-thermal effects. Understanding the underlying mechanisms of plasmon-enhanced catalysis is fundamental for the development of a next generation of photo-catalysts that can efficiently harvest light to drive chemical reactions.

[1] R. F. Hamans, M. Parente, G. W. Castellanos, M. Ramezani, J. Gómez Rivas, A. Baldi, Super-resolution Mapping of Enhanced Emission by Collective Plasmonic Resonances, ACS Nano 13, 4514-4521 (2019)

Indistinguishable single photon generation at telecom wavelengths from solid-state quantum emitters remains a significant challenge to scalable quantum information processing. Here we demonstrate efficient generation of indistinguishable single photons directly in the telecom O-band from aryl-functionalized carbon nanotubes by overcoming the quantum decoherence with plasmonic nanocavities. With an unprecedented single-photon spontaneous emission time down to 10 ps (from initially 0.7 ns) in the coupling scheme, we show a two-photon interference visibility at 4 K up to 0.79 without post selection. Cavity-enhanced quantum yields up to 74% and Purcell factors up to 415 are achieved with single-photon purities up to 99%. Our results establish the capability to fabricate fiber-based photonic devices for quantum information technology with coherent properties that can enable quantum logic.

The properties of topological insulators (TIs) have been widely explored due to their unique band structure. The energy states at the TI surfaces exhibit linear dispersion and spin-momentum locking. Carriers occupying these states are therefore two-dimensional, massless, and spin-polarized. Dirac plasmons comprising these carriers can be excited in TI thin films. Because most TI films are much thinner than the wavelength of light, plasmons excited on the top and bottom surfaces couple, resulting in an acoustic and an optical plasmon mode. Due to the spin-momentum locking characteristics of the TI surfaces, the optical mode is predicted to be spin-polarized. Before the spin properties of TI plasmons can be measured, propagating plasmons must first be excited. There are a variety of ways to excite plasmons in TI thin films. The most common way is to etch the film into an array of stripes to create localized plasmons. Although this technique has been highly effective, these localized modes cannot easily be used to understand the spin properties of TI plasmons. To excite propagating plasmons, we instead choose to use a grating coupler fabricated on the surface of the TI film.

We first grew a series of 50nm Bi₂Se₃ films using a molecular beam epitaxy (MBE) system. The TI films had the following structures: a layer of 5nm BiInSe₃ (BIS) on the top, 50nm Bi₂Se₃ TI in the middle, and 50nm BIS between the TI and the Al₂O₃(0001) substrate. The BIS layer on the top serves as a protection layer to prevent surface degradation and charge redistribution caused by the grating metals. The BIS layer on the bottom serves as a buffer layer to optimize the quality of the TI growth. After growth, 100nm gold/10nm titanium gratings with different grating periodicity are lifted off of the surface of the film. The periodicity of the gratings ranges from 100nm to 700nm. TM polarized transmission spectra are then taken in a Fourier Transform Infrared Spectroscopy (FTIR) system. From the extinction spectra, we observe a series of absorbing peaks that shift with the grating periodicity. We have ruled out any source other than the plasmons that could have possibly caused the peaks to show up in the range. TM transmission spectra were taken on a bare sapphire substrate, a single layer TI film directly grown on sapphire, a single layer BIS film grown on sapphire, and a sapphire substrate with gratings on top. None of the spectra shows the same peaks as those we see in TI films with top gratings. Hence, we have successfully demonstrated the existence of the propagating Dirac plasmons in the TI Bi₂Se₃ and the tunability of the plasmon frequency with grating period.

The next steps will be to launch the plasmon and detect the spin wave dynamically. TIs are the only known single-material system where such spin-polarized plasmons can be observed. The observation of propagating Dirac plasmons in TI is significant not only because it proves the theoretical predictions of TI Dirac plasmons, but also creates possibilities in developing future technology. A frequency-tunable propagating spin-density wave could be feasible for better memory or computing systems.

The integration of plasmonics and magneto-optics has led to the emergence of a new research field known as magnetoplasmonics. The main goal of magnetoplasmonic is twofold. First, the use of magnetic materials in plasmonic structures enables active light manipulation at the nanoscale via field-controlled breaking of time-reversal symmetry [1,2]. Second, the excitation of surface plasmons in magnetic materials can be used to resonantly enhance and spectrally tailor their magneto-optical response [3-7]. Despite its promise, magnetoplasmonics faces a challenge of overcoming optical losses. This holds particularly true for nanostructures containing ferromagnetic metals, whose losses are significantly larger compared to noble metals. Here we exploit surface plasmon polaritons (SPPs) excited at the interface of a SiO₂/Au bilayer to induce strong magneto-optical responses on the Ni nanodisks of a periodic array. Using a reference system made of Au nanodisks, we show that optical losses in Ni do not broaden the linewidth of the SPP-driven magneto-optical signals. Loss mitigation is attained because the free electrons in the Ni nanodisks are driven into forced oscillations away from their plasmon resonance. By varying the SiO₂ layer thickness and lattice constant of the array, we demonstrate tailoring of intense magneto-optical Kerr effects with a spectral linewidth down to ~25 nm. Our results provide important hints on how to circumvent losses in magnetoplasmonics via the design of off-resonance driving mechanisms.

[1] V. V. Temnov et al., Nat. Photonics 4, 107 (2010).
[2] V. I. Belotelov et al., Nat. Commun. 4, 2128 (2013).
[3] V. I. Belotelov et al., Nat. Nanotechnol. 6, 370 (2011).
[4] N. Maccaferri et al., Phys. Rev. Lett. 111, 167401 (2013).
[5] J. Y. Chin et al., Nat. Commun. 4, 1599 (2013).
[6] M. Kataja et al., Nat. Commun. 6, 7072 (2015).
[7] N. Maccaferri et al., Nat. Commun. 6, 6150 (2015).

In recent years, circularly polarized light (CPL) detectors and CPL emitters based on chiroptical effects arising from chiral molecules have attracted much attention as building blocks of advanced information technology. Their performances, however, have been shown to be severely limited by the trade-off between the external quantum efficiency (η_E) and the dissymmetry factor (g) that characterizes their asymmetric optical behaviors depending on the helicity of CPL. One way to overcome this is to utilize the supramolecular chirality, which translates a molecular chirality into a supramolecular system whose size is comparable to or larger than the wavelength of light. Although this approach can increase g without necessarily decreasing η_E, it is not applicable to optoelectronic devices whose vertical (with respect to the substrate) dimension is smaller than the wavelength, such as organic light-emitting devices and photodetectors.

Here, we numerically demonstrate that a nanophotonic platform consisting of a chirally patterned metal layer–insulator layer–metal layer (cp-M/I/M) structure can induce a strong chiroptical response of organic molecules positioned inside the insulator layer, and that a CPL-sensitive organic photodetector with both high g and η_E can be realized using this platform. In this nanophotonic structure, plasmonic hot spots are excited at an optical frequency only for a helicity of the incident CPL matched to the twisted direction of the chiral nanopattern, which is attributed to a selective excitation of a mode arising from the hybridization of a plasmonic mode near the top electrode and that near the bottom electrode. For full utilization of the plasmonic hot spots possessing high photonic densities of states, we examine the dependence of η_E and g_A on the molecular orientations. A CPL-sensitive photodetector incorporating in the cp-M/I/M structure vertically-oriented molecules, i.e., molecules whose transition dipole moment is normal to the substrate, features η_E = 24% and |g_A| =1.6, which are respectively 24 (or 240) and 1.7 (or 1) times larger than η_E and |g_A| of Schottky photodiodes using Z-shaped chiral nanostructures [1] (or helicene based photodetectors [2]).

References:
[1] Li, W.; Coppens, Z. J.; Besteiro, L. V.; Wang, W.; Govorov, A. O.; Valentine, J., Circularly polarized light detection with hot electrons in chiral plasmonic metamaterials. Nat. Commun. 2015, 6, 8379.
[2] Yang, Y.; da Costa, R. C.; Fuchter, M. J.; Campbell, A. J., Circularly polarized light detection by a chiral organic semiconductor transistor. Nat. Photonics 2013, 7 (8), 634-638.

The interaction of a magnetic field and the field generated by a localized plasmon resonance on Au nanorods is shown to be a measurable effect. Previously, the magneto-optical Kerr and Faraday polarization dependent effects have been observed on hybrid magnetic-plasmonic structures, and it has been theoretically demonstrated that under high magnetic field strength (>10T) a splitting of the plasmon absorption band occurs with circularly polarized light. Here the surface plasmon absorption of Au nanorods is measured utilizing a magneto-spectrometer with a magnetic field force up to 1.5T and non-polarized light. It is found that as the magnetic field strength increases, the energy and the lifetime of the plasmon resonance are altered. Coupled with theoretical work, our findings suggest either the Lorentz force is acting on the elections under a magnetic field effectively amplified by the localized plasmon resonance, or an alternative mechanism exists.

Enhanced and controlled light absorption as well as field confinement in an optically thin material are pivotal for energy efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films with near-zero permittivities, in their so-called epsilon near zero (ENZ) frequency regions, can support ENZ modes which may lead to perfect light absorption and ultra-strong electric field intensity enhancement (FIE) within the films. To achieve full control over optical absorption and FIE, one must be able to tune the ENZ material properties as well as the film geometries.
Here, we report a nano-engineering of ultra-smooth aluminum doped zinc oxide (AZO) films with tunable ENZ wavelength (1500-1725 nm), low optical losses, and thickness as small as 22 nm by using atomic layer deposition (ALD) technique. The ENZ properties of the AZO thin films are controlled by deposition conditions such as dopant ratio, deposition temperature, and number of macro-cycles. We experimentally demonstrate engineered absorption and FIE of AZO thin films via control on their ENZ wavelengths, optical losses, and film thicknesses. Furthermore, we introduce a simple mathematical formula for quick and accurate estimation of FIE when ENZ modes are excited in classical Kretschmann-Raether configuration. Finally, we demonstrate that under ENZ mode excitation, though the absorption and FIE are inherently related, the film thickness required for observing maximum absorption differs significantly from that for maximum FIE. This systematic study on engineering ENZ materials related enhancement properties by optimization of ALD deposition process will be beneficial for the design and development of next generation tunable photonic devices based on flat zero-index optics.

This work is supported in part by:

The Young Faculty Award Program from Defense Advanced Research Projects Agency (grant number N66001-17-1-4047), the Young Investigator Development Program and the Office of Vice Provost for Research at Baylor University,

CAREER Award Program from National Science Foundation (grant number: 1752295),

Robert A. Welch Foundation (Award number: AA-1956-20180324).

Epsilon-near-zero (ENZ) modes are supported in films of polaritonic materials with vanishing permittivity values and deeply sub-wavelength thicknesses. These novel optical excitations are associated with extreme electric field confinement, enabling control over light-matter interactions such as resonant perfect absorption and strong nonlinear interactions. Recent studies have demonstrated ENZ phenomena in doped transparent conducting oxides (TCOs). Unlike traditional metals that are not tunable in the IR and THz due to fixed, high carrier densities, TCOs allow for tuning of both carrier density and electronic mobility. One highly promising TCO is highly-doped CdO, which has been shown to achieve electron mobilities extending upwards to 500 cm²/V-s with carrier densities ranging from 10¹⁹ to 10²⁰ cm^-3. Unique to this material is a range of carrier densities where increasing values result in increasing mobilities.
ENZ modes are unique for their strong absorption/emission and narrow resonant linewidths that can be achieved without the need for nanostructuring. This behavior has been utilized to achieve perfect absorption in thin films as well as thermal emissivity control. These modes are attributed with a nearly-flat spectral dispersion that consequently results in low group velocities and short propagation lengths, diminishing their utility. However, these limitations may be mitigated through hybridization with other polaritonic modes, such as surface plasmon polaritons (SPPs) which are also supported in doped CdO films. Unlike ENZ modes, SPP modes propagate at a much higher group velocity but are hindered by carrier scattering losses and highly sensitive to surface morphology. Careful control over the carrier concentration during film growth has opened the door to achieving multilayer CdO films. Here we show that epitaxially-grown bilayer stacks of CdO, with bottom and top layers supporting SPP and ENZ modes respectively, exhibit cavity-free strong coupling between the ENZ and SPP modes and overcome the deficiencies of each constituent mode. The combined ENZ-SPP dispersion of these bilayer films displays a prominent anti-crossing with a separation that is on the order of the mode frequency, giving clear evidence of strong coupling. The degree to which these modes are strongly coupled together is dependent on both the spectral and spatial overlap of the modes. By carefully controlling the plasma frequency of the two individual CdO layers, this spectral overlap and thus the ENZ-SPP dispersion, can be tuned. We also show that tuning the oscillator strength of the individual modes is another approach towards manipulating the ENZ-SPP dispersion. This is controlled geometrically, by tuning the ENZ layer thickness.
In more recent work we have extended our focus to other geometries, such as CdO films where the SPP and ENZ films are spatially separated by a dielectric spacer layer. The dielectric layer thickness modifies the spatial overlap of the constituent electric fields as well as alters the effective index of the region above the SPP layer, providing additional tuning to the ENZ-SPP dispersion. We also examine the hybridization between localized SPP modes and an ENZ layer imbedded within a perfect absorber structure to monitor the impact of nanostructuring upon this strong coupling phenomena and how this can be used as independent knobs for tuning the resonant absorption, and alternatively thermal emission, of the structures. By fabricating such hybrid materials into nanostructure arrays, direct control of the emission polarization, spatial coherence and divergence is also engineered through careful design of nanostructure geometry and periodicity. This approach may lead to the realization of spectrally tunable thermal emitters for narrow-band, polarized and spatially coherent IR sources.

Transition metal nitrides have been considered as promising plasmonic materials due to the high thermal stability and tunable optical properties to replace noble metals. In this work, we used pump probe transient absorption measurements to study the ultrafast carrier relaxation processes in gold film (standard plasmonic materials) and alternative plasmonic materials, including sputtered titanium nitride, zirconium nitride, hafnium nitride and niobium nitride. The long lifetime of hot carrier in alternative plasmonic materials make it more beneficial than that in gold to cross the Schottky barrier between semiconductor and plasmonic materials, and further enhance the performance of plasmonic device. Here, we demonstrated that transition metal nitrides are possible to be utilized in efficient hot carrier extraction. With femtosecond Ti:Sapphire pulsed laser, we are capable of
distinguishing various carrier dynamics within a nanosecond. Two relaxation mechanisms take place during the period hot electrons relax probing at photon energy of 2.58 eV. One is the combination of electron-electron and electron-phonon interactions which occurs at femtoseconds to several picoseconds, and slow phonon-phonon relaxation with a hundreds of picosecond comes after the first short relaxation. Moreover, we not only confirmed that the relaxation process in transition metal nitrides are one order magnitude slower than that in gold, but also proved that niobium nitride has the longest first fast decay time. The more slowly the carriers relax, the more possibly they are captured. In addition, the relaxation time could be tuned with respect to quality of materials, thickness and substrate. Besides, we theoretically calculated the band structure of transition metal nitrides by first principles in order to study the carrier relaxation time. We conclude that the slow carrier relaxation time strikes an opportunity for hot carriers in transition metal nitrides to be efficiently transferred to semiconductor as hot carrier donor due to the long hot carrier lifetime. Thus, we believe the results pave the way for the applications of high performance optoelectronic devices with high efficient hot carrier extraction.

With the advent of integrated photonics, grating couplers offer a solution towards integrated optical isolation by coupling optic fibers to on-chip waveguides. In this context, we study the interplay between electro-optical and magneto-optical effects. The magneto-optical effect breaks the time-reversal symmetry and induces frequency shifts in the energy and angular spectra of plasmon resonances. As a result of these shifts, exceptionally large magneto-optic responses are can be achieved [1]. In optoelectronic devices modulation and switching capabilities can be accomplished by electric control of the amplitude and phase of electromagnetic waves, using architectures based either on Mach-Zehnder interferometers or micro-ring resonators. Combining the properties of the two aforementioned materials could enable the control of the properties of magneto-optically active devices by electric fields which is an attractive prospect for development of novel nanophotonic devices. Facing this challenge, we study a magneto-optically active Au/Co multilayer grating coupler fabricated on top of an electro-optically active substrate (BaTiO₃) as the basic device to uncover how these two active properties influence each other. We used Fourier optics microscopy to obtain the plasmonic band structures of our gratings [2]. The Fourier spectroscopy approach can be adapted to the study of magnetoplasmonic gratings by reducing the beam spot size in the objective back aperture, so that diffracted modes can be analyzed independently, enabling the exploration of the interplay between plasmonic resonances and diffracted light. We used this approach to study selectively surface plasmon polaritons propagating along backward or forward directions, enabling us to easily assess their non-reciprocal magnetic modulation. To assess the magneto-optic response, we measured the transverse magneto-optic Kerr effect (TMOKE) amplitude of Au/Co magnetoplasmonic gratings, measured from the ARR maps, obtained through the expression TMOKE = (I(H₊)-I(H_-))/I(H_avg), where I(H₊), I(H_-) are the reflected or diffracted intensity detected at opposite saturated magnetizations of Co (taken at H_+,- ≈ 150 Oe), and I(H_avg) is defined as the average of the absolute value of I(H₊) and I(H_-). To find out how the electro-optical effects influence plasmon propagation and magneto-optic responses, we measured ARR maps under applied electric fields. In line with our previous results, large magneto-optic signals – two orders of magnitude larger than intrinsic responses– arise from the SPP excitation. We show that, due to the presence of the ferroelectric layer, plasmon propagation can be modulated by electric fields enabling reversing the sign of magneto-optic signals. Thus, the combined integration of magneto-optical and ferroelectric materials enables control over non-reciprocal device properties by application of external electric fields, rather than magnetic fields, which could greatly simplify their integration into multifunctional nanophotonic devices.

[1] G. Armelles et al. Magnetoplasmonics: Combining Magnetic and Plasmonic Functionalities. Adv. Opt. Mater. 1: 10–35, 2013.
[2] R. Cichelero et al. Non-reciprocal diffraction in magnetoplasmonic gratings. Opt. Exp. 26: 34842-34852, 2018.

Organic semiconductors have attracted considerable research interest owing to their ease of processing and unique physical properties, such as compatibility with plastic substances, low temperature processing, large area coverage, mechanical flexibility and low embodied energy. Plasmonic metasurfaces, which are nanostructured metallic films capable of supporting and directing surface plasmons at visible wavelengths, have been shown to improve the performance of organic optoelectronic devices by, for example, enhancing light absorption in or emission from the active layer. However, large-area, low energy fabrication processes are required for plasmonic metasurfaces to be compatible with organic semiconductor device processing.
In this work, we describe a straight-forward fabrication process for Ag plasmonic metasurfaces based on thermally dewetting thin Ag films. We study the integration of plasmonic metasurfaces with the organic semiconducting polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), and the impact on the optical and electrical characteristics of the semiconductor thin film. F8BT is a fluorene-based conjugated polymer, that is widely employed as an active material in organic optoelectronic devices, especially organic photovoltaics and organic light-emitting diodes, because of its high quantum and power conversion efficiencies. By varying the size of the Ag nanoparticles (AgNPs) on the plasmonic metasurfaces from 20 nm to 150 nm in diameter, we observe pronounced increases in the scattering, absorption and photoluminescence spectra from F8BT film coatings. From the optical images, the light scattering intensity of metasurfaces with 150 nm diameter AgNPs is highest because of the high albedo (ratio of scattering to total extinction) of the large AgNPs. The extinction spectra are attributed to absorption and scattering by the localized surface plasmon resonances of the AgNPs. From the UV-visible extinction spectra, the extinction peaks of 20 nm- and 80 nm-diameter AgNP metasurfaces occur at a wavelength of 500 nm and the extinction peak of the 150 nm-diameter AgNP metasurface is around 550 nm. The red-shifted extinction of the larger AgNPs is due to their more extended and distributed dipole response to incident light which causes a lowering in the energy of the surface plasmon resonance. Similarly, the 150 nm-diameter AgNP metasurface caused the greatest enhancement in photoluminescence emission from F8BT film coatings because of the high albedo of the large AgNPs. In addition, using integrating sphere absorption measurements we find stronger absorption enhancement and red-shifted F8BT absorption spectra on metasurfaces with increasing AgNP diameterFinally, we investigate the effects of plasmonic metasurface electrodes on electrical properties of F8BT hole-only devices. We demonstrate working F8BT hole-only devices with plasmonic metasurface electrodes containing a wide range of Ag nanostructure sizes in an inverted device geometry.

For studies of molecular interaction with light such as tip-enhanced Raman spectroscopy (TERS), it is imperative that light be confined to a small space in the order of nanometers which can be achieved by means of localized surface plasmons.¹ In conventional TERS, which has seen dramatic advancements in recent years such as chemical mapping with atomic resolution,² surface plasmons are excited on the metallic probe by directly focusing the laser beam in free space in the vicinity of tip apex. Such excitation scheme has to overcome the inherent spatial mismatch between diffraction-limited, micrometer-sized focal spot and nano-scale tip-sample interaction volume, leading to low mode coupling efficiency and high background scattering in the detected field.³ Recently, studies have shown that optical fiber-incorporated plasmonic probes can be a platform for transporting electromagnetic energy in nanoscale with high efficiency.⁴ However, these devices rely on the higher order radial vector mode for coupling of photonic to plasmonic modes which requires a special type of fiber that maintains radial polarization and expensive radial polarizer in the excitation path.
In this work, we propose a fiber-plasmonic hybrid probe that efficiently couples linearly polarized fiber core mode to radial plasmons on gold tip for nano-scale confinement of light. Using finite difference time domain 3D simulations, we show that linearly polarized light can be utilized to create hot-spot at the apex of the tip by selectively exciting plasmons at only one side of the tip base from the optical fiber. The coupled plasmonic mode then propagates to the narrow apex where it gets localized and strongly focused. Then we discuss the polarization selectiveness of our excitation scheme by comparing two orthogonal source polarizations. We show that the enhanced field at the apex is due to the spiraling plasmons excited by the end-fire coupling of light from fiber core at the tip base. We also demonstrate the cavity-like resonant response of the device by spectrally mapping the field enhancement for varying tip sizes. The focusing efficiency of 900 nm long tip at resonant wavelength is 1.3%. In the collection mode, the radiated power from a dipole emitter is probed by the 800 nm long tip and 5.1% of the dipole radiation is transmitted through the fiber. The proposed device is highly desirable for a plethora of applications including medical procedures, biomedical imaging and near-field spectroscopy.

