Symposium EL05—Advanced Functional, Linear/Nonlinear and Quantum Materials for Metasurfaces, Metamaterials and Nanophotonics

A class of planar and wavelength-thick optical components exhibiting exceptional optical properties have emerged in recent years. These artificial interfaces, known as metasurfaces, can manipulate the wavefront of light in almost any desired manner, leveraging on the scattering properties of the subwavelength nanostructures. To further develop this technology towards dynamic tuning, broadband applications and industrial production, new materials and new fabrication methods have been proposed.

In this tutorial, I will discuss basic design and fabrication methods of metasurfaces and summarize various applications for beam steering, polarization control and monolithic integration of metasurfaces in opto-electronic systems. As an alternative of conventional bulky, the development of this technology is expected to create a positive disruption in modern optical technologies, in particular in the fields of imaging, holography, 3D dynamic image rendering, AR/VR and LiDAR systems.

Inverse design, in which the design process is performed through iterative optimization, has the potential to push metasurface performance to the physical limits of composite materials engineering. In this tutorial, we will discuss a range of state-of-the-art numerical optimization methods for metasurface design. We will introduce the objective-first and adjoint variables methods, which are gradient-based optimization concepts that can produce high performance freeform geometries. We will also provide an overview of machine learning techniques as applied to electromagnetics problems and show how generative neural networks can be harnessed as an effective global optimizer for photonic devices.

Dielectric metasurfaces are sub-wavelength diffractive optics, which can shape the phase, amplitude and polarization of the incident optical wavefront with high spatial resolution. While theoretically conceived decades ago, the availability of sophisticated nanofabrication techniques and computational techniques have recently rejuvenated this research field. In this tutorial, I will present the current state of the design and fabrication techniques of different metasurfaces. Going beyond single metasurface, I will present results with composite metasurfaces as well as tunable metasurfaces. Finally, I will show how computational post processing techniques coupled with metasurfaces can help create ultra-low-power and low-latency sensors.

Metasurfaces allow the manipulation of the amplitude, phase, and polarization of light on an ultrathin platform, and have started to be explored also in the context of digitizing optical holograms. To increase the bandwidth of a metasurface hologram, essential for high-capacity holographic memory devices, different properties of light including polarisation, wavelength, and incident angles have been exploited for holographic multiplexing; however, the bandwidth of a metasurface hologram has remained too low for any practical use.

We present the design of a complex-amplitude metasurface hologram for ultrahigh-dimensional OAM-multiplexing holography in momentum space. To realise a complex-amplitude Fourier hologram, we have introduced an OAM diffuser array with a random phase function, capable of scaling down the amplitude variation in a typical Fourier image as well as eliminating the coherence of holographic image channels. We will demonstrate first realizations of holograms based on this concept.

Transparent conducting oxides (TCOs) continue to play an important role in optoelectronic applications due to the unique combination of optical transparency and electrical conductivity but recently TCOs have become an emerging class of nanophotonic materials with tailorable optical properties by synthesis, post-processing and optically generated free carriers [1].
Here we report on the Nd doped ZnO thin films with tunable epsilon-near-zero wavelength in the near-infrared region obtained by controlling the doping and the growth parameters. Epitaxial wurtzite Nd doped ZnO thin films with a wide range of optical and electrical properties were grown by pulsed electron beam deposition (PED) on c-cut single crystal substrates at relatively low substrate temperatures and gas pressures. PED is a well-established ablation method to grow thin films and has common features with the pulsed laser deposition [2, 3]. Rutherford backscattering spectrometry, X-ray diffraction and pole figure measurements were performed to determine the precise texture and in-plane epitaxial relationships between film and substrate in correlation with the electrical resistivity, mobility, carrier concentration and optical properties of the films. The physical basis of the metallic conductivity at room temperature of ZnO thin films leading to the observed optical tunability will be discussed, in particular the ability to precisely control the oxygen deficiency in these films together with the role of the Nd doping in both the carrier concentration and the structural disorder. This work demonstrates that degenerately ZnO thin films are promising epsilon-near-zero materials for nonlinear optical applications and nanophotonics. [1] S.Saha et al, Materials Today (2020), in press; [2] M. Nistor et al., RSC Adv. 6, 41465 (2016); [3] M. Nistor et al. Mater. Sci. Semicond. Process. 88, 45 (2018).

As applications in fields like security or medicine require sensitive schemes in order to detect IR photons, an interesting strategy consists in converting weak IR signals into the optical domain where detectors with single photon sensitivity are readily available. We introduce here a novel platform for ultra-sensitive conversion and detection of far and mid-infrared signals, which is inspired by our previous work where we describe the interaction between molecular vibrations and plasmonic antenna using the model of cavity optomechanics. Our study quantified the nonlinear coupling rateand revealed that it could be as high as tens of THz. We also predicted signatures of optomechanical amplification that should be observable in state-of-the-art systems. Novel plasmonic platforms could thus enable the realization of protocols inspired by cavity quantum optomechanics.

The protocol that we suggest here benefits from the intrinsic ability of specific molecular vibrations to interact both with optical and IR fields as routinely observed in Raman and resonant absorption spectroscopy. To insure an optimal overlap between the two beams and the molecular system, doubly resonant nano-antennas confine the fields into similar mode volumes and increase therefore the efficiency of the conversion process.

In this conversion scheme, an incoming IR field drives resonantly a vibrational mode and modifies its excited state population, which is mapped onto the scattered anti-Stokes Raman signal produced during the interaction between the same vibrational mode and an optical pump beam. When the optical beam is red-detuned from the plasmonic resonance the interaction Hamiltonian reduces to a state swapping Hamiltonian:, enabling an efficient optomechanical conversion process. Consequently, the modified vibrational population gives rise to an additional emission of coherent optical photons on the anti-Stokes sideband that can be detected with existing single photon counting techniques.

Our study demonstrates that the noise equivalent power (NEP) can be as low as few pWxHz^-1/2, improving on the state of the art for devices operating at room temperature. In addition to its low noise figure the sub-wavelength dimensions of the proposed converter promises the development of multi-spectral systems designed for IR source recognition and novel technological platforms harnessing the coherent nature of the conversion process.

Since the development of diffractive optical elements in the 1970s research has focused on replacing bulky optical elements such as lenses and grating by thin counterparts. Over the last decade, nanophotonic metasurfaces rapidly advanced the development of flat optical elements based on the realization that resonant optical antenna elements enable local phase control. Present applications of metasurface flat optical elements include lenses, polarization control, and beam steering.

Next-generation applications of flat optics such as light detection and ranging (LIDAR), dynamic holography, and computational imaging require dynamic control over optical functionalities, e.g. the focal position or efficiency of optical elements. However, most nanophotonic structures are static after design and fabrication. Current approaches for dynamic control like electrical gating exhibit limited tunability due to the finite electrorefraction and electroabsorption effects in metals and semiconductors.

Here, we demonstrate actively-tunable and atomically-thin optical lenses by carving them directly out of monolayer transition-metal dichalcogenides (TMDs) like WS₂ with a strong excitonic resonance in the visible spectral range. This turns the 2D material into the antenna or metamaterial and incorporation of active materials into larger antenna structures will no longer be needed. Due to their sub-nm thickness, these materials are highly tunable through external control. We fabricate 1 mm diameter lenses with a 2 mm focal length by patterning large-area monolayer WS₂ on sapphire using nanolithography and reactive-ion etching. Using an electrochemical cell, we electrostatically control the carrier density in the monolayer WS₂ and thereby gain active control over the excitonic light scattering amplitude. Using confocal scanning microscopy, we characterize the focal shape and analyze the focal efficiency.

We demonstrate dynamic electrical tuning of the focusing efficiency with a 33% modulation depth through manipulating of the excitonic material resonance properties as opposed to tuning of antenna resonances. The highly tunable nature of these exciton resonances opens an entirely new approach for the design of dynamic flat optics and metasurfaces with applications in free-space beam tapping, wavefront manipulation, and augmented/virtual reality.