References
1. Stockman, M. I. Phys Rev Lett 2004, 93, (13).
2. Lee, J.; Crampton, K. T.; Tallarida, N.; Apkarian, V. A. Nature 2019, 568, (7750), 78.
3. Novotny, L.; Hecht, B., Principles of nano-optics. Cambridge university press: 2012.
4. Tugchin, B. N.; Janunts, N.; Klein, A. E.; Steinert, M.; Fasold, S.; Diziain, S.; Sison, M.; Kley, E. B.; Tunnermann, A.; Pertsch, T. Acs Photonics 2015, 2, (10), 1468-1475.

Acknowledgment
This work is supported in part by AFOSR-AOARD (FA2386-18-1-4099)

Topological insulators (TIs) are semiconductor materials, exhibiting topologically protected surface states that arise from strong spin-orbit coupling. These surface states form a Dirac cone band structure at Γ point, which exhibits linear dispersion. Electrons occupying these states travel at relativistic velocity and behave as massless Dirac fermions. In addition, these states demonstrate spin-momentum locking, which limits scattering from nonmagnetic perturbations. When perturbed with a sinusoidal electromagnetic field, these surface states can house two-dimensional Dirac plasmons at room temperature. The Dirac plasmons in TIs are predicted to be topologically protected and spin polarized, which means spin of electrons can be optically manipulated.
Instead of the normal bulk plasmon mode that arises in three-dimensional films, electrostatic coupling between the top and bottom surface states of a single layer of TI thin film causes an optical plasmon mode and an acoustic plasmon mode [1]. However, only the optical mode can be observed by optical methods due to its non-zero optical dipole [2,3]. Interestingly, the frequency of the optical mode lies in the untapped terahertz (THz) range. A superlattice structure of TIs and normal band insulators (BIs) is expected to bring coupling of multiple surface states, generating multiple optical modes in the THz. These modes are tunable by changing the layer structure of the superlattice, which is easy to do with molecular beam epitaxy (MBE). In our system, Bi₂Se₃ and (Bi_0.5In_0.5)₂Se₃ (BIS) were used as the TI and BI materials, respectively. BIS is a topologically trivial alloy material of Bi₂Se₃ and β-In₂Se₃, both of which belong to the group and share close lattice constants: 2.866nm and 2.823nm, respectively. It was shown that by using BIS as a buffer layer between a sapphire substrate and the Bi₂Se₃ film, the carrier mobility increased and the overall carrier density was reduced by a factor of at least two, indicating a reduction in trivial carriers and a Fermi energy closer to the Dirac point. Using BIS, indium diffusion into the TI layers is less of a concern than it would be with pure β-In₂Se₃ as the BI layers [5]
In this talk, we will demonstrate the experimental evidence of Dirac plasmon coupling in BIS /Bi₂Se₃/BIS/Bi₂Se₃/BIS stacked layers grown on sapphire (0001). The film was patterned into nanoribbon arrays with varying widths to excite localized plasmon modes. Fano-shape extinction spectra containing two distinct plasmon modes were observed with all samples in the range of 1.5THz-8THz through Fourier-transformed Infrared (FTIR) spectroscopy measurements. A three-oscillator, double-Fano-resonance model was adopted to extract parameters of the high/low frequency plasmon modes and to explain the quantum interference between the plasmons and the bulk phonon (α phonon=2.01THz). A blueshift in frequency was observed for both the high/low frequency modes as the nanoribbon width narrows, similar to what has been observed in the single plasmon mode in a single layer Bi₂Se₃ [2,3]. Understanding plasmon coupling in a multilayer TI/BI structure will provide a deep insight to the many-body interaction in low-dimensional strong spin-orbit coupling systems. It also lays the cornerstone for future tunable THz devices for detection, security screening and advanced communication.
Reference
[1] R. E. V Profumo, R. Asgari, M. Polini, and A. H. Macdonald, Phys. Rev. B 85, 085443 (2012).
[2] T. P. Ginley and S. Law, Adv. Opt. Mater. 1800113 (2018).
[3] P. Di Pietro, M. Ortolani, O. Limaj, A. Di Gaspare, V. Giliberti, F. Giorgianni, M. Brahlek, N. Bansal, N. Koirala, S. Oh, P. Calvani, and S. Lupi, Nat. Nanotechnol. 8, 556 (2013).
[4] Y. Wang and S. Law, Opt. Mater. Express 8, 2570 (2018).
[5] Y. Wang, T. P. Ginley, S. Law, Y. Wang, T. P. Ginley, and S. Law, J. Vaccum Sci. Technol. B 36(2), 02D101 (2018).

Titanium nitride has emerged as an alternative material to the noble metals gold and silver for optoelectronic and plasmonic applications due to its increased mechanical stability, thermal stability and spectral tunability.^1,2 As the number of TiN-based optoelectronic devices increases it is necessary to develop an understanding of the applicability of TiN within a range of operating environments. Key factors to consider include temperature stability and compatibility with application specific materials.

In this work, we investigate the applicability of TiN in conjunction with a range of materials commonly used within plasmonic and optical devices. TiN thin films were deposited by radio frequency (RF) and High-power impulse (HiPIMS) magnetron sputtering onto a variety of semiconductor- and industrial standard substrates including Si, MgO, glass and steel. Optical properties were measured using spectroscopic ellipsometry and correlated with the crystalline structure and surface morphology of the TiN thin films. Additionally, the spectral tunability of these materials was investigated with respect to lattice-mismatch-induced strain and film texture. This work also reveals the temperature range of applicability of the TiN based optoelectronic devices.

1. Wells, M. P. et al. Temperature stability of thin film refractory plasmonic materials. Opt. Express 26, 15726 (2018).
2. Braic, L. et al. Titanium Oxynitride Thin Films with Tunable Double Epsilon-Near-Zero Behavior for Nanophotonic Applications. ACS Appl. Mater. Interfaces 9, 29857–29862 (2017).

Metallic nanostructured films incorporated into optoelectronic devices such as photodetectors have attracted attention for their ability to support localized surface plasmon polaritons (LSPPs), which can enhance optical absorbance, internal electric field, and ultimately device performance. Metallic nanostructured films can also act as a device electrode, making them a cost-effective, flexible, high-performance alternative to the commonly used indium tin oxide (ITO), which is expensive and brittle. Photodetectors that are sensitive to UV illumination are important in a variety of applications including environmental monitoring, scientific research, imaging, and flame and missile detection. Despite their applicability, extension of plasmonic enhancement mechanisms to UV-selective devices has been relatively slow, because the common plasmonic metals of Ag and Au cannot support surface plasmon polaritons (SPPs) when illuminated by light with wavelengths shorter than about 350 nm. Al is well-suited to support and tune SPPs in the UV spectral range due to its carrier concentration and dielectric function, although oxide formation on the Al surface can interfere with and cause a redshift in its plasmonic properties. In addition, Al provides an attractive material option for UV photodetectors due to its relative abundance and low cost compared to Ag and Au.
In this work, Al nanohole arrays (Al-NHAs) were utilized as transparent conducting electrodes to optimize the UV-selectivity and response tunability of photodetectors having a conventional photodiode structure with organic active materials, which provide a low-cost, flexible alternative to inorganic materials. UV-selectivity was ensured through the material selection of Al, for plasmonic activity within the UV spectral range, and the polymer donor of poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2), for strong UV absorption with a cut-off around 515 nm. The active layer was a 350 nm film consisting of F8T2 and the fullerene-derivative electron acceptor PC₇₁BM, combined in a weight ratio of 100:4 based on previous reports. PEDOT:PSS and LiF were used as hole and electron transport layers, respectively, for an overall device structure of Al-NHA/PEDOT:PSS/F8T2:PC₇₁BM/LiF/Al. 3-dimensional finite-difference time-domain (3D-FDTD) electromagnetic simulations were conducted to design the Al-NHA to produce strong UV absorption in the active layer and enhanced internal electric field intensity. Al-NHA electrodes were successfully fabricated using nanosphere lithography and incorporated into photodetectors, which produced two narrow photoresponse peaks with specific detectivity (D*) values of 4.0x10⁹ and 4.6x10⁹ Jones under 340 and 515 nm illumination, respectively, and -2 V bias, and one broad photoresponse peak with a peak D* of 8.8x10⁹ Jones under 450 nm illumination and 2 V bias. Compared to control ITO-based devices, Al-NHA-based devices had similar response under reverse bias and superior response under forward bias, as ITO-based devices became unstable under forward bias. The bias-dependent photoresponse switching is believed to benefit from plasmonic enhanced internal electric field that increases the driving force for hole diffusion. The mechanism was further confirmed by the investigation of conventional devices with planar Al electrodes and hole-only devices with both planar Al and Al-NHA electrodes. This response switching improves the applicability of UV photodetectors through the incorporation of cost-effective, flexible, and performance-enhancing plasmonic Al-NHA transparent conducting electrodes.

Deep UV Photoluminescence (PL) Spectroscopy was employed to study optical properties of sapphire crystal. The sample was excited by the third harmonic of a Ti:sapphire pulse laser at wavelength ~ 266 nm which is a below bandgap photoexcitation. In the low temperature (12 K) PL measurements, we observed two sharp atomic-like peaks at 368.8 and 374 nm with line-width of 0.85 and 3.30 nm, respectively in the PL spectra. We also performed temperature and power dependent PL measurements. We will present the properties of the atomic-like transitions observed in sapphire at different conditions. The origin of these emission lines could be due to the point defects or incorporation of rare-earth elements. We will also discuss our exploration on the origin of atomic-like electronic states in sapphire crystal. Atomic-like emission in the sapphire could have potential applications in quantum information technologies for the development of solid state single photon source in UV region.

Bulk single crystal hexagonal boron nitride (h-BN) flakes were characterized by deep UV photoluminescence spectroscopy. We performed a comparative study of oxidized and unoxidized h-BN flakes at low and room temperatures. In the low temperature (15 K) PL of h-BN flakes, we observed strong phonon assisted band edge emission peaks at 5.451 and 5.755 eV. The sample was oxidized at 900 ^oC for 1 hour. The PL measurements of the sample after oxidation, in addition to the phonon assisted band edge emissions, we also observed a sharp atomic-like emission line at 4.082 eV along with its photon replicas at 3.88 and 3.68 eV which were not present in the unoxidized samples. The origin of the sharp atomic-like transition could be related to the incorporation of defect or impurity during the oxidation process. We will discuss the properties of these atomic-like transitions and its possible origins in the h-BN flakes. Our results could have potential applications in the solid state single photon source for quantum information technologies.

In this work, we prepared graphene-mediated surface-enhanced Raman scattering (G-SERS) substrates comprising few-layer graphene nanosheets decorated gold and silver nanoparticles for bio-nanotechnology. Raman spectroscopy is a surface-sensitive and nondestructive inelastic light scattering technique. SERS, a specialized form, is useful for rapid and precise identification of biological molecules, industrially relevant chemical dyes at ultralow concentration and monitoring DNA hybridization. This phenomenon is due to the enhanced Raman signals by several orders of magnitude on SERS-active surfaces. While the key point of SERS technology is the nanoscale metal particles, which generates localized surface plasmon resonances in response to laser excitation, the resulting electromagnetic enhancement of > 10⁴, controlled diameter and interparticle gap of metal nanoparticles on graphene supports offer an advance toward sensitive G-SERS substrates via localized hybridization at graphene-metal interfaces. We have used thermal reduction technique to produce functionalized graphene and wet chemistry for size tunable gold and silver nanoparticles as cost-effective facile synthesis approaches for strategic G-SERS platforms. Simple and high-throughput arrays (‘biochips’) are developed by decorating graphene nanosheets with gold and silver nanoparticles as well as sandwiching gold and silver nanoparticle and few-layer graphene for cascaded signal amplification to differentiate among nucleotide bases (adenine; A, thymine; T, cytosine; C, guanine; G), DNA hybridization through complementary and probe single-stranded DNA and to detect beta-carotene and malachite green chemical dye.

Hyperbolic phonon polaritons (PhPs) demonstrate great promise for the advancement of nanophotonics in applications such as subdiffraction focusing and flat optics. Due to the large optical anisotropy of hexagonal boron nitride (hBN), PhPs are intrinsically hyperbolic, meaning that they propagate with very large wavevectors, well below the optical diffraction limit. In comparison to plasmons, another class of propagating modes in metals, hyperbolic PhPs enable tighter confinement and experience lower losses in the mid-infrared regime. Isotopic enrichment of the polaritonic medium (e.g., hBN) has been shown recently to be an effective strategy to increase the propagation lengths and lifetimes of PhPs.
Photothermal induced resonance (PTIR) is a scanning probe-based technique that yields nanoscale IR absorption spectra and maps by transducing the light-induced thermal expansion of the sample with the probe tip. In this study, a novel tapping-mode PTIR measurement paradigm with heterodyne detection is implemented to measure PhP propagation and to determine the spectral dispersion in large flakes (up to 120 μm × 250 μm) of isotopically enriched hBN (¹⁰B). The high signal-to-noise ratio afforded by this measurement scheme enables imaging PhPs in real space over distances up to 40 μm. Such measurements, coupled with reciprocal space analyses, enables accurate evaluation of PhP propagation lengths, lifetimes, and in-plane wavevectors. Our results confirm for first time, experimentally, that isotopic enrichment provides over a eight-fold improvement in PhP propagation lengths and lifetimes, as predicted by theory. We believe that the measurement scheme developed here and the isotopic enrichment strategy will further foster the engineering and development of high performance PhP-based nanophotonic devices.

Graphene is a well-known 2D material, and its optical property can be tuned by changing its Fermi level [1]. This tunable property makes graphene useful to build novel optical devices, such as modulators. However, due to its atomically thin thickness, the interaction between pristine graphene and light is low, impeding its practical applications. To solve this problem, people have demonstrated that by combining graphene with plasmonic nano-structures, the interaction between light and graphene can be greatly enhanced, which significantly increases the tunability and efficiency of graphene-based optical devices. For photon energy larger or smaller than twice the Fermi Level, the absorption of light in graphene falls into interband or intraband interaction. Up to now, most of graphene-based modulators utilize intraband transition, corresponding to mid-infrared or terahertz operating wavelengths (e.g. [2]). In contrast, few works reported graphene modulators based on interband transition, corresponding to near-infrared and visible wavelengths. Although in the interband region, graphene plasmons cannot be excited, and surface conductivity is much lower compared with intraband region, the interband interaction has its advantages of insensitivity to graphene quality and step-like change of its surface conductivity, offering the potential to push the working wavelength of graphene modulators into the near-infrared or even visible range.
Based on the idea proposed above, we have fabricated a graphene-based modulator, in which graphene is hybridized with metal-insulator-metal (MIM) structures to confine the light intensity around graphene. The resonant wavelength is at 1.5 um and theoretical modulation depth is 50%. The measured modulation depth is close to 15%, higher than the reported modulation depth of graphene modulator using CVD graphene at the near-infrared wavelength ([3], [4]). By further optimizing the structure design as well as fabrication process, we believe that our device can provide a practical platform for near-infrared tunable optical devices.

[1] Koppens, Frank HL, Darrick E. Chang, and F. Javier García de Abajo. "Graphene plasmonics: a platform for strong light–matter interactions." Nano letters 11.8 (2011): 3370-3377.
[2] Kim, Seyoon, et al. "Electronically tunable perfect absorption in graphene." Nano letters 18.2 (2018): 971-979.
[3] Emani, Naresh K., et al. "Electrical modulation of fano resonance in plasmonic nanostructures using graphene." Nano letters 14.1 (2013): 78-82.
[4] Thareja, Vrinda, et al. "Electrically tunable coherent optical absorption in graphene with ion gel." Nano letters 15.3 (2015): 1570-1576.

Two-dimensional transition metal dichalcogenides (2D TMDCs) are a promising materials platform to integrate with nanophotonic structures due to their strong optical response in the visible spectral range. In particular, their strong exciton resonances are stable even at room temperature and allow facile coupling with resonant semiconductor and plasmonic nanostructures. Furthermore, the optical properties of 2D TMDCs are very susceptible to external stimuli because they are atomically thin. Their exciton resonance location (and hence the optical permittivity) can be controlled by methods such as electrical gating, dielectric screening, and strain. This paves the way towards the realization of practical and tunable nanophotonic devices.

In this work, we demonstrate a novel approach to tune the coupling of excitons in 2D TMDCs with plasmonic nanostructures using strain. By straining the monolayer TMDC, we can detune the exciton resonance location from the localized surface plasmon resonance. This then allows us to actively control the coupling between the two resonances and the resulting scattering spectra of the nanostructure. We study the spectral evolution and mode splitting of the coupled plasmonic-TMDC structure. Since we can control the scattering amplitude and/or phase of this coupled system, they act as building blocks for tunable nanophotonic devices and metasurfaces.

Recent experimental studies demonstrated that chemical reactions can be accelerated by adding plasmonic metal nanoparticles to the chemical reactants and illuminate them at their plasmon resonance. It was claimed that the enhanced reaction rate occurs via the reduction in the activation energy driven by the plasmon-induced non-thermal ("hot") electrons [Brongersm et al, Nature Nanotech (2015)].

In this breaking news, we show that these claims are extremely unlikely to be correct, and that instead, the faster chemical reactions are likely the result of mere heating. To do that, we derive a self-consistent theory of the electron distribution in metal nanostructures under continuous wave illumination [Dubi & Sivan, Light: Science & Application (2019)]. We show that only about one billionth of the energy provided by the illumination goes to creating non-thermal ("hot") electrons, and the rest goes to heating. Quite different from previous theoretical studies, we took account of the heat transfer from the illuminated nanoparticle to the environment via phonon-phonon coupling and ensured energy conservation in the electron-phonon-environment system (rather than just in the electron sub-system). This approach not only allows us to distinguish between the generation of high energy non-thermal ("hot") electrons and the regular heating of the nanoparticles, but also enables the determination of electron and phonon temperatures in a unique and unambiguous way. The theory is then used to compute the rate and energy distribution of electrons that tunnel out of the metal and can participate in a chemical reaction or enable photodetection [Sivan et al, Faraday Discussion (2019)].

Further, we develop a simple model based on the Fermi golden rule and the Arrhenius Law, which shows that the enhanced chemical reactions observed experimentally are highly unlikely to result from the generation of non-thermal non-thermal ("hot") electrons in the metal; instead, it is more likely originate from a purely photo-thermal effect [Sivan et al, Science (2019), Sivan et al, arXiv (2019)]. Specifically, we focus on a few of the seminal papers on this field and identify experimental errors in the temperature measurements that led the authors of these papers to underestimate the photo-thermal effect. Then, we show that the alternative theory of illumination-induced heating can explain the experimental data to remarkable agreement, with minimal to no fit parameters. Comprehensive thermal calculations (whereby we sum properly the heat generated by all particles in the system) confirm the temperature extracted from the experimental data, thus, showing that any claim in these papers related with "hot" electron action is not supported by the data.

Finally, we show that for sufficiently high temperature and/or illumination intensity, it is necessary to account for the thermo-optical nonlinearity due to the temperature dependence of the optical and thermal properties of the system [Sivan et al, arXiv (2019)]. We discuss the dominant contributions to the nonlinearity and the sensitivity to the various parameters of the sample and illumination. Our results provide the first ever comprehensive theory of plasmon-assisted photocatalysis and should become the basis for analysis of future experiments; it also reveals various routes for optimization of the chemical reaction acceleration. Our theory is also instrumental in quantifying experiments aimed to enable efficient photodetection.

Reference:
Brongersma, M. L., Halas, N. J. & Nordlander, P. Nature Nanotech. 10, 25 (2015).
Dubi, Y. & Sivan, Y. Light: Science & Applications; accepted (2019).
Sivan, Y., Un, I. W. & Dubi, Y. Faraday Discuss. 214, 215 (2019).
Sivan, Y., Baraban, J., Un, I. W. & Dubi, Y. Science 364, eaaw9367 (2019).
Sivan, Y., Un, I. W. & Dubi, Y. arXiv:1902.03169 (2019).