Metasurfaces rely on the resonances of nanostructures with dimensions in the nanometer range for devices working in the visible regime. The corresponding fabrication technology requires E-beam exposure and can be quite challenging. It is prone to defects associated with the many subtle process steps during fabrication,
resulting in missing, distorted, or displaced nanostructures within the metasurface. In this work, we analyze the robustness of gradient metasurfaces to a variety of such defects using both simulations and experiments on purposely miss-fabricated samples. Surprisingly, we observe that a "perfect" structure, without any defect, is not that which exhibits the best performance in terms of efficiency and angular response. We demonstrate that specific, well-controlled, defects can actually significantly enhance the performance of such a metasurface and explain this behavior using the resonance and near-field properties of the corresponding nanostructures. This work sheds very new light on the design strategies for metasurfaces and significantly alleviates some constraints on their fabrication accuracy.

Hybrid plasmonic waveguides are often considered as an alternative to traditional plasmonic systems in integrated photonics, as they exhibit significantly smaller losses while maintaining small structure size [1]. Nonlinear optics is another field of application for hybrid plasmonic waveguides [2]. Multiple groups have considered using both plasmonic and hybrid plasmonic waveguides to create nonlinear couplers [3,4,5]. Most of these configurations use silicon and silicon oxide in order to remain compatible with silicon-on-insulator technological framework [4]. Other proposals used highly nonlinear organic materials, such as DDMEBT or paratoluene sulphonate [5] to achieve stronger self-modulation response.
In this work we propose hybrid plasmonic waveguides based on high-index dielectrics, such as silicon, germanium, germanium telluride and germanium antimony telluride. High refractive index allows us to introduce materials with higher nonlinear response, although low refractive index contrast leads to increase of modal area. Additionally, high-index materials provide efficient coupling to dielectric waveguides with good mode confinement. We also propose fabrication method for "two-ridges-one-slab" nonlinear coupler, based on common deposition and litography technology. We use coupled mode theory based on Generalized Nonlinear Schrodinger Equation [6] to evaluate coupler performance. While nonlinear figure-of-merit remains relatively low (F=γ*L_prop ∼0.2 W^-1) due to two-photon absorption, we report significantly smaller coupling distances (down to several micrometers), which allows to minimize the coupler. Our simulations show that influence of both conductor nonlinear response and nonlinear coupling processes is negligible in hybrid plasmonic waveguides.

[1] Han, Z. H. and S. I. Bozhevolnyi (2013). "Radiation guiding with surface plasmon polaritons." Reports on Progress in Physics 76(1).
[2] Li, G. Y., et al. (2016). "Figure of merit for Kerr nonlinear plasmonic waveguides." Laser & Photonics Reviews 10(4): 639-646.
[3] Salgueiro, J. R. and Y. S. Kivshar (2010). "Nonlinear plasmonic directional couplers." Applied Physics Letters 97(8).
[4] Dai, D. X. and S. L. He (2009). "A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement." Optics Express 17(19): 16646-16653.
[5] Pitilakis, A. and E. E. Kriezis (2013). "Highly nonlinear hybrid silicon-plasmonic waveguides: analysis and optimization." Journal of the Optical Society of America B-Optical Physics 30(7): 1954-1965.
[6] Poletti, F. and P. Horak (2008). "Description of ultrashort pulse propagation in multimode optical fibers." Journal of the Optical Society of America B-Optical Physics 25(10): 1645-1654.

In this talk, I will present some of our most recent results in one of our ongoing research projects in my group, namely, how to manipulate and tailor light using four-dimensional (4D) metamaterials, i.e., structures with judiciously chosen spatial and/or spatiotemporal inhomogeneities, in order to provide exciting functionalities. Several topics, including metamaterial computing machines, “time-vs-space scattering”, temporal anisotropy, spatiotemporal control of diffusion, and temporal cladding will be presented. Connection of these concepts with the photonic networks such as the Mach-Zehnder interferometers will also be given, and possible merging of these platforms will be mentioned. Potential applications of these results will also be discussed.