We experimentally demonstrate the effect of the localized surface plasmon resonance (LSPR) of a single gold nanoparticle (AuNP) of 100nm in diameter on the mechanical resonance frequency of a free-standing silicon nitride membrane by means of optomechanical transduction. The key effect to explain the coupling in these systems is the extinction cross-section enhancement due to the excitation of the LSPR at selected wavelengths. In order to validate this coupling, we have developed an interferometric readout system with an integrated tunable laser source, which allows us to perform the first experimental demonstration of nanomechanical spectroscopy of deposited AuNPs onto the membrane, discerning in between single particles and small clusters by the frequency shift and polarization sensitivity.
The use of plasmonic structures as sensors is widely spread in recent scientific literature; thus, during the last years we have been witnessing a rising interest in the research on increasing the sensitivity of devices. The most widely used sensor type is based on the concept of surface plasmon resonance (SPR) which is especially sensitive to refractive index changes of the medium surrounding the metal structure, eventually detecting changes in the refractive index of 10-7. Despite the low-quality factor exhibited by the broad optical resonances, plasmomechanical systems represent an attractive alternative when compared with other optical cavities in literature because the SPR are easily excited by free-space light beams. This issue is of crucial importance because the free-space optomechanical coupling largely decays when the size of the mechanical system is below the wavelength[1,2], which, is needed to achieve high mechanical frequency regime.
We demonstrate the optomechanical coupling that emerges in the plasmonic cavity formed by a AuNP onto a free-standing SiN membrane by means of the excitation of the LSPR, which is related with an enhancement in the extinction cross section of the nanoparticle at a selected wavelength. The extinction is the combination of the absorption and the scattering of the particle; therefore, it becomes a hot-spot on the membrane consequently tuning the mechanical resonance frequency through thermomechanical effects. The LSPR can be tuned by changing the particle diameter or by clustering the particles, which allows the harnessing of the collective plasmonic modes. This cluster coupling usually vanishes when the separation between the particles exceeds the exponentially decreasing distance in which the evanescent field expands, which usually is in the order of few nanometers. However, we demonstrate the use of a membrane to efficiently extend the range of applicability of the coupling between nanoparticles to hundreds of nanometers[3].
We calculated the temperature profile by FEM simulations for different absorption powers from 10nW to 10uW. This temperature profile is translated into a frequency shift by means of the non-released stress generated in the structure due to the thermal expansion and the temperature-dependency of the material Young’s modulus. For integration times of about 3s, we have an Allan variance of 1.1x10-8, which means that the noise of our fundamental mechanical resonance at 8.6MHz is of 95mHz, which implies that the minimum detectable power is 72pW. On the other hand, the minimum power detectable by the state-of-the-art photodetectors is 2.2nW. Therefore, the proposed optomechanical device is two orders of magnitude more sensitive than a commercially available photodetector.
[1] Ramos, D., et al, “Silicon nanowires: where mechanics and optics meet at the nanoscale”, Sci. Rep., 3, 3445, (2013).
[2] Ramos, D., et al, “Optomechanics with Silicon Nanowires by Harnessing Confined Electromagnetic Modes”, Nano Lett., 12, 932-937 (2012).
[3] Ramos, D., et al, “Nanomechanical Plasmon Spectroscopy of Single Gold Nanoparticles”, Nano Lett., 18, 7165-7170 (2018).

Surface plasmon resonance (SPR) is a surface-sensitive analytical technique used in a variety of biological sensors.¹ The main problem with classical SPR technology is associated with the existence of a lower physical limit of detection (LOD) with respect to fluorescence. Various approaches have been proposed to overcome this limitation by developing different sensing concepts including phase-sensitive detection schemes, use of metallic nanostructures, line gratings and others. More recently, the addition of active functionalities to the SPR based devices have been proposed to enhance the intrinsic sensitivity of the SPR system. A magneto-optical (MO) SPR sensor, based on a magneto-plasmonic modulation technique produced in multilayers of noble and ferromagnetic metals has been lately proposed.²

The use of a trilayered thin film structure (e.g. Au/Ferro/Au) allows to provide a transverse magneto-optical Kerr effect(TMOKE) of the p-polarized light. This TMOKE signal exhibits an improved physical sensitivity over classic SPR measurements thanks to the non-reciprocal modification of the surface plasmon wave vector induced by the magnetization in the ferromagnetic layer which is controlled by the applied magnetic field.³ Beyond sensing techniques, this modulation of the surface plasmon wave vector in this kind of architecture allows for applications in active plasmonics such as optical switches.

Here, we propose a novel combination of metals based on exchange coupled ferrimagnetic multilayer thin films composed of TbCo₂ stacked with FeCo providing a well-defined in-plane and uniaxial magnetic anisotropy thus giving rise to an easy and a hard magnetization axis in the layer.⁴ This specific structure adds a new behavior to exploit the active control of surface plasmon resonance.
Simulationsof the Magneto-Optic SPR effect in our structures were performed using a finite element software. The dielectric tensor, including magneto-optical constants of the TbCo₂/FeCo multilayers was measured by ellipsometry, whereas standard parameters were used for the noble metal. Experimental studies were made by coupling our architecture with a modified-SPR commercial setup equipped with an external magnetic field and controlled by a Labview program. Measurements show a significant increase of sensitivity when used in a magneto-plasmonic configuration. Moreover, the measured plasmonic properties perfectly reflect the magnetization characteristics along the easy and hard axes allowing an indirect local measurements of the magnetic properties of the sample, and confirm the performed simulations. Methods, simulations and measurements will be shown and discussed at the conference. The interest of this novel sensing device for the detection of cardiac biomarkers such as troponin I⁵ will also be shown.


1. Homola, J. Surface Plasmon Resonance Sensors for Detection of Chemical and Biological Species. Chem. Rev. 108, 462–493 (2008).
2. Sepúlveda, B., Calle, A., Lechuga, L. M. &Armelles, G. Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor. Optics Letters 31, 1085 (2006).
3. Armelles, G., Cebollada, A., García-Martín, A. & González, M. U. Magnetoplasmonics: Combining Magnetic and Plasmonic Functionalities. Advanced Optical Materials 1, 10–35 (2013).
4. Tiercelin, et al. Low field anisotropic magnetostriction of single domain exchange-coupled (TbFe/Fe) multilayers. Journal of applied physics, 87, 2000, 5583.
5. Chekin, F. et al. Sensitive electrochemical detection of cardiac troponin I in serum and saliva by nitrogen-doped porous reduced graphene oxide electrode. Sensors and Actuators B: Chemical 262, 180–187 (2018).

Single layer graphene (SLG), a representative of 2D materials with outstanding electrical and mechanical properties, suffers from a low absolute optical absorption of ~1-2% depending on the substrate. [1] This significantly limits its efficiency as surface-incident photonic devices. Here we demonstrate a semimetal-dielectric-metal (SMDM) cavity structure that synergistically integrates near-field absorption enhancement with a Fabry-Perot cavity to address this challenge. The SMDM cavity structure consists of self-assembled, close-packed ultrahigh refractive index semimetal Sn nanodots (n=8~9 at λ=1600–5000 nm [1]) on a SLG/SiO₂/Al stack. The nanogaps (~10 nm) between Sn nanodots effectively funnel the incident light into the underlying SLG regions with >100x electromagnetic field enhancement, and scatter the incident light into oblique angles. SLG absolute absorption has already achieved 5-15% over the visible and near infrared light regimes without a SiO₂/Al stack. Reflection by the backside Al layer and coupling into the SLG (high index and a 2D Dirac semimetal at room temperature)/SiO₂/Al cavity further enhances the SLG absorption to up to 25% in a broad spectral regime of λ=500-2500 nm. The enhanced SLG absorption has also been confirmed by field-enhanced Raman scattering and significantly increased photo-conductivities. This is much advantageous over the conventional plasmonic approach which uses costly materials (typically Au or Ag) and has a narrow resonant peak, as well as the nanophotonic design which requires a complicated and precise lithography in nanoscale. [2, 3] Further tuning the thickness of SiO₂ dielectric optical medium and the size/gap of Sn nanodots, our design even shows an optimal SLG absorption of 40-45% at λ=1500-2500 nm. This work offers a new approach for high-efficiency, broad-band, nanoscale photon management in 2D photonics, which is also compatible with nano-fabrication foundries in terms of scaling up photonic device fabrication.

[1] S. Fu, H. Wang, X. Wang, Y. Song, J. Kong, J. Liu, ACS Photonics 2019, 6, 50.
[2] Y. Liu, R. Cheng, L. Liao, H. Zhou, J. Bai, G. Liu, L. Liu, Y. Huang, X. Duan, Nat. Commun. 2011, 2, 579.
[3] J. Liu, N. Liu, J. Li, X. J. Li, J. Huang, Appl. Phys. Lett. 2012, 101, 052104.

The advance on hybrid metamaterial design using bottom-up fabrication technique brings multiple advantages towards sensing and large-scale nanophotonic device integration. In our work, a novel two-phase plasmonic framework with Au nanoantenna arrays being embedded inside a titanium nitride (TiN) matrix was demonstrated, with easy access of controlling the packing density or aspect ratio. Advantages include sub-10 nm nanoantenna arrays, high crystalline quality, inch-scale fabrication as well as high durability. Such geometrical flexibility brings tunable resonance frequency and anisotropic dielectric dispersion at optical regime. We demonstrate effective molecular sensing affected by the surface-enhanced plasmonic substrate with built-in Au antenna array. Our functional hybrid thin film template, as a first step, can be applied to many three-dimensional metamaterial designs, we will show some of our latest research progresses on nanostructures and applications using such hybrid template.

The spectral behavior of localized surface plasmon resonances is often studied using ensemble spectroscopy techniques, which provide average values of resonance energies and broadened line-widths due to varying sizes and shapes of the nanostructures. Single particle measurements can offer a sharp and distinct spectral fingerprint of plasmonic nanostructures but they often probe only the scattering profile or a mixture of multiple light-matter processes. Here we introduce integrating sphere microscopy as a method to probe directly and simultaneously the scattering and absorption behavior of single plasmonic nanostructures. We use this technique to quantitatively characterize the energy, line-width and extinction cross-section of the localized surface plasmons of single nanostructures. Our measurements are compared with theory and used to trace hot-electron chemical reactions on the surface nanostructures. The realization of a direct measurement of the scattering and absorption of a single plasmonic entity allows to probe with superior resolution physical and chemical processes that are induced by localized surface plasmon resonances.

The ability to control the optical properties of photonic materials on demand is essential for the operation of a variety of devices ranging from active color filters and routers to switchable mirrors and sensors. Here we will present our latest work on actively controllable photonic devices using metal hydrides. We have developed an apparatus for in situ measurements of hydrogen content and optical and mechanical response. Using this apparatus, we will show the dynamically tunable optical properties of several metals and alloys over a broad spectral range (250 - 1690 nm) and discuss a number of devices including structures that have five orders of magnitude change in reflectivity, as well planar systems for physical encryption and counterfeit detection. Finally, we investigate the hydrogen-loading amounts for several metal alloys and find higher loading amounts when compared to previous measurements for alloys with low atomic percent Pd, resulting from film stress and microstructuring. These results have led to new insights in the dynamic behavior of metal-hydrides and will help in the design of next-generation hydrogen sensors and tunable photonic devices.

Over the past years, there has been rapidly growing interest in non-Hermitian photonic systems. In 2003, Bender et al. proved that a Hamiltonian is not necessarily Hermitian to have a real spectrum(1). For instance, a Hamiltonian with Parity-Time (PT) symmetry can show real spectrum below a certain threshold, known as the exceptional point, accompanied with novel phenomena above the EP. Thanks to the equivalence in mathematical forms between the Schrödinger equation in quantum mechanics and the paraxial wave equation in optics, we can conveniently explore unique phenomena and properties of PT systems in optics, which will accelerate the development of novel optical devices.
Here we propose a plasmonic meta-grating with PT symmetry to excite unidirectional surface plasmon polaritons (SPPs). The basic principle is based on the behavior of the PT system around the exceptional point, where only directional propagation mode of SPPs is allowed. We have designed realistic grating array consisting of passive gratings (i.e., without gain materials), which can significantly reduce the challenges in fabrication and optical experiments. In addition, instead of using ideal sinusoidal permittivity modulation in simulation in former paper(2), here we demonstrate in both simulation and experiment that using discrete grating array can also achieve excellent contrast between SPPs in opposite directions at the working wavelength around 1150nm.
In addition to the unidirectional excitation of SPPs, we have designed PT symmetric meta-gratings to realize other intriguing functionalities, such as a unidirectional plasmonic distributed Bragg reflector, that is, a plasmonic diode, that shows selective reflection/transmission for SPPs with high contrast. To achieve this function, we need to modify the 2^nd order Fourier coefficient for permittivity modulation, which is proportional to the reflectance of SPPs incident from the left and right.
In summary, we have demonstrated a sub-wavelength plasmonic grating structure with PT symmetry for unidirectional excitation and transport of SPPs. These results can be potentially employed as a new approach to designing transformative nanoscale optical devices, such as low-loss plasmonic routers and isolators for efficient optical computation, communication, and information processing.


1. C. M. Bender, D. C. Brody, H. F. Jones, Must a Hamiltonian be Hermitian? American Journal of Physics 71, 1095-1102 (2003).
2. W. Wang et al., Unidirectional Excitation of Radiative-Loss-Free Surface Plasmon Polaritons in PT-Symmetric Systems. Phys Rev Lett 119, 077401 (2017).

Near-zero refractive index systems, i.e., with vanishing permittivity and permeability values, have been shown to exhibit unique and extreme optical properties. Ultrathin epsilon-near-zero (ENZ) layer can support plasmon polariton mode with enhanced and highly-confined optical field [1]. Recent studies on ultrathin transparent conducting oxide (TCO) ENZ film have proposed various optical applications such as broadband perfect absorber [2], enhanced ENZ nonlinearity [3] and electrically tunable meta-devices [4, 5], etc. However, most of the recent studies on ENZ materials are limited in planar structures or metasurfaces with a relatively short length of light-matter interaction and operation, which restricts the excitation platform for developing novel optical devices with advanced functionalities.
In this work, we report the first experimental demonstration of ENZ mode excitation in D-shaped optical fiber nano-coated with aluminum-doped zinc oxide (AZO) layer via atomic layer deposition (ALD) [6]. The ultrathin AZO layer possesses ENZ property (real part of permittivity crosses zero at a wavelength of 1546 nm) in near-infrared wavelength regime. The evanescent optical field of the guided core mode of D-shaped fiber interacts with the ultrathin AZO ENZ layer on the side-polished portion to excite the ENZ mode. Highly polarization- and wavelength-dependent transmission with on and off ENZ resonance difference of ~20 dB at a wavelength of ~1600 nm in ENZ regime is observed in a 1.7 cm-long ENZ-optical fiber with a 30 nm-thick AZO layer. The measurements show a good agreement with full-wave numerical simulation and phase matching condition between fundamental core mode of D-shaped fiber and ENZ mode supported by ultrathin AZO layer. Compared to the excitation of ENZ mode on the planar substrates, the hybrid ENZ mode on the optical fiber exhibits much lower loss and relatively long light-matter interaction length of a few centimeters. The hybrid ENZ-optical fibers provide a novel platform for zero-index photonic applications, for instance, studying enhanced ENZ nonlinearity in fiber, quantum emission in ENZ media, and subwavelength mode enhanced in-fiber optical- and bio-sensing.
This work was supported in part by the Young Faculty Award Program from Defense Advanced Research Projects Agency (grant number N66001-17-1-4047), Robert A. Welch Foundation (award number: AA-1956-20180324), and the Vice Provost for Research at Baylor University.
1. Liberal, I.; Engheta, N. Nature Photonics 2017,11, (3), 149.
2. Anopchenko, A.; Tao, L.; Arndt, C.; Lee, H. W. H. ACS Photonics 2018,5, (7), 2631-2637.
3. Alam, M. Z.; Schulz, S. A.; Upham, J.; De Leon, I.; Boyd, R. W. Nature Photonics 2018,12, (2), 79.
4. Lee, H. W.; Papadakis, G.; Burgos, S. P.; Chander, K.; Kriesch, A.; Pala, R.; Peschel, U.; Atwater, H. A. Nano Letter 2014,14, (11), 6463-6468.
5. Huang, Y.-W.; Lee, H. W. H.; Sokhoyan, R.; Pala, R. A.; Thyagarajan, K.; Han, S.; Tsai, D. P.; Atwater, H. A. Nano Letter 2016,16, (9), 5319-5325.
6. Anopchenko, A.; Gurung, S.; Tao, L.; Arndt, C.; Lee, H. W. H. Material. Research Express 2018,5, (1), 014012.

CdO thin films have generated interest due to high electron mobilities with tunable carrier concentrations via substitutional doping with Y (Y:CdO). This combination of mobility and carrier concentrations have been used to support plasmonics in the mid-infrared spectrum spanning from 2.5 µm to 8 µm in wavelength. As the thickness of a film is reduced far below the skin depth of the material, a plasmonic mode known as an epsilon-near zero (ENZ) mode becomes active. This plasmonic mode is highly absorbing and highly confining of the incident electric field. One can create a Y:CdO/Au thin film junction, where the work function difference results in a small EMF. Illuminating this structure with IR light at ω_IR=ω_ENZ produces strong absorption and an increased T_E in Y:CdO. As electrons equilibrate with the lattice, the Y:CdO temperature increases thus changing the junction EMF. Y:CdO films were fabricated via HiPIMS sputtering, and junctions were created via photolithography techniques. The thermopile output voltage increases with increasing incident laser power, and is linear with the number of thermopile junctions. 30 µV were measured between 10 junctions with an incident laser power of 350 mW without optimization of the illumination area.

The field of plasmonics and metasurfaces has advanced by leaps and bounds in the last 10-15 years, but ohmic losses continue to slow technological advances and scientific understanding. There is a need for plasmonic metasurfaces with tunable conductivity, instead of relying on fixed conductivity of ohmic metals. Here, we explore a novel material – a crystalline PbS/InP heterojunction – that is voltage-tunable, exhibits single-electron tunneling (SET), and has possible applications to neuromorphic computing. A neuromorphic computing architecture would go beyond traditional von Neumann linear architecture to a bio-inspired (neural) network of non-linear devices and weighting elements responsible for data processing, allowing much faster computation [1,2]. We report on tip-based tunneling measurements and analysis of SET in self-formed oxide interfaces between the two materials, which exhibit single-electron Coulomb-blockade staircases along with memory and memory-fading behaviors. This gives rise to both short-term and long-term (seconds and hundreds of seconds, respectively) plasticities as well as a convenient non-linear response, making this structure attractive for neuromorphic computing applications. We predict typical behaviors relevant to the field, obtained by an extrapolation of experimental data in the SET framework. The estimated minimum energy required for a synaptic operation is in the order of 1 fJ, while the maximum frequency of operation can reach the MHz range. Comparatively, the human brain synapses consume more energy (~pJ) and operate at a slower rate (1 ms or 1 kHz) [3], which demonstrates the potential of the proposed structure for such applications.


[1] P. A. Merolla, J. V. Arthur, R. Alvarez-Icaza, A. S. Cassidy, J. Sawada, F. Akopyan, B. L. Jackson, N. Imam, C. Guo, Y. Nakamura, B. Brezzo, I. Vo, S. K. Esser, R. Appuswamy, B. Taba, A. Amir, M. D. Flickner, W. P. Risk, R. Manohar and D. S. Modha, "A million spiking-neuron integrated circuit with a scalable communication network and interface," Science, vol. 345, pp. 668-673, 2014.
[2] B. V. Benjamin, P. Gao, E. McQuinn, S. Choudhary, A. R. Chandrasekaran, J. Bussat, R. Alvarez-Icaza, J. V. Arthur, P. A. Merolla and K. Boahen, "Neurogrid: A Mixed-Analog-Digital Multichip System for Large-Scale Neural Simulations," Proceedings of the IEEE, vol. 102, pp. 699-716, 5 2014.
[3] J. Harris, R. Jolivet and D. Attwell, "Synaptic Energy Use and Supply," Neuron, vol. 75, pp. 762-777, 2012.

In this work, we have designed and fabricated an array of plasmonic nano-ellipse that interacts with different types of quantum emitters (QEs) in the visible range of wavelength. The proper geometry of our design provides such absorption-reflection properties which spectrally overlap with emission spectrum of the QE. Alongside such spectral overlap, a thin layer of the dielectric layer between the plasmonic structures and a gain medium provides the possibility of spatial overlap. The interaction between the strong subwavelength localized field at the edges of the gold nano-ellipses and QEs, enhances Purcell factor towards the modification of the fluorescence and decay time of QEs. This approach allows enhanced emission from different emitters embedded in hybrid quantum systems. In this work, we study the energy transfer between the fluorescent dye molecules and CdSe/ZnS hydrophobic QDs with the array of plasmonic nano-ellipses.

Many biological processes are associated with conformational changes of macromolecules such as enzymes, proteins, and DNAs. Currently, these dynamic processes can be observed indirectly through techniques such as optical tweezers, förster resonance energy transfer (FRET), and optical beacon or directly with ultra-high frequency atomic force microscopy (AFM). While the last example points to the promises using AFM to directly visualize the molecules of interest, it only limits to certain processes where the speed is slower or comparable with the mechanical scanning of AFM. In this talk, I will discuss a new approach to visualize the conformational changes of biomolecules in situ. We constructed the technique based on the force exerted by polarized light on biomolecules. To enhance the light-molecular interactions, we created a plasmonic cavity formed between the plasmonic AFM tip and a plasmonic nano-aperture. We have shown that the plasmonic nano-aperture can exert differential optical forces down to piconewton ranges on chiral nano-structures when illuminated with circularly polarized light of opposite handedness. Here, instead of a nano-structured AFM tip, we functionalize DNA molecules on the plasmonic AFM tip with a controlled density. We investigate both single strand and double strand DNAs at 20-base pair length with various densities. We are able to measure the difference in optical forces with a sensitivity of ~2.1pN/100uW/um². This sensitivity is associated with the optical forces exerted on single strand and double strand DNA molecules embedded between the plasmonic cavity. In addition, by introducing denaturing agent to the solution, we observed in-situ dynamic conformational changes of the double-strand DNA molecules, directly measured with the optical force microscopy technique.

The decay of surface plasmons generates hot carriers that can be injected into semiconductors or molecular systems, opening a new pathway to drive photo-induced chemical reactions. Recent studies demonstrate that by tuning plasmon resonance of metal nanoparticles hot carriers may be injected into specific anti-bonding orbitals of an adsorbed molecule and therefore the product selectivity can be achieved. In this talk, we will demonstrate surface plasmon polariton (SPP)-induced hot carrier generation that has a potential for energy-tunable photocatalysis. Unlike metal nanoparticles that suffer from resonance inhomogeneity and limited spectral tunability, metal films support SPPs that are homogeneous in-plane and accessible by simply tuning an illumination angle in a broad spectral range. We measured electrochemical responses of a metal/semiconductor heterofilm (Ag/TiO₂) and a bare metal film (Au) that were contacted with a mixed solution of sodium hydroxide and methanol. A strong photon energy dependence in the photon-to-carrier conversion efficiency was obtained from the heterofilm and this was further confirmed by the hole injection efficiency calculated using a Schottky transport model. Hot carriers generated in the bare metal film were energetically positioned near the metal's Fermi level, and therefore, chemical reactions were controlled by tuning electrode potential or solution pH. Both of these results have important implications for plasmon-induced photocatalysis, especially for energy-tunable chemical reactions.
[1] Ahn, W., Vurgaftman, I., Pietron, J. J., Pehrsson, P. E., and Simpkins, B. S. “Energy-Tunable Photocatalysis by Hot Carriers Generated by Surface Plasmon Polaritons” J. Mater. Chem. A., 2019, 7, 7015 - 7024.

Besides its fundamental importance, manipulation of light at the nanoscale is of great interest for the prospect of real-life applications, such as energy harvesting and photovoltaics, wave-guiding and lasing, optoelectronics, biochemistry and medicine. Novel optical designs and architectures that modify the optical power flow through plasmonic nanostructures represent another crucial step towards a nanoscale manipulation of light-matter interactions. In this framework hyperbolic metamaterials (HMMs) have received great attention due to their unusual properties at optical frequencies that are rarely or never observed in nature [1-2].
Here, we report about unconventional optical properties of metal-dielectric meta-antennas supporting type II hyperbolic dispersion, which enable almost pure and spectrally separated absorption and scattering channels in the visible/near-infrared spectral range [3]. We demonstrate that the physical mechanism responsible for the control of scattering and absorption lies in the different nature of the plasmonic modes excited within the hyperbolic meta-antennas. We also show that scattering is the dominating electromagnetic decay channel, when an electric super-radiant dipolar mode is induced in the system, whereas strong light absorption occurs when a magnetic sub-radiant dipole is excited. Importantly, both modes can be excited directly by coupling with far-field radiation, thereby making the proposed architecture suitable for practical applications. In this framework, we demonstrate also that HMM meta-antennas could find promising applications in photo-thermal therapy. Our findings open the pathway towards novel routes for exploiting light to energy conversion channels beyond what is offered by current plasmon-based architectures, possibly enabling applications including thermal emission manipulation, thermoplasmonics-based theragnostic nano-devices, novel nano-antenna designs and plasmon-enabled enhanced molecular spectroscopy.

[1] Poddubny, A.; Iorsh, I.; Belov, P.; Kivshar, Y. Nat. Photon. 7, 958-967 (2013)
[2] Narimanov, E. E.; Kildishev, A. V.; Nat. Photon. 9, 214-216 (2015)
[3] Maccaferri, N.; Zhao, Y.; Isoniemi, T.; Iarossi, M.; Parracino, A.; Strangi, G.; De Angelis, F. Nano Lett. 19(3), 1851–1859 (2019)

Bimetallic nanoparticles have emerged as a promising class of photocatalysts for chemical processing, energy harvesting, and environmental remediation. By alloying multiple metals together within a nanoparticle, significant improvements in reactivity as well as product selectivity have been realized. However, to date, most measurements are conducted on particle ensembles, where limited spatial resolution and averaging effects conceal important details about the photocatalytic mechanisms and optical properties of individual nanoparticles. To improve the efficiency of bimetallic photocatalysts, it is crucial to understand how their nanoscale properties such as shape, size, crystallinity and composition, affect performance.
Here, we investigate the photocatalytic efficiency of individual bimetallic nanoparticles, focusing on the light-driven dehydrogenation of AgPd single crystalline prisms. The particles are synthesized through the reduction of palladium and silver salts by formaldehyde, resulting in monodisperse prisms with edge lengths of approximately 35nm. We systematically vary the concentration of Ag in the nanoparticle from 0% to 2%, 5%, 10%, 20%, 30%, and finally 40%. The particles are dispersed onto a SiO2 grid, and a combination of electron imaging, diffraction, and electron energy loss spectroscopy (EELS) are used to follow the photocatalytic transformation. First, using monochromated EELS, we determine how the Ag concentration affects the plasmonic modes in individual AgPd triangular prisms. For the corner, edge, and bulk plasmon modes, we see dramatic shifts in the plasmon resonance energies with increasing Ag composition: 7.5eV to 6eV for the bulk, 5.8eV to 4.1eV for the edge, and finally 3eV to 1.9eV for the corner mode, for a composition change of 0% to 30% Ag. Although resonance shifts are observed, we see minimal change in comparative extinction cross-sections. Next, we illuminate the particles on their plasmon resonances and follow the dehydrogenation of theses individual nanoparticles in an environmental TEM. Our TEM setup consists of an optical fiber coupled to a parabolic mirror for in-situ sample illumination, while concurrently allowing gas flow to the sample. We introduce H2 gas at pressures of ~20Pa - 40Pa and observe the light-driven transformation of individual AgPd nanoparticles from the H-rich �� phase to H-poor �� phase. Using both direct imaging of the propagating grain boundary defect, as well as the change in bulk plasmon resonance via low-loss EELS, we show that the phase transformation progresses to minimize the planar defect area at the phase boundary. While the dark-state kinetics decrease with increasing Ag concentration, illumination on resonance can lead to an order-of-magnitude increase in reaction kinetics. Importantly, the transition speed is linear in time and is therefore not limited by hydrogen diffusion but rather by surface-reaction-rates. Furthermore, the speed is correlated with excitation of the edge plasmon mode. We discuss how the transformation kinetics and mechanism vary with illumination wavelength and power, and we can correlate these changes with an individual nanoparticle’s specific plasmonic modes and chemical composition. Our results help elucidate the combined roles of electronic and chemical contributions to bimetallic catalysis, informing optimized alloy concentration and nanostructure for record efficiency.

Augmented Reality (AR) is a technology which superimposes information or optical images onto the real-world environment of an observer. It is challenging to create optical elements that can seamlessly overlay images on top of a real scene and offering high transparency across the visible spectrum. Typically, bulky optical systems are required, but recent advances in holographic diffractive optical elements (DOE) have precipitated new approaches to realize high-performance AR displays. Unfortunately, high-diffraction-efficiency DOEs tend to produce undesired rainbows and image distortions. Many of the same opportunities and challenges hold true for eye-tracking systems.
Here, we demonstrate an ultracompact, near-unity transmission, and rainbow-free optical eye tracking DOE based on a guided-mode resonance structure. To realize this optical element, we first prepare an anti-reflection coating on a quartz substrate by depositing a 200-nm-thick Si₃N₄ slab capped with a 100-nm-thick SiO₂ layer. This dielectric stack affords high transmission (~90%) over the entire visible spectrum. The nitride layer also serves as a waveguide that support a single transverse electric and transverse magnetic mode in the frequency range of interest. For this reason, this system can be turned into a guided-mode resonator by placing 3-nm-thick Si gratings between the Si₃N₄ and SiO₂ layers with 1-μm-long periodicity. Upon the planewave incidence, the scattered light from the Si grating elements can couple light into quasi-guided modes supported by the nitride layer. This guided light is primarily absorbed or diffracted, depending on the frequency of the incident light. We observe a high quality (Q~1000) guided-mode resonance at 870 nm wavelength and rigorously characterize the dispersion properties with a home-built angle-resolved confocal spectroscopy microscope. The spectrally sharp (<1 nm) resonant diffraction with a high peak diffraction efficiency (=13%) to first-order diffracted beams is also directly measured under the normal incidence illumination. In contrast, across the visible spectrum the guided-mode resonance is dampened severely due to light absorption by the Si grating. This achieves the successful suppression of visible rainbows and diffraction efficiencies into the diffracted-orders are below 0.1% across the visible spectrum.
As a prototype eye tracking device, we fabricate a 2-cm-wide guided-mode resonance glass and attach it to a regular eyeglass frame. A small 870 nm light-emitting diode (LED) suspended on the eyeglass frame illuminates an artificial eye in front of the glasses. The scattered light from the eye is redirected by the optical element into a 1.6-mm-wide endoscopic camera located at the temple arm of the frame. The images formed by the diffracted rays allow the endoscopic camera to capture the full front-view of the eye even viewing at an oblique angle (60°). We also mathematically formulate all the relevant physics of the image formation with this element. We anticipate that our guided-mode resonance platform opens a promising route to realize new types of AR optical devices and elements for three-dimensional holography.

The field of optical meta-surfaces is rapidly growing due to its great potential to enable thin optics implementation with relatively complex and flexible functions. In particular, high power laser systems could benefit from meta-optics that could implement beam shaping, e.g., for wave-front aberration correction, but with the advantages of smaller accumulation of nonlinearity and lighter weight. Additionally, meta-surface technology could enhance laser optics with improved anti-reflective layer designs.
Current meta-surface technology is limited with respect to high power laser optics, which requires both scalability and laser intensity durability. The principal challenge arises from the necessity of patterning sub-wavelength features (to control the local optical properties by modifying geometrical properties) while being able to modify the structural parameters on the large optics scale used in high power laser systems (e.g., National Ignition Facility, Laser MegaJoule).The current patterning methods are either limited in scalability (e.g., FIB, e-beam lithography) or limited in robustness due to the usage of soft-materials (e.g., nanoimprint).
We are developing novel technology capable of generating robust and scalable all-dielectric based meta-surfaces. In this talk we will describe the method, show results of fabricated meta-surfaces, and discuss the various levels of control that we have with this process. This method extends the current application field of interest for meta-surfaces to high power lasers, with the potential to stimulate meta-surface utilization in additional fields requiring large and robust optics.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-CONF-777421.

The extraction of hot carriers at a Schottky barrier interface has attracted enormous attention for example due to the possibility to create a photovoltage in a semiconductor/metal junction without being limited by the semiconductor bandgap energy. In particular the use of hot holes for these types of devices has not yet been studied extensively while recent studies predict an increase in extraction efficiency compared to their hot electron counterparts¹.
Here², we perform hot hole photodetection measurements on a simple p-Si/metal thin film junction using various metals such as Ti, Au and Titanium nitride (TiN). TiN is of particular interest as it constitutes a refractory alternative to other materials used for plasmonic applications where high field intensities exceeding the melting point of commonly used metals can occur. Resonantly excited plasmonic structures lead to strong fields in the vicinity to the metal surface which has been demonstrated to give rise to energetic and large hot carrier distributions.
We show that a backside illuminated p-Si/TiN system can be used for efficient hot hole extraction allowing for a responsivity of 1 mA/W at an excitation wavelength of 1250 nm and at zero bias. Our results unambiguously demonstrate that a few nanometer thin TiO_2-x interfacial layer forming during growth at the Si/TiN interface allows for an increase in photoresponsivity of about one order of magnitude compared to a clean p-Si/metal interface. The direct comparison between a p-Si/Au and p-Si/TiO_2-x/Au further underpins this observation and additionally shows that the photoresponse dispersion relation of a TiO_2-x thin film containing system deviates clearly from the response of a p-Si/metal system and thus from the commonly used Fowler description. Despite the fact that based on a bandgap energy of about 3.2 eV, TiO_2-x is expected to act as a hole blocking layer, our results show a strong photoresponse. Thus, we propose a trap state mediated carrier transfer mechanism which is further supported by results obtained via electrical transport measurements.

References:
(1) Sun, Q.; Zhang, C.; Shao, W.; Li, X. Photodetection by Hot Electrons or Hot Holes: A Comparable Study on Physics and Performances. ACS Omega 2019, 4 (3), 6020–6027. https://doi.org/10.1021/acsomega.9b00267.
(2) Güsken, N. A.; Lauri, A.; Li, Y.; Matsui, T.; Doiron, B.; Bower, R.; Regoutz, A.; Mihai, A.; Petrov, P. K.; Oulton, R. F.; et al. TiO 2– x -Enhanced IR Hot Carrier Based Photodetection in Metal Thin Film–Si Junctions. ACS Photonics 2019, acsphotonics.8b01639. https://doi.org/10.1021/acsphotonics.8b01639.

Plasmonic nanolasers are important for fundamental studies of light-matter interactions and applications in on-chip photonic integration. Metal nanoparticle arrays supporting surface lattice resonances (SLRs) can provide optical feedback for directional lasing emission at room temperature. However, current SLR lasers rely on organic dye molecules as the gain media and show limited photostability. Colloidal quantum dots are promising gain materials because of their long-term stability, bright photoluminescence, convenient solution processing, and size-controlled spectral tunability. Here we show quantum dot lasing from SLR-based nanocavities. Our quantum dot-plasmon lasers exhibit low thresholds and engineered far-field properties. In addition, we demonstrated the energy transfer between excitons and plasmons by investigating the lasing dynamics.

The possibility to create and manipulate nanostructured materials encouraged the exploration of new strategies to control the electromagnetic properties without the need to modify its physical structure, i.e. by means of an external agent. An approach is the combination of magneto-optically active and resonant materials (e.g. plasmonic modes), where it is feasible to control the optical properties with magnetic fields in connection to the excitation of resonances [1] (magnetoplasmonics). It has been shown that these nanostructures can be employed to modulate the propagation wavevector of SPPs [2], which allows the development of label free sensors with enhanced capabilities [3-5] or to enhance the magneto-optical response in isolated entities as well as films, in connection with a strong localization of the electromagnetic field [6-8].
Here we will show that they also play a crucial role in the active control of thermal emission and the radiative heat transfer (RHT) [9-11]. In particular Near Field RHT between two MO particles can be efficiently controlled by changing the direction of the magnetic field, in the spirit of the Anisotropic Magneto Resistance in spintronics [11]. This phenomenon, which we term anisotropic thermal magnetoresistance (ATMR), stems from the anisotropy of the photon tunneling induced by the magnetic field. We discuss this effect through the analysis of the radiative heat exchange between two InSb particles, and show that the ATMR can reach amplitudes of 100% for fields on the order of 1 T and up to 1000% for a magnetic field of 6 T. These values are several orders of magnitude larger than in standard spintronic devices. More importantly, this thermomagnetic effect paves the way for exploring heat transfer physics at pico- and even subpicosecond time scales, which are even shorter than the relaxation time of heat carriers. Moreover, we show that the heat flux is very sensitive to the magnetic field direction, which makes this effect very promising for the development of a new generation of thermal and magnetic sensors.

References:
[1] G. Armelles, et al., Adv. Opt. Mat. 1, 10 (2013)
[2] V.V. Temnov et al., Nat. Photon. 4, 107 (2010)
[3] B. Sepulveda, et al., Opt. Lett. 31, 1085 (2006)
[4] M.G. Manera, et al., Biosens. Bioelectron. 58, 114 (2014)
[5] B. Caballero, et al., ACS Photonics 3, 203 (2016),
[6] N. de Sousa et al., Phys. Rev. B 89, 205419 (2014)
[7] N. de Sousa et al., Sci. Rep. 6, 30803 (2016)
[8] M. Rollinger et al., Nano Lett. 16, 2432-2438 (2016)
[9] E. Moncada-Villa, et al., Phys. Rev. B 92, 125418 (2015).
[10] R. M. Abraham Ekeroth, et al., Phys. Rev. B 95, 235428 (2017)
[11] R. M. Abraham Ekeroth, et al., ACS Photonics 5, 705 (2018).

Organo-lead halide perovskite materials have recently received considerable attention for achieving an economic and tunable diode laser, owing to the use of solution-processable materials and the exceptional optical attributes of long carrier lifetimes and diffusion lengths, high fluorescence quantum yields, wavelength tenability, high optical gain coefficients<span style="font-size:10.8333px"> </span>and promising two-photon absorption properties. However, reducing the volume of such lasers to the nanoscale is the challenge nanophotonics, with potential applications in arrays of ultra-compact lasers on a chip. Nanoplasmonic devices facilitate strong light-matter interactions at nanoscales, thus providing unique opportunities to control and manipulate radiative emission.
Here, we report a novel plasmonic enhanced upconversion nanolaser consisting of a subwavelength organo-lead-halide perovskite nanocrystal on a 5-nm Al₂O₃ film on top of a plasmonic TiN film. A pump-probe transient absorption spectroscopy was used to study the photon-recycling-effect of perovskite nanocrystals as excellent optical gain media. Hence, we experimentally and theoretically demonstrated the localized strong optical filed enhanced single-mode lasing emission through two-photon absorption process in single perovskite nanocrystal coupled with plasmonic cavity. With the plasmonic cavity, the measured upconversion lasing threshold (200 nJ/cm²) was reduced by more than two orders of magnitude. Finally, we will discuss the outlook for upconversion plasmonic nanolasers in applications including, on-chip coherent light sources for of bio-imaging, optical communication applications.

Noble metal nanoparticles are one of the most interesting and well-studied nanomaterials because of their versatile preparation and their attractive electrical and optical properties. Alloyed noble metal nanoparticles have opened up new possibilities in applications such as in heterogeneous catalysis, electro-catalysis, imaging and surface enhancement Raman spectroscopy ^[1] due to their tunable plasmonic properties. This tunability can come from controlling both the composition and the size in the synthesis of nanoalloys. Herein, we introduce a seeded growth synthesis of gold-silver alloy nanoparticles by chemical co-reduction in aqueous solution. Using gold acetate or gold chloroauric acid and silver nitrate as precursors and sodium citrate as the main reductant and stabilizer, we successfully synthesize stable gold-silver nanoalloys with diameters from 15 nm to 80 nm. The size of alloy nanoparticles can be controlled by the number of growth steps and the amount of precursor ions, which also determines the elemental composition of alloy nanoparticle. In addition, bis(p-sulfonatophenyl) phenylphosphane (BSPP) is utilized to etch the nanoalloys to obtain novel nanostructures, where smaller nanoparticles surround a larger "mother" nanoparticles of different composition. These novel nanostructures might be of significance in SERS since the periphery mainly contain silver on EDX-mapping.
Reference:
[1] Ristig S, Prymak O, Loza K, et al. Nanostructure of wet-chemically prepared, polymer-stabilized silver–gold nanoalloys (6 nm) over the entire composition range[J]. Journal of Materials Chemistry B, 2015, 3(23): 4654-4662.

Recent experimental studies demonstrated that chemical reactions can be accelerated by adding plasmonic metal nanoparticles to the chemical reactants and illuminate them at their plasmon resonance. It was claimed that the enhanced reaction rate occurs via the reduction in the activation energy driven by the plasmon-induced non-thermal ("hot") electrons.

In this contribution, we show that these claims are extremely unlikely to be correct, and that instead, the faster chemical reactions are likely the result of mere heating. To do that, we derive a self-consistent theory of the electron distribution in metal nanostructures under continuous wave illumination. We show that only about one billionth of the energy provided by the illumination goes to creating non-thermal ("hot") electrons, and the rest goes to heating. Quite different from previous theoretical studies, we took account of the heat transfer from the illuminated nanoparticle to the environment via phonon-phonon coupling and ensured energy conservation in the electron-phonon-environment system (rather than just in the electron sub-system). This approach not only allow us to distinguish between the generation of high energy non-thermal ("hot") electrons and the regular heating of the nanoparticles, but also enables the determination of electron and phonon temperatures in a unique and unambiguous way. The theory is then used to compute the rate and energy distribution of electrons that tunnel out of the metal and can participate in a chemical reaction or enable photodetection.

Further, we develop a simple model based on the Fermi golden rule and the Arrhenius Law, which show that the enhanced chemical reactions observed experimentally are highly unlikely to result from the generation of non-thermal non-thermal ("hot") electrons in the metal; instead, it is more likely originate from a purely photo-thermal effect. Specifically, we focus on a few of the seminal papers on this field and identify experimental errors in the temperature measurements that led the authors of these papers to underestimate the photo-thermal effect. Then, we show that the alternative theory of illumination-induced heating can explain the experimental data to remarkable agreement, with minimal to no fit parameters. Comprehensive thermal calculations (whereby we sum properly the heat generated by all particles in the system) confirm the temperature extracted from the experimental data, thus, showing that any claim in these papers related with ``hot'' electron action is not supported by the data.

Finally, we show that for sufficiently high temperature and/or illumination intensity, it is necessary to account for the thermo-optical nonlinearity due to the temperature dependence of the optical and thermal properties of the system. We discuss the dominant contributions to the nonlinearity and the sensitivity to the various parameters of the sample and illumination. Our results provide the first ever comprehensive theory of plasmon-assisted photocatalysis and should become the basis for analysis of future experiments; it also reveals various routes for optimization of the chemical reaction acceleration. Our theory is also instrumental in quantifying experiments aimed to enable efficient photodetection.

Metal-containing nano-composites have numerous advanced optical, mechanical, electrical and photovoltaic properties. Thus, recent advancement in the diverse fields of plasmonics, metamaterials, flexible electronics, biosensors, artificial implants and solar cells have generated a significant demand for the fabrication of novel metallic nanoparticle containing nano-composites. However, the traditional techniques for the production of such nano-composites are either inherently limited to two-dimensional (2D) processing and/or involve multiple, time- and cost-intensive synthetic steps. Two-photon lithography based 3D printing technology has been shown to overcome these limitations and can fabricate arbitrary 3D micro/nano structures with resolution in the region of 100 nm. In this work, we report our progress in fabricating complex 3D gold-/silver-/copper-containing nano-composite structures by simultaneous two-photon polymerisation and photoreduction. The success of the two-photon induced (a) polymerisation is verified by Scanning Electron Microscopy (SEM) and Raman spectroscopy, and (b) metal salt reduction is verified using Transmission Electron Microscopy (TEM). This confirmed the presence of small metallic nanoparticles (diameter of gold nanoparticles: 4.3 ± 2.8 nm; diameter of silver nanoparticles: 5.8 ± 1.5 nm) embedded within the polymeric matrix. UV-vis spectroscopy defined that they exhibit the property of localised surface plasmon resonance (LSPR), whilst X-ray Photoelectron Spectroscopy (XPS) confirmed that they exist in the zero valent oxidation state. The capability demonstrated in this study opens up new avenues for a range of applications, including plasmonics, metamaterials, flexible electronics and biosensors.

In quantum optics, different types of light-matter interactions are studied through the coupling of optical emitters to microcavities with the aim of developing high performance optoelectronic devices. In the weak coupling regime, when the coupling strength is less than the damping rate of emitter or cavity, the spontaneous emission rate of the emitter is enhanced, known as the Purcell effect. With increasing coupling strength, the scattered light of the systems starts exhibiting a Fano-like, coupling-induced transparency-dip in scattering, which grows into a vacuum Rabi splitting in the strong coupling regime, when energy transfer between the cavity and the emitter is reversible.

While plasmonic particles coupled to single emitters like dyes or quantum wells represent auspicious candidates for single photonic sources, systems with multiple emitters like quantum wells promise high coupling strengths, as the coupling strength scales with the number of emitters. Two-dimensional organic-inorganic perovskites, which grow naturally in multi-quantum well structures, combine large oscillator strengths with large exciton binding energies (> 250 meV), distinguishing them as promising emitters for use in cavity coupled systems. Two-dimensional perovskite have been successfully integrated into dielectric Bragg reflector cavities, but have not yet been integrated with plasmonic particles.

Here we study the coupling between solution processed, two-dimensional butylammonium lead iodide perovskite (BAPI) and individual silver nanoprisms. Consisting of defined layers of PbI₄ separated by organic spacers, the thin layer n=1 BAPI multi-quantum wells feature narrow excitonic absorption and emission linewidths (ca. 100 meV). Through sequential spin coating of nanoprisms and two-dimensional perovskite emitter, we are able to combine individual silver nanoprisms with BAPI multi-quantum wells. The area around individual nanoprisms exhibit an enhancement of the emission intensity in photoluminescence. Using single particle dark field scattering spectroscopy, we observed a suppressed scattering in the single nanoprism spectra corresponding to the excitonic feature of the BAPI, indicating a coupling between the cavity and emitter. The observed transparency dip induced a peak splitting of approximately 300 meV, approaching the strong coupling limit.
Additionally, by placing the BAPI layer between a gold mirror and the silver nanoprisms, we explored the effect of different geometries on the plasmon-perovskite coupling, further modifying the coupling interaction between the plasmonic cavity and the emitters.

We realized a high-density array of “accordion-like” plasmonic silver nanorods over a large area (~cm²) exhibiting multiple electromagnetic responses in visible and near-infrared (NIR) wavelengths. This array of “accordion-like” silver nanorods was fabricated by confining lamellar-forming polystyrene-block-poly (methyl methacrylate) copolymer (PS-b-PMMA) inside cylindrical pores of aluminum oxide (AAO) template grafted by thin neutral brush layers. PS and PMMA lamellar nanodomains with the sizes of ~15 nm were alternatively stacked along the nanorod direction. After the AAO template was removed, a 5 nm thick layer of silver was thermally deposited on only PS nanodomains. Due to the multiple resonances exhibited in the visible and NIR regimes, the array could be used for multi-analyte detection. Furthermore, this concept of fabricating sophisticated nanoscale architectures by utilizing block copolymer self-assembly and incorporating plasmonic metals into one nanodomains could be applied to realize large-scale metamaterials working at visible and NIR wavelengths.

Thermal engineering in the pursuit of attaining radiative coolers, thermophotovoltaics, optical clocks as well as classical incandescent lamps is currently being studied intensively. An appropriate radiation spectrum must be engineered fitted for specific applications. In this study, we established a passive cooling strategy which tunes a spectral irradiance of objects in mid-infrared wavelengths (e.g., λ = 2.5 – 30 μm). For solar photovoltaics, the device temperature is much higher than an atmospheres temperature under direct sun light in daytime, which deteriorates the power conversion efficiency (e.g., 0.042%/K [1]). Therefore, a passive radiative cooling strategy without consuming external energy is considered a highly promising approach as a new environmentally friendly technology.
For thermal radiation cooling, dielectric materials (e.g., SiO₂, HfO₂, Si₃N₄, Al₂O₃) are widely used due to the existence of phonon-polariton resonance at specific mid-infrared wavelengths. According to Kirchhoff’s radiation law, one needs to design a structure having large absorptivity over broadband mid-infrared wavelengths. Thus, we tailored a dielectric structure which is absorptive in considered mid-infrared wavelengths and also visibly transparent (e.g., λ = 0.4 – 0.8 μm).
We fabricated a two-dimensional array of micron optical cavities covered with dielectric shells on Si substrates using standard photolithography, in which hollow voids were conformally covered with thin dielectric layers. The dielectric shells were composed of 300-nm-thick Al₂O₃ and 500-nm-thick SiO₂ layers. Fourier-transform infrared spectroscopy equipped with an integrating sphere showed greatly enhanced, broadband absorptivity, although the total thickness of the dielectric shells is far below than the center wavelength of radiation (e.g., λ ~ 10 μm). We evaluated the thermal capability of developed radiative coolers through outdoor experiment in daytime and nighttime. Then, we observed a temperature drop of 8 K (daytime) and 5 K (nighttime) on a Si substrate.

[1] S. Bensalem et al., “Solar cells electrical behavior under thermal gradient,” Energy Procedia (2013).

In transparent or translucent films, similar transmissions in wavelengths are generally observed, however, plasmonic scattering of metal nanoparticles is known to alter the transmitted light. Likewise, nanospherical particles have been demonstrated to exhibit the same properties without the inclusion of metallised particles. Here we propose a design methodology for films which can exhibit asymmetric scattering behaviour via directed self-assembly. The results of two polymorphic approaches for nanospheres are reported, molecular-induced asymmetry and topological asymmetry.

In both cases we embedded the nanospheres within an ultra-thin a-Si:H or a-SiC:H layer, coated with TiO₂ thin film to match polarisation between samples. On one surface either a molecular-altered nanosphere or topological nanosphere was selected for front incidence, with either selected for rear incidence. To date we have experimentally prepared a series of different samples that scatter light of different wavelengths and to different degrees from the front and rear surfaces.

The detection and measurement of THz radiation presents an ongoing challenge in many potential applications mainly due to the difficulty in building devices suited to detection within the THz region. In many devices some level of cryogenic cooling is required in order to obtain sufficient signal/noise ratio, or alternatively resolution is sacrificed by the use of a GaAs antenna. Given that most materials have properties in the THz range different to their properties in the visible range, THz-detection and imaging potentially opens up a vast range of applications.
Therefore, development of a new micromechanical devices suitable for detection of terahertz radiation at or close to room temperature while offering significantly faster measurements than conventional thermal sensors is worthwhile.
In this study, a unique bridge design, adapted from our MEMS e-sensors, detects and measures the incident THz radiation by analysing the fluctuations in the mechanical resonance frequency of the bridge which result from thermal expansion under incident THz radiation. Three thin-layer coatings were tested on the bridges, Nickel-Chromium (NiCr), Silicon-Gold (Si-Au), and Silicon-Titanium (SiTi), for their distinct sensitivity to THz radiation and ease of fabrication. Standard semiconductor fabrication process are used to produce our MEMS e-sensors, as well as form the thin layer coating on the bridges. Our devices demonstrated high THz–induced thermal sensitivity, with electrical noise approximately 110pW/√Hz.
A hierarchical neural network was utilised to fingerprint spectra based on the variations in resonance for single bridge configurations and between parallel bridge configurations. Although at an early stage, this new approach has advantages over current alternatives for THz radiation detection and measurement. The thermomechanical sensor in this work operates uncooled while remaining sensitive to THz, ideally suited for real-world applications, for example real-time, non-destructive integrity testing of electrical wiring and electronic trace-lines.

Plasmonic nanostructures provide enhanced light-matter interaction due to the excitation of collective free-carrier oscillations, so-called localized surface plasmons (LSPs). LSPs cause enormous electric-field enhancements in the near-field. They also may decay into energy-rich electron-hole pairs due to Landau-damping. These ‘hot-carriers’ are exploited in various applications including photocatalysis, photovoltaics, and optoelectronics in general. Consequently, combining such plasmonic nanostructures with two-dimensional layered materials that are discussed for next generation (opto)-electronic devices should drastically enhance their capabilities: such hybrid-materials could offer the unique and extraordinary properties known from 2D materials like graphene and benefit from the enhanced light focusing, large absorption cross sections, and hot-carrier generation provided by the plasmonic nanostructure. For instance, ‘hot carriers’ in graphene-based semiconductor-devices reportedly generate photocurrents or locally increase the electron density leading, for example, to the formation of pn-junctions. However, those plasmonic effects strongly depend on the particle’s dielectric environment and geometry mandating the investigation of individual structures. Here, we study the injection of hot-electrons that arise from non-radiative localized surface plasmon decay in a model system, i.e., a single gold nanoparticle, on a monolayer graphene substrate. We reveal the intricate interplay between hot-electrons injected from plasmonic nanoparticles into graphene and show how this induces a quantifiable altering of graphene's phonon dispersion relation. Spatially resolved micro-Raman spectroscopy is a virtually ideal tool for this study as the generation of hot-electrons due to the excitation laser and the detection of the corresponding Raman-spectra occur simultaneously. Furthermore, micro-Raman spectroscopy provides easy access to single particle measurements, which is desirable since the plasmonic properties of such particles vary drastically with the particle’s size and shape. In addition, we present an analysis procedure yielding further information on occurring temperature and strain distributions solely from the captured Raman shift maps. Raman-mappings on single spherical nanoparticles, either consisting of pristine gold or encapsulated by silica which are positioned on single-layer graphene on quartz substrates with different excitation wavelengths and under variation of the incident laser power have been carried out and will be compared in terms of hot-carrier injection and heating effects.

The fabrication of thin dielectric layers is essential in the development of micro-photonic devices. The disposition of the layered materials is based on their refractive index contrast. This allows determined architectures (waveguides) to confine light and selectively transfer specific wavelengths through different types of on chip optical devices. Furthermore, future planar photonic platforms need the ability to construct stacked layers of such photonic devices. However, this has been proven to be economically restricted by the current methods of transferring crystalline Silicon layers. Therefore, it opens opportunities for other materials such as Silicon Nitrate (SiN) and amorphous Silicon (a-Si) to be deposited through standard CMOS processes.
In this work we present the fabrication of low loss hydrogenated amorphous silicon that can be used to build planar photonic devices. Thin films of a-Si were deposited through low temperature Plasma Enhanced Chemical Vapor Deposition (PECVD). Different levels of hydrogenation were used to passivate the deposited layers and both undoped and n-doped (with phosphorous) films were studied. Additionally, the films were optically and electrically characterized, their surface topology was analyzed through AFM, and their vibrational modes studied through Raman spectroscopy. Simple micro-ring resonators (with 15 µm in radius) were also fabricated in these films using e-beam lithography and Deep Reactive Reactive Ion Etching (DRIE) and used to study their propagation losses. We attained an optical loss of 15.88 dB/cm at 1537 nm for single mode waveguides with width and thickness of 500 nm and 200 nm, respectively.

Many researchers have extensively studied optical properties of porous anodic alumina in recent years. Apart from the interference colors appearance due to the multiple reflection of the vertical and skew incident light,[1] photonic effects due to the light scattering and propagation into lateral directions have also gained much interest.[2] Although considerable research has been devoted to the well-ordered porous alumina photonic crystal structures, rather less attention has been paid to the systems with deliberately introduced tailored disorder. Such planar random photonic alumina films, however, have numerous potential applications ranging from structural coloration, light entrapping, to lasing and diverse optical devices. In any realistic experimental situation, anodic alumina will always naturally contain intrinsic defects in nanopore layout. The challenge in preparation of alumina-based 2D random photonic nanomaterials is, however, to introduce different predetermined degrees of disorder into highly ordered photonic crystal by a controlled manner, gradually varying the experimental parameters. A number of approaches have been developed in attempts to tune the nanopore and cell sizes and the degree of pore ordering independently, such as imprint and electron beam lithography.[3] Such methods commonly rely on the artificial creation of pore nucleation sites on aluminum and yield only limited nanochannel length due to the spontaneous rearrangement of pores during the long-term growth. Here, we demonstrate the feasibility of preparation of mechanically stable and transparent 2D photonic waveguides with predetermined degree of disorder introduced into alumina by pulse anodization method. The resulting nanochannels have invariable diameters, length up to 166 µm, as well as clearly controllable and predetermined randomness in their arrangement.

1. Pashchanka, M.; Yadav, S.; Cottre, T.; Schneider, J. J., Porous alumina-metallic Pt/Pd, Cr or Al layered nanocoatings with fully controlled variable interference colors. Nanoscale 2014, 6 (21), 12877-12883.
2. Masuda, H.; Ohya, M.; Asoh, H.; Nishio, K., Photonic band gap in naturally occurring ordered anodic porous alumina. Jpn. J. Appl. Phys. 2-Lett. 2001, 40 (11B), L1217-L1219.
3. Choi, J.; Nielsch, K.; Reiche, M.; Wehrspohn, R. B.; Gosele, U., Fabrication of monodomain alumina pore arrays with an interpore distance smaller than the lattice constant of the imprint stamp. J. Vac. Sci. Technol. B 2003, 21 (2), 763-766.

A new class of semiconductor materials, MAPbX₃ (where MA=CH₃NH₃⁺, X = I^-, Br^- or Cl^-,) perovskites, has emerged as a promising candidate for high-performance photovoltaic and optoelectronic devices, exhibiting a range of excellent properties including strong light absorption, direct bandgaps, long carrier lifetime, high balanced hole and electron mobilities, and long electron-hole diffusion lengths. Patterning has been proposed to form periodic structures on the surface of perovskites, for applications such as improving the power conversion efficiency for solar cells, enhancing photodetector performances, fabricating photonic crystals and distributed feedback lasers. Here we report high-performance nanoscale surface patterned perovskite monocrystalline thin film photodetectors. The nanopatterned metal-semiconductor-metal photodetectors demonstrate significantly improved performance compared to the nonpatterned monocrystalline thin-film devices.

Fluorides exhibit unique optical features, such as low phonon energy and high transparency in UV region. Fluoride doped by rare-earth (RE) makes them excellent for optoelectronics and photonics applications. Nano-structured materials, where the metallic nano-particles (NPs) are distributed in a dielectric matrix, represent new type of material with unique optical properties such as local surface plasmon resonance LSPR. Most of the plasmonic research has so far focused on “classical” materials Ag and Au. However, other less used metals could bring significantly new functionalities. For example Al NPs yield an LSPR within the deep UV optical range and by tuning the size of NPs the resonance frequency could be shifted up visible spectral range. Despite potential and low cost of Al, the exploitation of its plasmonics is very recent and still facing both scientific and technical challenges. One of the crucial problems is the degradation of plasmonic properties induced by rapid oxidation. This problem could be solved by using fluorides matrices, which will embed the Al NPs. In our work we demonstrated successfully fabrication of Ag, Al as well as Rh and Bi NPs embedded by CaF2 and Eu³⁺:CaF₂ films fabricated by Pulsed Laser Deposition techniques (NPs) with auxiliary Electron Beam Evaporation (fluoride) at UHV conditions. By alternating of PLD and evaporation the photonic crystals consist of multilayers of metals nanoparticles and fluoride dielectric were fabricated.
The fabrication of metallic NPs in UHV conditions embedded in fluoride matrix prevent the oxidisation, which could degrade of plasmonic properties of NPs. The size of the NPs was controlled by the number of the laser pulses focused on the metallic target and varied between 5 and 20 nm. The distance between them in perpendicular direction is easily controlled by laser triggering.
The analysis of the measured date revealed an absorption band in the range from 200nm up to 450 nm corresponds to LSPR of incorporated metallic NPs depending on the metals and size, respectively. Results were compared and discussed with the results of analysis structural properties performed by SEM, TEM, AFM and XRD. The calculated absorption cross absorption effective cross-section will be compared with experimental results. The plasmonic behaviour of metal NPs will be compared with those presented for metal oxides nanocomposites. The effect of NPs on luminescence and down conversion properties of Eu³⁺:CaF₂ films will be presented as a function of their structural properties.

The emerging field of noble metal nanostructures has gained a lot of interest in the recent years. Their ability to induce localized surface plasmon resonances (LSPR) when excited by incident light shows much potential for various applications reaching from biomedical technologies over sensing to optical circuits. Plasmonic structures interacting with optically active materials like quantum emitters or J-aggregates can lead to light-matter interactions like strong coupling and fluorescence enhancement. [1] If such plasmonic systems are arranged in a periodic manner, coupling between the grid resonances and the LSPR can lead to coherent energy transfer and Surface Lattice Resonance (SLR) modes of high optical quality. Their properties can be tuned over a large spectral bandwidth by varying parameters like periodicity, composition and lattice structure. There was significant progress in both, manufacturing and tailored properties of such plasmonic nanosystems lately. Nevertheless, it still remains challenging to develop efficient and scalable ways to produce such devices. In this work, we show a procedure to generate plasmonic grids on transparent and flexible materials by using Laser Interference Lithography (LIL). This enables us to fabricate a square lattice of gold nanodiscs in a highly time- and cost-efficient way whose properties can be easily adjusted to meet the desired demands. The SLR of these periodic arrays can be coupled to gain materials to induce phenomena like coherent energy coupling on macroscopic scale. With this scalable approach based on LIL, nanophotonic devices can be moved closer to implementation in future optoelectronic devices.

[1] J. Phys. Chem. C 2019, 123, 6745−6752

We have observed solid state dewetting [1-3] of gold formed by vacuum vapor deposition onto a paper substrate. Solid state dewetting occurs resulting in minimization of total free energy of the gold-air, gold-paper, and paper-air interfaces. We previously reported plasmonic color changes with solid state dewetting of gold nanoparticles on a coated paper surface [4]. In the case that the isolated gold nanoparticles are formed all over the paper surface just after deposition, coalescence of the nanoparticles proceeds even though ambient condition at room temperature. In the case that the gold nanolayer with defects is formed all over the paper surface, growth of the defects proceeds and finally the gold particles are formed. The shape changes of gold due to solid state dewetting bring about dynamic color changes.
In this study, we investigated the relationship between gold coverage on the coated paper surface and dynamics of solid state dewetting. A gold nanolayer with a metallic color was deposited on the coated paper with 1.0 Å/s for 10 nm. The gold coverage is about 90% on the paper surface according to analysis on the image collected with a scanning electron microscope. Heating of it at 40°C induces growth of the defects, resulting in formation of particles after 72 h. The surface color changes to magenta after 72 h. The growth of defects and the color change at 40°C proceed faster than at 80°C. We guess that the speed of the growth of defects might be determined by losing the balance between wetting and dewetting of gold.

[1] C. V. Thompson, Annu. Rev. Mater. Res. 42, 399-434 (2012).
[2] P. R. Gadkari, A. P. Warren, R. M. Todi, R. V. Petrova, and K. R. Coffey, J. Vac. Sci. Tech. A 23, 1152-1161 (2005).
[3] A. L. Giermann and C. V. Thompson, Appl. Phys. Lett. 86, 121903 (2005).
[4] N. Fukuda et al. MRS Adv. 4, 325-330 (2019).

One of the most critical challenges to high-performance nanowire (NW) photodetectors is the detectability of weak signals that noise obscures. Cloaking is a proposed method to reduce the noise generated due to the probed field being disturbed by light scattered by the photodetector itself. Here we study and compare theoretically a traditional plasmonic material gold (Au) with an emerging plasmonic material zirconium nitride (ZrN) as a plasmonic cloak for silicon nanowire photodetectors, using Mie formalism for scattering and absorption efficiency and near-field contours of the electric field. Compared to Au, ZrN has real permittivity values similar in magnitude to common dielectric components, an ideal condition for generating a polarization vector in the shell that is antiparallel to that in the core, resulting in cloaking, and the cheaper availability of raw materials makes it a good replacement for the noble metal. We have predicted the performances across the entire visible spectrum, and have shown that though ZrN cloaks produce a significant decreasing the scattering, greater than 10 times compared to a bare nanowire, and twice as better compared to Au in the wavelength region of 400-500 nm, their performances become comparable at 550 nm, with Au providing twice as good a scattering cancellation as compared to ZrN over the wavelength region of 600-700 nm. By taking the absorption efficiency into account, we have defined a figure of merit (FOM) to determine the overall performance of the cloaked Si photodetector, and show that a ZrN cloak provides up to 3 times enhancement over the performance of a bare Si NW and a 60% improvement over a Au cloaked NW, in the wavelength region of 400-500 nm, while a cloaked Au NW shows up to 30 times improvement in the wavelength region of 600-700 nm over bare Si NW and up to a 4 time improvement over a ZrN-cloaked NW. We have also predicted the optimal dimensions for the cloaked NWs at various wavelengths between 400-700 nm.

Gold nanoparticles are very attractive for biomedical applications due to their enhanced light absorption by collective oscillations of their surface electrons at the surface plasmon resonance wavelength, λ_SPR [1]. For example, their high λ_SPR sensitivity on the refractive index of the medium shows great potential for detection of small organic molecules and proteins [2]. The λ_SPR shifts from the visible light range (e.g. 530 nm) for single spheres to the near infrared spectrum (e.g. 780 nm) for non-spherical particles, such as nanorods. The λ_SPR sensitivity to refractive index of the medium also increases as with decreasing particle sphericity, ranging from 44 nm per refractive index unit (RIU) for single spheres to 224 nm/RIU for rods [3].
Here, discrete element modeling (DEM) is coupled with discrete dipole approximation (DDA) [4] to investigate numerically the evolution of gold light absorption during nanoparticle agglomeration. The DDA model is validated against simulations and experiments for single spheres and nanorods [5]. The DEM-derived agglomerate sphericity is quantified by the fractal dimension, D_f, evolving from 3 for single spheres to 1.91 for ramified agglomerates having more than 15 monomers. The gold λ_SPR shifts from 530 to about 650 nm during agglomeration due to plasmonic coupling effects [6], in good agreement with data [7]. The evolution of λ_SPR sensitivity to the medium is elucidated as a function of agglomerate size and benchmarked against those of single spheres and rods [3].
References:
[1] Willets, K. A., & Van Duyne, R. P. (2007) Annu. Rev. Phys. Chem., 58, 267-297.
[2] Acimovic, S. S., Keuzer, M. P., Gonzalez, M. U., & Quidant, R. (2009) ACS Nano, 3, 1231-1237.
[3] Chen, H., Kou, X., Yang, Z., Ni, W., & Wang, J. (2008) Langmuir, 24, 5233-5237.
[4] Kelesidis, G. A., & Pratsinis, S. E. (2019). Proc. Combust. Inst., 37, 1177-1184.
[5] Qin, Z., Wang, Y., Randrianalisoa, J., Raeesi, V., Chan, W.C.W., Lipinski, W., & Bishof, J.C. (2016) Sci. Rep. 6, 29836-1-13.
[6] Sotiriou, G. A., Starsich, F., Dasargyri, A., Wurnig, M. C., Krumeich, F., Boss, A., Leroux, J. C., & Pratsinis, S. E. (2014) Adv. Funct. Mater., 24, 2818-2827.
[7] Zook, J. M., Rastogi, V., MacCuspie, R. I., Keene, A. M., & Fagan, J. (2011) ACS Nano, 5, 8070-8079.

ErAs is a semimetallic rare earth monopnictide which has a plasmonic response in the IR-region. ErAs has a rock salt crystal structure and is known to grow epitaxially in zinc-blende GaAs. This ability has led to numerous potential application such as thermoelectrics, plasmonics, and photonics. In the case of plasmonics, patterning the ErAs is required in order to design particular resonances in a metasurface. However, Er oxidizes extremely easily in air which has prevented ErAs use in these application. Due to the difference in crystal structure of ErAs and GaAs, a high energy interface exists between ErAs and GaAs. ErAs nanoparticles form on the GaAs As surface via an embedded growth mode in which the ErAs NP are partially buried in the GaAs surface. We recently observed a dewetting of GaAs films from the ErAs NP-GaAs composite interface. In this presentation we study this phenomena as a vehicle for the formation of tunable plasmonic structures. Previously we have shown that the dewetting process depends on several variable such as ErAs NP concentration, the dewetting temperature, dewetting time, and the thickness of the GaAs cap. We explore the ability to form a patterned surface by controlling the thickness of the GaAs. Specifically we produce a cap covering an ErAs NP growth (0.5 monolayers) which is sufficiently thick to prevent the surface from dewetting. By ex-situ etching we thinned the GaAs cap back to a thickness between 2 and 5nm which will allow dewetting. We then perform in-situ cleaning followed by dewetting of the structure in an MBE chamber. We dewet at a temperature greater than 550°C for at least 15 min. We show that we can form dewetted regions of exposed ErAs NPs. Using a low flux growth process, we control the ErAs growth to only the exposed region and produce large areas with continuous ErAs. This demonstration is a first step in producing definable resonances in a metasurface composed of ErAs using this approach.

Metamaterials interact with electromagnetic radiation to produce exotic resonant optical responses that arise as a result of meta-building block geometry and arrangement, rather than chemical composition alone. Such exotic optical responses include near-zero permittivity, negative permeability, and negative refractive index, which find applications in high resolution super flat-lenses, perfect absorbing super-flat antennae, and cloaking materials, among many others. In order for resonances to occur, the size of the meta-building blocks and the spacing between them need to be significantly smaller than the target radiation wavelength. Optical metamaterials therefore require the synthesis of highly uniform nano-sized building blocks as well as their 2D or 3D nano-scale arrangement. Currently, the large-scale synthesis of uniform nano-sized objects faces difficulties due to the costly and time-consuming nature of high-resolution lithographic techniques. Furthermore, the periodic arrangement of these nano-sized objects in 3D over extended length-scales presents yet another challenge. To address these challenges, we employed viral proteins that self-assemble into robust supramolecular scaffolds which can host a variety of plasmonic metallic nanoparticles for the preparation of nano-sized and uniform meta-building blocks. These buidling blocks were further assembled in 3D in a polyelectrolyte multilayer (PEM) system, using a layer-by-layer deposition technique.

One particular meta-building block, an annular ring composed of discrete plasmonic metallic nanoparticles, has previously been predicted to display a dominant magnetic dipole moment at optical frequencies. Once assembled in a 3D square lattice, these rings collectively displayed effective negative permeability resonances. Most excitingly, this material produced a negative refractive index at frequencies where the negative permeability and negative permittivity resonances overlapped. The permeability resonance frequency exhibited tunability across the entire visible regime, as a function of the number of nanoparticles in the ring. Our study investigated the optical properties of in-house fabricated rings as a function of the number of nanoparticles. Optical scattering spectra of individual rings were correlated with ring morphology and nanoparticle number via transmission electron microscopy (TEM)-correlated hyperspectral dark-field microscopy. In order to compare single-ring with ring-ensemble properties, polarization-dependent absorption measurements were investigated for solutions of rings as well as 3D ring assemblies in a PEM system. Finally, to probe the putative magnetic dipole moment of individual rings, Lanthanide complexes were chemically conjugated onto rings and employed as fluorescent magnetic moment reporters. Individual and ensemble measurements were also compared for fluorescence studies, using both TEM-correlated hyperspectral dark-field microscopy and solution-phase fluorescence.

Low-emissivity (low-e) glass innovation is intended to decrease the loss of infra-red radiation produced from articles at the inside of a structure with an ambient temperature of 300 K. Consequently, the reflection characteristic for the low-e glass is appropriate to reflect ~72% of the radiation from around 3.5 μm and up to ~9.7 μm. However, claiming the reflectivity characteristic drops at wavelengths below 10 μm, about 28% of the radiation discharged is yet lost to the exterior. In addition, this glass covering isn't appropriate at all to reflect sun oriented NIR radiation. Practically 49% of sun-based vitality is appropriated in the range from 0.7-2.5μm for which the low-e glass-sheet reflectivity is beneath 10%. Along these lines, the advancement of a robust film that can reflect close infrared and infrared warmth radiation in the range 0.7-10 μm while transmitting over 90% of the obvious light power is highly desirable.

Metal oxides have been utilized to deal with the light reflectance over a constrained wavelength extend. In such materials, electron plasma recurrence characterizes the beginning of the high reflectivity routine. To move the plasma recurrence to higher energies (shorter wavelengths), the doping convergence of the metal oxide must be expanded over the dimension of 1x1021 cm-3. At these focuses, in any case, the dopant particles have been appeared to create transporters with heavier powerful mass, diminished portability, and more profound benefactor expresses that fundamentally decline the transmittance.

The metamaterial proposed circumvents the need for excessive dopant incorporation through field-enhancement of the electron concentration at the interface of the moderately doped ZnO thin film with SiO2. To achieve this, a structure was designed that alternates insulating and conducting layers that, when voltage biased, produces a 2-dimensional electron accumulation in a field-enhancement device (FED). The device structure deposited by magnetron sputtering may also find application in a variety of optoelectronic applications.

Because silica nanoparticles scatter but do not absorb light in the visible wavelength range, their spectral profile gradually increases in intensity with a decrease in illumination wavelength. Upon adding small gold nanoparticles (5-10 nm diameter) to a much larger mesoporous silica particle (100s nm in diameter), the latter gradually takes on enhanced optical properties that reveal the presence of the gold. In a series of experiments, the degree of gold loading into mesoporous silica was gradually increased, in order to characterize the optical effects. Two methods of gold loading were employed with slightly different end results. In the first method, gold was directly plated into the silica structure. In the alternative technique, gold nanoparticles were attached to the silica structure through molecular linkers. In an additional series of experiments, the optical signals of mesoporous silica particles were monitored in real time as molecularly-linked gold nanoparticles were removed from their surface. This was performed through the use of chemical cleaving agents in two different settings: a controlled experiment within a microchannel and naturally within an A549 cancer cell. All data were captured by means of filtered imaging via differential interference contrast (DIC) microscopy. These findings demonstrate that these dual-nanoparticle systems provide a valuable method for monitoring activities within dynamic environments.

Three-dimensional graphene architectures namely, graphene-based polyhedral pyramids, cubes, and hollow cylindrical tube, and cubes have recently been realized using a surface-tension self-folding mechanism. The spatial coverage of the plasmon-enhanced electric-field in two-dimensional graphene ribbons needs to be extended into the bulk space through volumetric enhancements to overcome the surface and edge-limited efficiencies induced in 2D nanoribbons currently used for conventional plasmonic devices. Unlike patterned 2D graphene where the conventional and coupled enhancement modes are isolated only to the edges of the materials, the self-assembled multi-faced polyhedral graphene exhibits hybrid plasmon modes caused by multi-dimensional coupling between the graphene vertices, edges, and surfaces over an extra spatial degree of freedom. While 2D graphene can only demonstrate bi-directional coupling through the two edges; the 3D nanomaterials benefit from fully-symmetric 360° coupling at the apex of pyramidal graphene, orthogonal four-directional coupling in cubic graphene, and uniform cross-sectional radial coupling in tubular graphene. Each of these coupling mechanisms exhibits a corresponding plasmonic enhancement mode with unique optical features and advantages. Here, we discuss the fabrication and characteristics of each 3D graphene nanomaterial and the corresponding advantage for plasmonic sensing mechanisms. 3D graphene nanomaterials induce bounded volumetric field that senses minute quantities of targeted substances even away from the graphene surface with almost a 60% shift in resonant frequency that is far superior to the 10% change exhibited by 2D graphene ribbons. The ease of incorporation of the 3D nanomaterials with microfluidic channels has the capability of delivering optofluidic sensing mechanisms that far exceed the performance constraints faced by current plasmonic sensors.

Circular dichroism (CD) is a spectroscopy method based on the differential absorption of left and right circularly polarized light, which enable identification and quantification of structures at molecular and nanometer scales. Until now, however, available spectral range for CD is physically limited by the development of dynamic polarization modulators. Among not yet fully explored region, of particular interest is the far infrared or terahertz (THz) region of the electromagnetic spectrum. This is because terahertz circular dichroism (TCD) could offer multifaceted spectroscopic capabilities for understanding mesoscale chiral architecture and low-energy molecular vibrations. Here we show that reconfigurable optical modulators fabricated from patterned sheets with periodic kirigami cuts enabled dynamic range of polarization rotation modulation in the THz region over thousands of cycles. Under mechanical application, the plasmonic stripes transformed into a topologically equivalent helix structure. We measured TCD spectra of several representative biological samples using kirigami modulators and found distinctive TCD peaks. Kirigami modulators will also play an indispensible role for other applications, such as polarization resolved THz imaging and phase-encrypted THz telecommunication.

The appealing capacity of plasmonic nanoparticles to efficiently harvest, scatter and emit light has recently garnered attention for application of these materials in plasmon-based photocatalysts, solar cells and thermo emitters. Gold and silver have been extensively used to successfully drive hydrogen dissociation and CO oxidation reactions by injecting hot electrons into molecules adsorbed to their surface. However, due to the low thermal stability and the high cost of both metals, the necessity of studying alternative plasmonic materials, that potentially will expand the field towards more ambitious and cost-effective applications, has been growing in the last years. Titanium nitride (TiN) is a conductive ceramic with high hardness and bulk melting point (2930 °C). Its plasmon resonance located in the visible-NIR region, low cost (relative to gold and silver), and the well-understood properties as a thin film in the semiconductor industry, make it a strong alternative to precious plasmonic metals.

The present work encompasses a comprehensive study of the plasma-based synthesis of TiN nanocrystals and highlights the potential for their in plasmonic-driven catalysis and as a high-temperature-resistant photothermal absorber.

TiN particles were synthesized via a scalable, modular, non-thermal plasma method. Titanium and nitrogen precursors were transported into a RF frequency plasma where TiN particles nucleate and grow. Platinum nanoparticles were subsequently deposited on the TiN by photo-induced reduction of an aqueous solution of chlorplatinic acid (H2PtCl6). The reduction of the precursor metal was driven by electron hole pair generation via plasmon decay. The addition of methanol as a hole scavenger increased the electron lifetime, obtaining metallic platinum. This reaction occurred at temperatures below 40°C under visible light illumination.

In addition, a novel TiN@SiOxNy core-shell structure was produced by taking advantage of the modular capabilities of the non-thermal plasma synthesis method. The synthesized core shell particles displayed a 60% higher plasmon peak in the extinction coefficient with respect to the uncoated TiN particles. To probe the potential of these heterostructures, core-shell and uncoated TiN particles were deposited on SiOx substrates by coupling the plasma reactor with a nozzle. This simple modification enables to deposit a thin film by jet impaction method. The emissivity (absorptivity) of both samples were measured under vacuum conditions, demonstrating the high temperature resistance of TiN@SiOxNy core-shell films, as their optical properties at 700 °C remained stable. This simple experiment demonstrates the stark promise of this material to generate a tunable-band-like emission for applications in thermo photovoltaic systems.

This work strengthens the case for alternative plasmonic materials in fields dominated by precious metals, and heavily driven by materials cost.

Plasmonic nanomaterials and devices have attracted enormous and multidisciplinary interest and efforts over the last decade. However there are few examples of plasmonic devices mainly due to materials issues, since gold and silver, the “traditional” plasmonic materials, are expensive, with limited possibilities to tune their plasmonic response across the spectra and low melting point, which makes them incompatible with CMOS growth and processing for application in microelectronics.
Transition metals nitrides (TMN) emerge as alternative plasmonic materials and principal candidates for plasmonic applications. The TMN are conductive ceramics with exceptional properties such as substantial electronic conductivity, high melting point (>3000 K) and tunable work function values. Consequently, they are particularly stable in hostile chemical environments, high temperatures, and strong electric fields, such as in lasers. These traits make them suitable for a wide range of applications from microelectronics to photonics and medicine. Among them, titanium nitride (TiN) is recently emerging as significant candidate for plasmonic applications (biosensors, catalysis and photochemistry, solar energy harvesting, photo-detection, and optical storage of information). In this work, TiN nanostructures with controlled spacing and tunable dimensions were fabricated using Nanosphere Lithography (NSL) and DC magnetron sputtering (MS) for the growth of TiN nanostructures.
NSL appears as a very promising approach, due to its rapid implementation and compatibility with wafer-scale processes. NSL combines the advantages of both top-down and bottom-up approaches and includes: (a) preparation of the nanospheres colloidal mask and (2) the deposition of the desired material in the empty space between the spheres. The mask is then removed and the layer keeps the ordered patterning of the mask interstices.
Specifically, a suspension of monodisperse polystyrene nanospheres (diameter, d=552 nm) was spin coated onto the Si (001) substrate to form the mask. Subsequently, the selective growth of TiN was made by: (i) rf biased DC MSin Ar/N atmosphere by varying the TiN thickness from 10 to 30 nm. The arrays of ordered TiN nanostructures appear after the lift-off of the nanospheres mask.
Atomic Force Microscopy showed the fabrication of relatively large scale (> 40X40 μm) and well-ordered plasmonic surfaces, where the TiN nanostructures with thickness lower than 20 nm form a honeycomb template over the Si (001). The optical response of the TiN plasmonic surface was examined by Optical Spectroscopy and Spectroscopic Ellipsometry.

Ref.: P. Patsalas, N. Kalfagiannis, S. Kassavetis, G. Abadias, D.V. Bellas, Ch. Lekka, E. Lidorikis, Materials Science and Engineering R 123 (2018) 1–55.

Thin films of degenerately doped metal oxides such as those of Sn-doped In₂O₃ (Sn:In₂O₃) are commercially significant for their broad utilization as transparent conducting electrodes in optoelectronic devices. Over the last decade, nanocrystals (NCs) of Sn:In₂O₃ and other doped metal oxides have also attracted interest for localized surface plasmon resonance (LSPR) that occurs in the near to mid-infrared region. The suitability of this LSPR for some applications depends on its capacity to concentrate light in small regions of space, known as near-field hot spots. This efficiency to create near-field hot spots can be judged through an LSPR figure-of-merit such as Quality factor (Q-factor), defined as the ratio of LSPR peak energy to its linewidth. The free electron density determines the LSPR peak energy while the extent of electron scattering controls the LSPR linewidth and hence these factors together essentially dictate the value of the Q-factor. Here, we describe the properties of aliovalent cationic dopants that allow both high LSPR energy and low LSPR linewidth and, subsequently, high LSPR Q-factor. In this context, we identify Zr⁴⁺ as a model aliovalent dopant for high LSPR Q-factor in the In₂O₃ lattice. The resulting Zr-doped In₂O₃ NCs exhibit one of the highest LSPR Q-factors reported in the mid-infrared region while also performing equivalently to the recognized materials for either high dopant activation (Sn:In₂O₃ NCs) or low LSPR linewidth (Ce-doped In₂O₃ NCs), simultaneously.

ZrB₂, which belongs to family of ultra-high temperature ceramics (UHTC), is known in the literature by its high melting point, oxidation and ablation resistance, chemical reactivity and erosion resistance. These properties make ZrB₂ a great candidate for applications in extreme environments, such as aeronautics and astronautics. In such applications, device temperatures can reach up to elevated temperature, therefore an effective cooling approach is required. Due to the unavailability of the conduction and convection heat transfer, task of controlling the radiative heat transfer becomes crucial. To improve the radiative heat transfer rate from the surface, effects of mixing and doping on the emissivity of the ZrB₂ have been widely investigated. Improved emissivity is reported in these studies, by which heat transfer from the surface is increased.
As an alternative to mixing and doping, we investigated the effect of patterning of ZrB₂ on the emissivity of it in 0.5 – 1.5 µm spectrum interval. As a starting point, we considered grating structures which are heavily utilized for coupling between incident waves and the surfaces. We found out that by arranging the periodicity of the patterns on the surface, emissivity is improved due to the increased coupling. Two resonant behaviors are observed as well as the broadband emissivity enhancement in the 0.5 -1.5 µm spectrum interval. Parametric analysis is carried out to optimize the emittance of the surface in broadband spectrum. Underlying physical mechanisms are investigated and source of broadband enhancement is explored. Initial findings yield that one distinct peak stems from the surface geometry and other one occurs due to dielectric function of ZrB₂, which approaches to zero around the second peak wavelength.
After the parametric analysis, an initial design is determined. To further improve the emittance of that structure, we used adjoint method to optimize the topology it. In recent years, adjoint method is adapted to field of electromagnetics and utilized to design superior structures via computationally efficient topology optimization. When adjoint method is applied, final structure’s emissivity approaches to 1 around peak wavelengths and reaches up to 85 % in 0.5-1.5 μm spectrum interval. These findings show that emissivity of the pure ZrB₂ can be improved by patterning. When parametric analysis is combined with adjoint method, very high emittance is obtained. Such high emittance demonstrates the possibility of an alternative approach for emittance enhancement, thus thermal management.

Graphene and related two-dimensional materials have attracted intensive attention for electronic and photonic applications due to their unique structures and band energy aligns over a wide wavelength range, despite being atomically thin. However, most of the existing active graphene devices suffer from low light-matter interactions, resulting in either slow responsivity or limited detectivity. Integrating graphene with the intelligent substrate to excite graphene surface plasmons offers a promising approach for improving the photoelectric performances of graphene-based devices.
Here, we demonstrated the graphene carriers can be easily modulated by ferroelectric domains and firstly proposed its potentials for tunable infrared photodetector and micro-spectrometer applications. Compared with the silicon-based graphene plasmonic devices using a complex process of micro-nano fabrication, our proposed devices provide the advantages of more convenient and controllable technique without the need of patterning graphene, and lower energy consumption due to nonvolatile properties of the ferroelectrics free of additional contact electrode. The simulated results show the photodetector features a tunable absorption peak, modulated by periodically polarized ferroelectric domains at nanoscale, with an ultra-high responsivity up to 6.72×10⁶ A W^-1 in the wavelengths ranging from 5 to 20 μm at room temperature. The potential mechanism for the prominent performances of the proposed photodetector can be attributed to the highly confined graphene surface plasmons excited by the local electrical field across the interface of graphene and ferroelectric layer resonant to the incident wavelength, which could be easily controlled by the features of the ferroelectric domains. The tunable spectral response and the ultra-high responsivity make the photodetector based on graphene plasmon tuned by the ferroelectric domains promising in practical applications of micro-spectrometer and other light sensing devices.

Rare-earth activated upconverting nanoparticles (UCNPs) have been studied due to their energy conversion property based on multi-photon absorption process. Often, these nanoparticles are combined with plasmonic nanostructures to overcome the intrinsic limit, especially, low internal quantum efficiency. However, recent plasmonic structures suffer from complicated process and restricted resonance region. In this study, we demonstrated a novel plasmonic platform to manipulate resonance wavelength via lattice control method using sequential metal-contact nanolithography. The platform employed metal nanostructure-UCNPs embedded insulator-metal reflector (MIM), which exhibited various optical characteristics including confinement of near-infrared (NIR) light, and facile extraction of converted visible light through plasmonic resonance excitement. Consequently, we observed over 200-folded enhancement for upconverted luminescence. Unlike the previous research, which tends to enhance specific wavelength, the lattice-controllable plasmonic platform shows broadband enhanced upconversion luminescence providing a convenient and versatile application for photo-isomerization reaction.

Efficient nonlinear optical materials are rare yet critical platforms to enable photon coupling. Here, we report an anomalous third harmonic generation (THG) effect universally found in the epitaxial films of a family of V-VI chalcogenide topological insulators (TIs) with drastically different optical density of states. Unlike the regular scaling behavior and without any optical resonances, THG generated from these films decays exponentially as the film becomes thicker. At the minimum thickness required for the topological states at the top and bottom surfaces to remain decoupled, the THG intensity reaches its maximum which is almost four orders of magnitude stronger than current leading materials. We attribute this unusual effect to the topological bands with nonlinear dispersion velocities and nontrivial Berry connections. This discovery provides new insights into the connection between band topology and optical nonlinearity and also a unique venue for realizing strong nonlinear optical performances on chip.

Nanocomposite film composed of plasmonic nanoparticles and matrix of metal-oxide semi-conductor is promising materials in future application for conversion of solar light into chemical energy with high efficiency. This background motivates the research on fabrication of noble-metal (Au, Pt, Ag, Pd) nanoparticles/semi-conductor metal oxide (TiO2, ZnO) nanocomposite film. Generally, the fabrication process employs one of the following conventional coating method; sol-gel, sputtering, vacuum evaporation. In these, sputtering and vacuum evaporation have an advantage of chemical wastewater free, while these require costly high-vacuum vessel.
In this paper, we present the novel method based on sputtering performed in open air, which is expected to be alternative to the conventional sputtering and evaporation method. The method has the following features; 1. Non-use of harmful source for operation in open air, 2. Synthesis of nanoparticles of noble-metal with narrow size distribution, 3. Direct deposition of the synthesized nanoparticles on film surface. We have investigated its potential as a method for easy manufacturing of plasmonic nanocomposite film. Recently, we confirmed that the fabricated nanocomposite film reveals high photocatalytic activity for pollutant degradation under sunlight exposure. The detail of the developed method and the performance of the nanocomposite film will be presented in our presentation.

Integrating micro-scale light sources with passive components in a single chip has been one of critical challenges in photonics and optoelectronics. In addition, driving the active optical elements by direct current injection has been a long-standing goal since it’s been great challenge to inject current into micro-sized optical devices without affecting the important optical characteristics. In this work, we use epitaxially grown III-V semiconductor compounds (AlGaInP) with vertical p-i-n junction including multi-quantum wells (MQWs) structures with central emission wavelength of ~680 nm, and successfully fabricate an individually transferrable microdisk (MD) array with diameters of 5 and 7 µm, respectively, which can serve as micro-scale light sources at visible frequency. In addition, we utilize the CVD-grown multilayer graphene (MLG) that shows high charge transmission and optical transparency at the given frequencies as the transparent electrodes to electrically pump the device. In fabrication, we first transferred the CVD-grown MLG on a Si₃N₄ device substrate using wet transfer method. Then we exploited PDMS micro-tip transfer technique to pick up the pre-fabricated MDs from the semiconductor wafer and transferred them onto the pre-transferred MLG on Si₃N₄ substrate. We used the electron-beam (E-beam) lithography and O₂ plasma etching to pattern top- and bottom-MLG electrodes that are directly in contact with p-doped top and n-doped bottom layers of the MDs. A final metallization carried out on these MLG electrodes completed the fabrication of the electrically driven MD devices. We performed the electroluminescence (EL) experiment by applying pulsed currents with a repetition rate of 1MHz and duration of 19.5 ns. A clear EL emission from the device was captured by the charge-coupled device (CCD), and subsequently observed in the measured spectrum. We believe that our device can be essential building blocks for the compact integrated optical circuits.

Despite their unique properties including CMOS-process-compatibility, high chemical stability, catalytic activity, etc, lossy metals are generally shunned from use in structural colors because their damping losses compromise the color vibrancy. In this work, we demonstrate a strategy to create highly vibrant and high contrast color pixels from lossy metals (e.g., Pt and W). Our pixel is of the reflective type and consists of a metal substrate supporting a near-wavelength dielectric grating oriented 45 degrees to the input polarization. This allows the incident light to be decomposed into s-polarized (s-pol) and p-polarized (p-pol) light with equal strengths. The reflected signal is filtered by a cross-polarizer, cancelling the background signal and keeping the pixel contrast high. Each s-pol and p-pol component excites a resonance at different wavelengths through the coupled waveguide-array modes in the grating [1,2]. At each resonance, a phase difference between the s-pol and p-pol light arises, abruptly rotating the total polarization vector by π/2 over the wavelength. As a result, the cross-polarized filter transmits two spectrally distinct and sharp signals from the two resonances. We find that the metal loss severely degrades the p-pol resonance (i.e., surface plasmon polariton) due to the spatial overlap of the resonant field with the metal interface whereas its effect on the s-pol resonance, located between the grating and air interface, is relatively small. By measuring the cross-polarized reflection from gratings on lossy metals, we obtain spectrally pure and sharp signals, translating to a high color vibrancy and large gamut range, as only the spectrally sharp s-pol resonance contributes to the reflected spectrum. In fact, W which is one of the lossiest metals, provides a larger gamut coverage than that of Ag. These results offer intriguing routes for realizing high-contrast structural colors from an expanded range of metals that host interesting chemical and physical properties [3].


References
[1] C. J. Chang-Hasnain, W. Yang, Adv. Opt. Photonics. 2012, 4 379
[2] P. Lalanne, J. P. Hugonin, P. Chavel, J. Lightwave. Technol. 2006, 24, 2442
[3] Y. Kim, K. Jung, J. Cho, J.K. Hyun, ACS Nano, 2019, https://doi.org/10.1021/acsnano.9b05382,

We show that the plasmonic response of InSb spherical nanoparticles splits into two resonances when an external magnetic field is applied. This splitting is akin to a Zeeman splitting with the frequency separation of the satellite resonances depending on the magnitude of the applied magnetic field.
InSb has a homogeneous dielectric function, and the extinction of the nanoparticle shows a resonance in the frequency range of THz. The application of the external magnetic field induces an anisotropy in the dielectric function. To understand the splitting of the plasmonic resonance, we show that the anisotropic sphere is topological equivalent to a spheroid with an isotropic dielectric function. The shifting of the plasmonic peaks is equivalent to having a spheroid of varying eccentricity. The plasmonic response and peak splitting also depends on the direction of the applied magnetic field.
Finally, applications to active plasmonics will be discussed.

Partial support from DGAPA-UNAM IN110819

Slow light enables spatiotemporal manipulation of electromagnetic waves at the nanoscale and allows access to a plethora of nonlinear optical phenomena. Although the guided waves in plasmonic waveguides are known to inherently possess a slow energy velocity, their ultimate light-trapping performance remains unknown as the effect of the waveguide’s shape alteration has not been considered systematically so far. In this work, we search for the optimal geometry of a free-form metal-insulator-metal (MIM) plasmonic waveguide for light trapping, and reveal its unique properties that are significantly different from the conventional linear taper structures. We optimize the waveguide geometry by using three different numerical optimization algorithms combined with the transfer matrix method, and confirm the results with full-wave simulations based on the finite element method (FEM). The optimized light trapping structure possesses a dramatically enhanced quality factor, almost reaching the theoretical limit imposed by the material loss. Interestingly, it also exhibits a distinctive mode dynamics and a unique dependence on the material loss, both being fundamentally different from those of the conventional linear taper structures. Unlike linearly tapered waveguides, whose characteristic length (minimum necessary length to reach half of the maximum quality factor) is inversely proportional to the material loss, the quality factor of the optimized structure saturates at much shorter length, and its characteristic length scales logarithmically to the inverse material loss. Further, in order to demonstrate the feasibility of proposed design approach, we analyze the optimization results obtained with realistic materials at visible and mid-IR frequencies.

Plasmonics enhanced photodetectors have been well explored in visible to THz spectrum. Several plasmonics materials have been given promised results in visible to THz regime, Silver and gold are most extensively used noble metal for plasmonics applications . A wide range of ultraviolet spectrum is less discovered due to the limitation of the intrinsic behaviour of these metals. Where silver suffers from rapid oxidation and poor quality factor of Surface plasmon resonance (SPR) and Localized surface plasmon resonance (LSPR) below 350 nm besides gold has an interband transition around 500 nm.To explore the plasmonic properties in Ultraviolet (UV) regime, aluminum has enormous potential and high quality factor to operate in the deep UV to near UV spectrum, so we present localized surface plasmon resonance driven UV photodetection through aluminum metasurface on gallium nitride (GaN) substrate. For the generation of UV range LSPR and formation of Schottky contact at the surface of GaN, the single crystalline aluminum film is grown by plasma assisted Molecular beam epitaxy (PA-MBE) method on undoped GaN/Al₂O₃ substrate. As a metasurface, periodic nanoholes were designed. CMOS compatible techniques are used to make subwavelength metasurface and Schottky diode. Furthermore, FDTD simulation and micro UV reflectance measuremnet were perofmed to realize the behaviour of these periodic nanoholes.The hot electrons generated by the decay of LSPR in these periodic subwavelength nanoholes and atomically smooth metal semiconductor interface contribute to enhance photoemission efficiency and wide UV spectral enhancement coverage. The ultrahigh responsivity (670 amp/Watt) and detectivity (1.48 × 10¹⁵ cm Hz^-1/2 W^-1) are observed at 355 nm. This CMOS compatible plasmonics based ultraviolet photodetector can replace the traditionally GaN-based photodetector and open a new era in ultraviolet detection regime.

Photonic micro-structures, including microring resonators, photonic crystal defect cavities, and whispering gallery resonators, have been by far the most successful platforms for boosting light matter coupling, which is largely thanks to their huge Quality (Q) factors, spanning thousands to millions. The associated long photon residence times and enormous amplification of the local light intensity has led to highly-efficient lasing[1], frequency comb generation[2], optical signal modulation[3], optical isolation[4], quantum generation[5], and single molecule biosensing[6]. At the same time, nanoantennas provide an unprecedented level of control over the scattering of freespace optical signals, especially when arranged into metasurface arrays, enabling flat optical elements such as beam steerers, lenses, and holograms[7]. However, researchers typically face a trade-off between antenna size in relation to wavelength and resonant lifetime, with subwavelength structures limited to quality factors (Q) less than 100, keeping more exotic nonlinear phenomena out of reach without the use of extremely high power femtosecond lasers.

By utilising Mie and guided mode resonance (GMR) to control the scattering profile of high Q, Q>1000, resonant nanostructures, here, we demonstrate experimentally that this trade off is not in fact fundamental. We show that GMR in ultrathin dielectric metasurfaces can be employed to bring strong field amplification to a wide class of freespace nanophotonic systems, producing efficient optical nonlinearities in a nanoscale footprint. Combining wavefront shaping with subtle structural symmetry breaking, arbitrary freespace scattering can be achieved alongside GMRs with lifetimes that can be increased almost indefinitely. As a proof of principle, we provide the first theoretical and experimental demonstration of two high Q phase gradient metasurface functions, one capable of efficiently steering an infrared plane wave to a predetermined angle and another splitting a plane wave into two beams, with both devices supporting GMR Q factors greater than 1000.

The metasurfaces were patterned into a silicon on sapphire wafer with a 600nm silicon layer, using electron beam lithography followed by reactive ion etching. To realize the different metasurface phase profiles, nanowires of varying widths were arranged within a 2121nm supercell. Subwavelegnth periodic notches were also etched into particular nanowires, giving rise to GMRs with Q factors controlled via notch depth. Using a home built angle resolved microscope coupled to a grating spectrometer, we report efficient beam steering and beam splitting between 1350-1500nm, accompanied by GMRs with Q factors as high as 2500. To the best of our knowledge, this is the highest Q factor observed to date in a phase gradient device. We have numerically confirmed that the huge electric fields associated with these resonances can excite efficient nonlinearities, including the Kerr effect and stimulated Raman scattering, opening the possibility for novel functionalities such as subwavelength nonreciprocity. While our proof of principle demonstrations involve beam steering and beam splitting, the design principle we present could easily be extending to other types of wavefront shaping applications, such as lensing.

(1) Spillane, S. M.; Kippenberg, T. J.; Vahala, K. J. Nature 2002, 415 (6872), 621–623.
(2) Stern, B.; Ji, X.; Okawachi, Y.; Gaeta, A. L.; Lipson, M. Nature. Nature Publishing Group October 18, 2018, pp 401–405.
(3) Wang, C.; Zhang, M.; Yu, M.; Zhu, R.; Hu, H.; Loncar, M. Nat. Commun. 2019, 10 (1).
(4) Fan, L.; Wang, J.; Varghese, L. T.; Shen, H.; Niu, B.; Xuan, Y.; Weiner, A. M.; Qi, M. Science 2012, 335 (6067), 447–450.
(5) Guo, X.; Zou, C.; Schuck, C.; Jung, H.; Cheng, R.; Tang, H. X. Light Sci. Appl. 2016, 6 (5), e16249.
(6) Vollmer, F.; Arnold, S. Nat. Methods 2008, 5 (7), 591–596.
(7) Yu, N.; Capasso, F. Nat. Mater. 2014, 13 (2), 139–150.

A wide range of optic and optoelectronic applications require surfaces with specific properties, such as antireflection, self-cleanliness, electro-optic tuneability, etc. In this talk, we will review recent efforts from our group in developing optical structures based on ultrathin materials and nano-structuring, which offer unprecedented flexibility in molding light propagation, from the visible to the infrared range, and producing self-cleaning effects. In particular, we will show how to use graphene, ultrathin metals, scalable metal dewetting nano-patterning and nano-imprint lithography, to produce transparent surfaces for transparent electrodes, displays, optical windows and infrared sensing.

Optical bandpass filters allow or reject the light within a specific wavelength range to prevent damage from unfiltered light or to utilize the quarantined light. The terrestrial atmosphere or complex biological fluids are among the examples which provide either broad or narrow transmission windows to accomplish certain functions. Metallic nanorods and their assemblies are a promising platform for optical bandpass filter due to their intense light-matter interactions with tunable optical cross-section greater than the particle volume. With our innovative synthetic strategy, we achieve to fabricate optical filters with independently tunable center wavelength and a variable bandwidth by carefully selecting the aspect ratio of a collection of AuNRs with narrow resonances that allow to create ideal flat top profile for targeted absorption and a sharp roll-off at the filter edges. Laser irradiation can be a simple tool to transcribe a transmission window at desired wavelength by reshaping a population in resonance with the LSPR. It is possible to write a single or multiple transmission window using polydisperse nanorods or nanowires with dense broadband spectral distribution over visible to mid-IR. We examine the impact of laser irradiation conditions on the spectral position, depth, and width of the transmission window and validate the results with theoretical study. The rational design can be achieved in various systems including solutions and nanocomposites to fabricate complex optical materials with properties not found in natural media.

The high reflectivity of typical semiconductor is a major factor in deteriorating the efficiency of solar cells. The conventional approach to minimize these losses is to add a quarter-wavelength-thick antireflection coating with an optimized refractive index. However, this technique has limitations as single-layer antireflection coatings are relatively narrow band and do not trap light in the cell by redirecting it into the plane of the cell. New high-performance photon management approaches are required to achieve high-efficiencies with optically-thin and low-cost cells.
In this research, we propose and experimentally test the performance of broadband light-trapping layers for few-micron-thick Si cells using dense arrays of Si nanostructures. Light reflected from Si substrate can be canceled out by the scattered light from the array of Si nanostructures. While dense arrays of nanostructure can be viewed as homogeneous medium with a broad Fabry-Perot resonance, each individual nanostructure can also support Mie resonances to of destructively interfering with the reflected light from the substrate. By judiciously optimizing the structure parameter of the Si nanostructures in an array, both broadband antireflection and light trapping can be achieved. The increased absorption of light is demonstrated experimentally with a 3 um thick solar cell. A 10.3% power conversion efficiency is achieved, representing a boost in the current density as well as enhancing cell’s efficiency over 48% compared to a reference cell without an antireflection coating.

Photonic cooling using coherent laser radiation has enabled reaching temperatures close to absolute zero, which plays a key role in identifying new states of matter, and led to novel strategies for solid-state refrigeration. The low entropy of laser radiation makes these cooling processes thermodynamically feasible. Recent theoretical studies have suggested the possibility of photonic cooling using incoherent light from semiconductor diodes. However, such photonic cooling using incoherent light has not been experimentally achieved. In this talk we will describe a first demonstration of photonic refrigeration by controlling the chemical potential of photons. Leveraging the large heat-transfer rates of near-field radiation and sensitive, high-resolution nano-calorimetry, we were able to experimentally observe net cooling of a planar object when it was separated from a reverse-biased photodiode by a nanoscale gap. Such cooling arises from a combination of a suppression of thermal radiation from photodiode at reverse bias, and an enhancement of photon emission from the planar device due to near-field tunneling. We will also describe the dependence of cooling power on gap distance and voltage bias. Finally, the competing effects of negative luminescence and radiative heating will be considered. Our study points to the new and promising direction of combining nanophotonics and optoelectronic devices for achieving solid-state refrigeration.

Thin film magnetooptical materials are enablers for integrated non-reciprocal photonic devices such as isolators and circulators, as well as magnetoplasmonic and magnetooptical heterostructures. Iron garnets, A₃Fe₅O₁₂ where the A-site includes Y, Bi and Ce or a rare earth, offer particularly good magnetooptical figure of merit (Faraday rotation (FR)/optical absorption) in the near-IR communications wavelengths, with FR and absorption both increasing at visible wavelengths. We describe the growth and magnetooptical properties of films of polycrystalline bismuth-, cerium and terbium-substituted yttrium iron garnet (BiYIG, CeYIG, and Tb,Bi,CeYIG), deposited on silicon substrates and waveguide devices using pulsed laser deposition. These materials were incorporated as cladding materials on Si or SiN waveguides to make integrated isolators based on both ring resonators and on Mach-Zehnder interferometers (MZIs). For TM-mode devices, isolation ratios and insertion losses are up to 40 dB and 3 dB respectively for a ring resonator and up to 30 dB and 6 dB for an MZI, with the latter having a significantly higher bandwidth of 20 dB isolation ratio over a 2 nm wavelength range. TE-mode devices, which require growth of the magnetooptical material on the sidewall of the waveguide, were also fabricated and tested. Garnet films are also useful in active photonic or magnonic devices based on the propagation of spin waves, and programmable spin wave logic devices have been demonstrated in YIG films. In BiYIG films capped with Pt, spin-orbit torque has been used to translate domain walls at velocities exceeding 4 km/s, enabling rapid magnetic switching or transmission of data. The potential of these complex oxides in enabling new types of photonic devices and structures will be assessed.

Cadmium oxide (CdO) is one of the most promising candidates for low-loss mid-infrared plasmonics to date, as it demonstrates a tunable range of carrier concentrations (from 10¹⁸ – 10²¹ cm^-3), while maintaining mobilities between 300 cm²/vs and 500 cm²/vs over the entire range. These high mobility values lead to plasmonic structures with very low loss and very narrow frequency bands. Fully accessing the spectrum of plasmonic applications, including surface plasmon oscillations, epsilon-near-zero (ENZ) modes, and strong coupling phenomena requires that one maintains these appealing transport properties over a film thickness range spanning a few tens to a few thousands of nanometers.

Extensive CdO fabrication experiments using high-power impulse magnetron sputtering reveal a strong dependence of carrier density and mobility on film thickness, particularly when thickness values drop below 100 nm, a particularly interesting and important range for ENZ modes. For unintentionally doped material in this thinness range, mobilities fall and carrier concentrations rise, both of which can be attributed to surface donors. There is a widely reported surface accumulation phenomenon in CdO thin films that accounts for this phenomenon. Conversely, if CdO is donor doped to values where the Fermi energy is above the surface donor energy this effect can be reversed. In this case, as film thickness falls below 100 nm, carrier concentration drops due to surface depletion.

In this presentation, we demonstrate the reversal between surface accumulation and depletion in CdO by comparing thickness series sets of intrinsic CdO and In-doped material. Transport properties from this sample collection reveals a carrier concentration at which the thickness-dependence relationship is no longer present. At this carrier density, the bulk Fermi energy aligns with the surface defect energy so as to eliminate accumulation or depletion zones. Finally, we show how optical properties, i.e., perfect absorber ENZ layers are affected by these surface effects, and how they can be eliminated by sandwiching doped CdO layers between intrinsic (and lower doped layers), or by capping CdO surfaces to change the surface defect formation energies.

While surface accumulation has been reported in CdO thin films, this is the first demonstration of the reversal between surface accumulation and depletion at varying dopant levels, allowing for tunability of surface charge. This offers potential for tunable quasi-2-dimensional electron gas (Q2DEG) behavior at CdO surfaces.

Owing to high sub-diffraction limit resolution (~100 nm) and a true three-dimensional printing, two-photon polymerization (2PP) direct laser writing has opened the possibility of fabricating near-infrared complex 3D optical components such as micro-lenses, micro-ring resonators and others. In this work, by combining 2PP direct laser writing and inverse design optimization methods, we have been able to demonstrate a compact, thin and broadband free-space polarization beam splitter working in infra-red range. We have optimized a 2λ-thick (in the direction of propagation) device capable of splitting parallelly and perpendicularly polarized light to be directed to the left by +θ° and right by -θ°, respectively. From our simulations, we observe a relatively large bandwidth operation ranging from 1.3 µm to 1.7 µm for the beam splitting behavior. In the experimental demonstration, we successfully fabricated the device using a commercial 3D direct laser lithography system based on a femtosecond laser at 780 nm. We have also performed optical characterizations for the fabricated device operating at 1.3 µm and 1.55 µm. The device exhibits good polarization splitting while also maintaining low transmission loss of 2 dB (1.3 µm) and 2 dB (1.55 µm).

In order to fulfil the promise of plasmonics, it has been viewed that one of the most disruptive technologies will be the “all dielectric plasmonics”. This, by itself, contradicts the definition of plasmonics (interaction of light with collective charge oscillations at metallic nanostructures). However, some interesting approaches can be exploited. In that respect, polar dielectrics can be used to couple an electromagnetic field to collective lattice oscillations, namely optical phonons. Those polar crystals can support optical modes that are confined either to the surface of the material or can be highly confined within or in the vicinity of sub-wavelength geometries, resulting in strong field enhancement. Similar to their metallic counterparts these oscillations are only supported when the real permittivity is negative and in the case of the polar dielectrics, this happens at the so-called Reststrahlen band. Naturally, the extent of this band defines the spectral range of operation.
In this work, we focus our attention to a family of very important technological materials; that of metal oxide perovskites. Metal oxide perovskites have been investigated for many years due to their excellent dielectric, piezoelectric, ferroelectric and optoelectronic properties. When a small number of electrons are introduced in the lattice (e.g. in oxygen-deficient samples or doped with metals, such as Fe or Nb) many other attractive properties arise such as superconductivity, ferromagnetism, high thermoelectric coefficient, blue and green light emission as well as accommodation of a two-dimensional electron gas. We present their exceptional capabilities, in terms of their bulk optical constants, which we have measured with an IR Spectroscopic Ellipsometer and which we compare with the most prominent polar dielectrics (h-BN, Sapphire, InP, GaN and GaP) and metals (such as Au) in an effort to establish a baseline for their performance. A particular focus is given to SrTiO₃which demonstrates a unique capability to maintain negative real permittivity for an extended spectral range due to an effective overall between two Reststrahlen bands and could throw it directly into centre stage for future infrared nanophotonic applications. We then calculate the localised modes on subwavelength structures and we: (i) clarify the fundamental nature of the modes, (ii) we demonstrate an extraordinary tunability that SrTiO₃ offers in the mid and far infrared wavelengths and (iii) we present a practical example of IR photodetection based on this material.

Carbon Nanotubes have been extensively studied in more than two decades for their physical and chemical properties, and more recently in their industrial applications, including wafer scale nano-electronic devices and nanocomposite structures. The interest in carbon nanomaterials has been greatly enhanced by the rise of graphene, the two dimensional analog of the one-dimensional tubes. Due to their extreme scaled-down geometry, particularly the diameter, a small perturbation in their structure could induce large changes in their electronic properties. However, such control of their structure is difficult to achieve due to processing complexity in a growth reactor. In this work, we use a fast, scalable and room temperature process to alter the nanotube structure by exploiting the light-matter interactions from a femto-second laser radiation. Multi-wall carbon nanotubes are irradiated with pulsed laser beam to modify the tube structures into ribbon-like geometry that may lead to unique electronic and photonic properties. In this work, we use scanning electron microscopy, atomic force microscopy and Raman spectroscopy to study the structural properties of these carbon nanomaterials that may be suitable for coupling with photons.

Controlling the micro- and nanostructure of metals offers the opportunity to investigate and engineer the properties of metal nanotechnologies, which are essential components in advanced optical and electronic systems. The electronic, optical, and thermal properties of these systems are closely related to the presence of strain, defects, and grain boundaries in the metal. Carrier transport, band structure, electromagnetic field enhancements, and thermoelectric properties are all dependent on crystal quality, but there is still an incomplete understanding of the role that individual crystal defects play.
Here, we report a study of the thermoelectric properties of thin-film gold single crystals and bicrystals on amorphous insulating substrates. The crystalline gold microstructures are created via rapid melt growth, which supports the high-yield, high throughput growth of thousands of single crystals and bicrystals on the wafer-scale [1,2]. Using the crystalline gold microstructures as a model system, we can enable systematic studies that reveal the relationship between the micro- and nanostructure of gold and its properties. We studied the spatial variation of the photothermoelectric voltage across the gold stripes using a focused scanning laser probe, which locally heats the metal via direct absorption and excitation of a plasmon resonance. In the bicrystals that each possess a single grain boundary, we observe a thermoelectric voltage distribution where the local Seebeck coefficient varies at the grain boundary, indicating strong carrier interaction with the boundary. Furthermore, variations in Seebeck coefficient along the length of the single crystal gold grains suggest that other crystal defects and strain play a role in determining the thermoelectric properties.

[1] K. Zhang, X. B. Pitner, R. Yang, W. D. Nix, J. D. Plummer, J. A. Fan, Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 685.
[2] L. T. Gan, R. Yang, R. Traylor, W. Cai, W. D. Nix, J. A. Fan, Adv. Mater. 2019, accepted.

Imaging in short-wave infrared (SWIR, nominally 1 to 3 μm) range has many applications in optical communications, chemical sensing, night vision, and so on. Current solutions to SWIR applications are mainly based on III-V materials (InGaAs, InSb), lead salts (PbS, PbSe, and PbTe)， and mercury cadmium telluride (HgCdTe). Group-IV material based photo detector monolithically grown on Si enjoys benefits from low cost and large volume CMOS compatible process.
Ge based photo detectors have been widely investigated for Si photonics. However, the response of Ge based photo detectors drops significantly at ~1.55 μm, corresponding to its direct bandgap. Incorporation of Sn into Ge could effectively enhance the absorption of Ge and at the same time extend the absorption cutoff to longer wavelength.
Among SWIR, 2 μm has recently been proposed as the next communication window. Optical communication has long been the flagship application of Si photonics. Advancement of low loss hollow-core photonics bandgap fibers and thulium doped fiber amplifiers have shown the great potential of this new spectral window. Thus, research on photonic components operating at this range is highly desired.
In this presentation, we report the demonstration of GeSn high speed photo detector and GeSn avalanche photo detector with operation at 2 μm window. Both photo detectors are directly grown on Si substrate. For the high speed photo detector, low leakage and beyond 10 GHz operation are demonstrated. The avalanche photo detector shows a high optical responsivity as compared to p-i-n photodiode due to internal avalanche gain, which is beneficial for low-light-level detection. In addition, Ge-on-insulator (GeOI) photonics platform has been proposed for 2 μm applications. GeOI based photonic components could have better performance than silicon-on-insulator (SOI) counterparts at this new spectral window, due to the superior optical properties of Ge over Si. High performance GeSn photo detector has been demonstrated on this platform. Ge has also been considered as a promising channel material to replace Si for high performance logics applications. This platform also enables the potential of integrating photonics devices with Ge CMOS devices.

The ability to tune the optical spectra of nanostructures such as nanowires (NWs) through optical coupling is valuable for optoelectronic applications. The optical coupling of two Si NWs horizontally placed without active tunability has been studied. In this case, a part of optical resonant modes (symmetric mode) is only excited under top planewave illumination. However, the optical coupling of vertically stacked suspended NWs has not yet been explored due to its difficulty to reliably fabricate the system. This configuration is particularly advantageous in that both symmetric and asymmetric modes can be simultaneously excited when illuminated because they naturally receive different phase information. Moreover, the active tuning of the system to engineer the strength of optical coupling has not been demonstrated. In this study, we demonstrate a novel method to fabricate vertically stacked (3D stacking) suspended SiGe NWs. Furthermore, photodetectors are realized with active tunability by electrostatic force applied between the two suspended NWs.
High quality pseudomorphic SiGe (30% Ge)/Si multilayers are epitaxially grown on Si substrate by chemical vapor deposition (CVD) and patterned by e-beam lithography. Then, the SiGe nanobeams are released by tetramethylammonium hydroxide (TMAH), followed by electrode formation on the SiGe NWs. Here, Ge composition in NWs can be increased by high temperature oxidation of the NWs. Electrostatic force applied between the SiGe NWs is used to modulate the spacing in between them. Spectral changes in both scattering and absorption through photocurrent measurement are monitored with various spacings ranging from 200nm to 50nm.
As a result, the proposed meta-photodetectors with the active tunability pave a new way for the next generation CMOS compatible on-chip optoelectronic devices that can be monolithically integrated with nanoelectronics.

Phase-contrast imaging is an optical method which visualizes the optical phase differences that light acquires as it traverses structures with similar transparency. The inventor of phase-contrast microscopy, Frits Zernike, demonstrated successful imaging of unstained cells. The underlying key principle of this technique is to interfere the scattered light by a specimen with the high spatial-frequency content with an attenuated, 0.5π phase-retarded illumination containing the low spatial frequencies (i.e. DC component). This maximizes the intensity contrast in the interference images. Conventionally, this requires a set of bulky Fourier optics components for filtering and selective phase retarding elements, limiting its use in certain applications.
Here, we propose ultracompact phase-contrast Fourier optics elements with unprecedented high angular accuracy by using a planar guided-mode resonance structure. We designed 40-nm-thick, 160-nm-wide Si₃N₄ gratings on a 130-nm-thick Si₃N₄ slab waveguide with a lattice constant of 370 nm. The structure can be used to perform phase-contrast microscopy by interfacing the normally incident planewave to the guided-mode resonance at a 645 nm illumination wavelength. This facilitates 90% amplitude reduction and 0.5π phase pickup for the DC incident illumination while leaving the scattered, non-DC Fourier components from a specimen passing through with near unity transmission. We deposit the Si₃N₄ waveguide on a quartz substrate by low-pressure chemical vapor deposition (LPCVD) and fabricate the grating with electron beam lithography and reactive ion etching (RIE). We also characterize the optical dispersion properties of the fabricated samples in a home-built angle-resolved-spectroscopy setup and observe the high quality guided-mode resonances (Q~100). This Q corresponds to ~0.5° angular accuracy in selecting the Fourier components for amplitude reduction and phase retardation. Our guided-mode resonance structures can be used in a conventional optical microscopes and we expect this opens a promising avenue for compact, flexible, and high-quality phase-contrast microscopy.

In this talk we give an overview of our work on solar and laser sailing for deep-space travel in our solar systems and beyond. We will show that with the use of novel materials and lightweight photonic designs, radiation pressure forces may be harnessed to enable ultrafast maneuvering and travel. We will show that both solar and laser sailing may be used to enable revolutionary in-space propulsion. Hence, we will discuss conceptually new solar sail designs that making use of solar gravity assist and can be propelled to unprecedented >30 AU/year velocities. These sails maybe ideally fitted for interstellar probe missions. The close solar approach requires design of novel photonic materials capable of withstanding high solar flux. Laser sailing, on the other hand, may pave the way to near speed of light (20% of the speed of light with a ~100 GW laser) space travel enabling exploration of interstellar space and neighboring stars. We will highlight the conditions needed, discuss sail materials challenges, and provide a brief overview of problems such a mission might face. We will than show that with a smart photonic design many of the challenges facing laser propulsion may be alleviated.

Self-organized multiscale porosity in terms of precise pore size, shape, and orientation has been achieved in many base materials. Here we exemplify that in combination with self-assembly of soft fillings in pore space this provides particularly versatile pathways for the engineering of functional materials [1]. First, we present a nanoporous semiconductor/polymer hybrid with tunable electro-strain and will relate the macroscopic deformation to meso- and nanoscale electroactuation. Second, we show that embedding liquid crystals in nanoporous solids provides novel opportunities for subwavelength control of light-matter interactions on the single-pore scale and thus to fine-tune the optics of these materials. To that end we present reciprocal space mappings employing synchrotron-based 2D X-ray diffraction in combination with high-resolution birefringence experiments on disk- and chiral rod-like molecules confined in monolithic nanoporous silicon, silica and alumina. As a function of pore hydrophilicity and thus distinct molecular anchoring at the pore walls we observe a remarkably rich self-assembly behavior, unknown from the bulk state, such as a quantized formation of concentric discotic rings [2], a transition from axial to radial aligned discotic columns and the formation of pore-axis aligned supermolecular helices [3]. Intimately related with this surprising self-organization at the nanoscale the soft-hard hybrid materials exhibit novel metaphotonic functionalities encompassing optical anisotropy step-wise changing with temperature [2], enhanced light rotation and extremely fast electro-optically active Goldstone excitations typical of para-to-ferroelectric phase transitions [3]. Self-assembly of radial aligned rings and axial aligned columns of disk-like molecules in cylindrical alumina nanopores as evidenced by X–ray diffraction. This results in an either prolate or oblate ellipsoid of refractive indices (indicatrix) aligned to the pore axis direction. Thus, linear polarized light is split up by a parallel array of such pores into two beams with perpendicular polarizations and distinct propagation speeds. Their relative phase shift (retardation R) after passing the birefringent pore array is positive or negative, respectively, and vanishes upon heating to the isotropic liquid state.
References:
[1] P. Huber: Soft matter in hard confinement: phase transition thermodynamics, structure, texture, diffusion and flow in nanoporous media. Journal of Physics: Condensed Matter 2015, 113, 103102.
[2] K. Sentker, A.W. Zantop, M. Lippmann, T. Hofmann, O.H. Seeck, A.V. Kityk, A. Yildirim, A. Schoenhals, M.G. Mazza, and P. Huber: Quantized self-assembly of discotic rings in a liquid crystal confined in nanopores. Physical Review Letters 2018, 120, 067801.
[3] M. Busch, A.V. Kityk, W. Piecek, T. Hofmann, D. Wallacher, S. Calus, P. Kula, M. Steinhart, M. Eich, and P. Huber: A ferroelectric liquid crystal confined in cylindrical nanopores: Reversible smectic layer buckling, enhanced light rotation and extremely fast electro-optically active Goldstone excitations. Nanoscale 2017, 9, 19086.

The free charge carrier concentration of semiconductors, such as in silicon, is tunable over several orders of magnitude. Thus, the plasma wavelength of these materials can be adjusted over a wide spectral range by controlling the doping concentration. Comparable to noble metals in the ultraviolet/visible spectral region, highly doped semiconductors possess 'metal-like' optical properties but typically limited to the mid infrared region due to the solubility limit of dopants.

We demonstrate that the range of accessible plasma wavelengths can be extended far into the near-infrared region reaching the telecommunication wavelength by hyperdoping of silicon – doping beyond the solubility limit - using two non-equilibrium processes: high fluence ion beam doping and subsequent pulsed laser annealing. Laser annealing using high intensities over very short time scales leads to surface layer melting followed by rapid resolidification, which allows exceeding the solubility limit.

Further, we demonstrate how area selective activation of dopants by focused laser annealing can be used as a fabrication-friendly platform for flat optical and plasmonic devices. Detailed characterization of the optical properties of silicon is performed at various doping levels, and diffractive optical elements and plasmonic surfaces that operate in the near-to-mid infrared regime are presented. Our resulting devices are monolithic, flat, resilient to thermal and physical damage, and can be easily integrated into other silicon-based platforms.

Planar nano-optical elements beyond the diffraction limit are envisioned to replace the conventional refractive optics and are already providing needed functionalities to build lenses, beam deflectors, holograms and polarizing interfaces [1]. Their key elements are the nanoantennas that operate via plasmon or Mie-type optical resonances and are able to confine light on the nanoscale and modify its fundamental properties such as scattering, directionality, polarization and phase. However, a significant demand exists for the real-time steering of light in such optics. Here, we combine for the first time Mie-resonant and plasmonic nanoantennas and the light-responsive elastomers to create a fundamentally new dynamic / real-time tunable optical platform for reflected or transmitted light control.

We build a tunable optical platform based on nanoantennas and stimuli-responsive polymers, already well known to the fields of haptics and soft robotics, and also as artificial muscles, due to their strong yet soft actuation properties. For that, we employ liquid crystalline elastomers (LCEs) that are a special type of elastic polymers with self-organization and the ability to reversibly change their microscopic shape in response to the external stimuli such as temperature, irradiation with light or the electric fields (via resistive heating or piezo-like response) [2]. We further functionalize the LCE with photoswitchable molecules (azo- derivatives) incorporated to the molecular network, which makes elastomers light-responsive and capable of producing large mechanical strains upon illumination. By spacing the nanoantenna’s elements with LCE we change their mutual positioning upon actuation, thus providing the change in scattered light’s propagation, polarization and phase. We also demonstrate that the desired photomechanical strains and modified optical response can be programmed by careful chemical engineering of the molecular network. For that, we employ lithographic techniques such as direct laser writing (DLW) to produce high-resolution three-dimensional (3D) nano- and micro-scale structures and add the nano-mechanical functionality to optically resonant nanoantennas [3].

[1] Genevet, P. et al. Optica, 4, 1, 139 (2017).
[2] Ohm, C. et al. Adv. Mater. 22, 3366 (2010).
[3] Zeng, H. et al. Adv. Mater. 26, 2319-2322 (2014).

Since successful demonstration of graphene as a monolayer 2D material, scientific community significantly expanded layered van der Waals materials family with different electronic, optical, mechanical and thermal properties which are pivotal in the quest for miniaturized photonics. Here, we investigate the optical properties of α-MoO₃ both theoretically and experimentally.
Due to strong anisotropy of α-MoO₃ in all 3 dimensions, it is expected to show three distinct Reststrahlen (RS) bands in mid-IR wavelengths which can give rise to the excitation of phonon polaritons within these RS bands. Phonon polaritons have significant implications for optical device design due to their unique properties such as wavelength shrinking and low loss. The optical properties of α-MoO₃ can adequately be described using the phenomenological Lorentz oscillator model with parameters estimated in literature. Applying this relation, the complex dielectric constant of α-MoO₃ is calculated which is in turn used to obtain the dispersion relation of α-MoO₃ in x,y and z directions. After confirming our optical model, we propose a thin film multi-layer structure in order to experimentally examine the traces of the mentioned three RS bands through enhanced absorption as a result of phonon polariton excitation. The proposed structure, from bottom to top, is composed of thick Au, Ge and transferred α-MoO₃. In particular, two samples with two different thicknesses of Ge (400 nm and 800 nm) are fabricated. The reflectance (R) versus wavenumber measurements are carried out using FTIR to obtain the total absorption (A). Since the thick Au suppresses any transmission, the total absorption can easily be calculated using the R data. The FTIR results illustrate absorption peaks around 800 cm^-1 (12 μm) and 550 cm^-1 (18 μm) which represent the x and y direction (in-plane) phonons respectively. Furthermore, polarized incident light measurements in FTIR emphasize polarization-dependent absorption in α-MoO₃ which is an explicit outcome of perpendicular x and y phonons, where one of the in-plane resonance peaks is maximized only when the other one disappears.
In order to explain the observed results in further depth, simulations are carried out using TMM and FDTD methods which are in agreement with the experiments. In order to have accurate simulation results, the thickness of α-MoO₃ flakes are measured by AFM and introduced to simulation. Our simulation results show Fabry-Perot (FP) effect in the Au/Ge/α-MoO₃ structure which enhances the absorption. Specifically, for the sample with 800 nm thick Ge, the FP resonance is intentionally designed to occur in the vicinity of the x-phonon peak. Therefore, the x-phonon peak is intensified which further supports our discussion.
Since our measurement method relies on normal incidence measurement, there is no electric field component in z-direction. As a result, excitation of z-phonons is not viable. In order to circumvent this issue, we have designed and fabricated a new structure. A patterned layer of periodic Au nanodisks is added to the top of α-MoO₃ in Au/Ge/α-MoO₃ structure. When illuminated, the nanodisks diffract light and facilitate the coupling of radiation to the z-phonons of α-MoO₃. The absorption peak near 10 cm^-1 (10 μm) can be spotted in both experiments and simulations which justifies the observation of z-phonon polariton excitation. The electric field simulations are in line with the expected enhancement of z-component of the electric field.
The outstanding anisotropy of α-MoO₃ in three directions can open new paths for mid-IR optics and it can provide unprecedented opportunities to engineer low-loss optical devices for the crucial mid-IR atmospheric window (8 – 12 μm).

Metasurfaces are 2D metamaterials; they have exciting optical behaviors and interesting applications as ultrathin optical devices for nanoscale-resolution imaging, sensing, holograms and nanodisplay. Plasmonic metasurfaces, studied so far have inherent metallic-loss in the visible regime, so they have poor color saturation. Instead of plasmonic coloring, all-dielectric metasurfaces such as silicon metasurfaces offer an alternative way to generate colors by exploiting Mie resonances [1,2]. Nevertheless, for the next step toward real-life applications, the metasurface coloration demand a broad color spectrum as well as dynamic/tunable functionality which many of research are dealing with recently [3]. Therefore, we should consider three main aspects: material selection, structural design and the dynamic colour mechanism. All-dielectric materials are preferred to avoid optical losses that exist in conventional metals. The metasurfaces should be designed based on deep physical understanding of the nanoantenna’s scattering behavior to achieve vivid and bright colours. Lastly, the dynamic colour mechanism requires a fast response and the compatibility to work with electrically driven devices.
Here, we would like to introduce our significant effort which realize cryptographic nanoprints by metasurface coloration and thus will achieve a substantial step forward towards enhanced security printing applications [4]. 1) Our hydrogenated amorphous silicon (a-Si:H) metasurfaces achieve diverse colours due to high refractive index and near-zero extinction coefficient of a-Si:H. 2) We deploy nanocuboid-antennas which is designed based on Kerker’s condition for tunable functionality. Unidirectional scattering tuned by the asymmetric nanoantenna’s spatial dimensions and polarization state enable a wide colour gamut and modulation range. 3) We design and experimentally demonstrate cryptographic nanoprints that contain encrypted optical information under unpolarized light, where the hidden information can be decoded under polarized light of a specific angle. This dual optical data storage enables doubly-secured QR codes as cryptography and microprints as steganography, respectively. We success to scan those QR codes with smartphone scanner. Consequently, we were able to connect nano planar optics with smartphone technology spread out all over the world. This research hastens the date when the meta-prints technology will be deployed on a commercial use.

References
[1] S. Sun, et al. “Real-Time Tunable Colors from Microfluidic Reconfigurable All-Dielectric Metasurfaces,” ACS Nano 12, (2018).
[2] Z. Dong, et al. “Printing Beyond sRGB Color Gamut by Mimicking Silicon Nanostructures in Free-Space,” Nano Lett. 17, 7620–7628 (2017).
[3] X. Duan, S. Kamin, N. Liu, “Dynamic plasmonic colour display,” Nat. Commun. 8, 14606 (2017).
[4] J. Jang, et al. “Kerker-Conditioned Dynamic Cryptographic Nanoprints,” Adv. Opt. Mater. 7, 1801070 (2019).