The generation of optical frequency combs has fundamentally advanced optical metrology and high precision measurements. Recently, frequency combs of acoustic phonons have been demonstrated in nonlinearly driven micromechanical resonators [1]. We take the theory underlying this comb generation to the atomic scale and investigate the possibility of optical phonon frequency combs [2]. Ultrashort laser pulses can be used to coherently excite infrared-active phonons, leading to atomic vibrations with large amplitudes. Nonlinear couplings between different phonons then arise from anharmonicities in the interatomic potential of the crystal lattice, which can be used for comb generation. A particularly challenging factor plays hereby the short lifetime of optical phonons, often on the order of several picoseconds. Using a combination of first-principles calculations and phenomenological modeling, we determine the atomistic parameters of materials that are required for a clear discrete spacing of frequency lines in the comb spectrum of the optical phonons. We propose a set of promising material candidates for displaying optical phonon frequency combs that would set a milestone in nonlinear phononic control of material properties.

[1] A. Ganesan, C. Do, and A. Seshia, Phys. Rev. Lett. 118, 033903 (2017)
[2] D. M. Juraschek, A. Ganesan, and P. Narang, in preparation

This work is supported by the Swiss National Science Foundation (SNSF) under Project No. 184259, the DARPA DSO under the Driven Nonequilibrium Quantum Systems (DRINQS) program, Grant No. D18AC00014, and by the Department of Energy ‘Photonics at Thermodynamic Limits’ Energy Frontier Research Center under Grant No. DE-SC0019140. This research used resources of the National Energy Research Scientific Computing Center (NERSC) under Contract No. DE-AC02-05CH11231.

The mid-infrared (MIR) spectral range is often referred to as the molecular fingerprint region due to the multitude of molecular vibrational signatures it contains. As such, research focused on developing MIR optical sources of sufficiently narrow bandwidth, minimal power demands and small form factors are of great interest for potential spectroscopic and sensing applications such as bio- and chemical sensing, as well as the detection of harmful gases. One approach for such applications that has garnered significant attention recently has been frequency-selective thermal emitters. Here, by judiciously selecting and/or structuring semiconductor materials, the thermal photonic density of states can be tailored such that frequency-dependent far-field impedance matching, and therefore absorptivity, is achieved. Reciprocally, through Kirchhoff’s law, this results in an emissivity of equivalent direction and magnitude. Here we report on a powerful approach towards realizing narrowband thermal emitters with a high degree of frequency selectivity, without sacrificing narrow emission linewidths that is typically an issue with plasmonic-based emitters, through the inverse design of aperiodic Tamm plasmon (TP) devices.

TPs are optical interface states that form between a distributed Bragg reflector (DBR) and a metal or between two dissimilar DBRs. These excitations exhibit a parabolic dispersion that falls within the photonic bandgap of the DBR and the air light cone and are therefore accessible from free space without the need for expensive and time-consuming lithographic and etching fabrication steps. Here, we employ a gradient descent regression (GDR) algorithm to design TP-supporting films in the metal-DBR geometry and grow films that reproduce the predicted spectral features of our designs with great success. We utilize n-CdO deposited through high-power impulse magnetron sputtering as our metal layer. This highly-promising transparent conducting oxide (TCO) has been demonstrated to exhibit broad spectral tunability of the plasma frequency, while maintaining exceptionally low optical losses. This is due to CdO possessing both a low effective mass (ranging from 0.12 – 0.26 in epitaxially-grown films with carrier densities ranging from 10¹⁹-10²⁰ cm^-3) as well as electron mobilities extending upwards to 500 cm²/V-s. As TP modes correspond with the impedance-matched condition of the DBR and the metal film, having such control over the impedance of both the DBR (through changes to individual layer thicknesses and dielectric index), and the CdO layer (through changes to the carrier density and layer thickness), grants significant flexibility to our design.

Inverse design has been used to design TP-supporting films in the past, however, these efforts have relied on computationally-expensive techniques, such as genetic algorithms, Bayesian optimization or deep learning, often requiring several hours or days to reach a final solution for a single-peak emission spectrum. In contrast, our GDR approach is capable of arriving at a solution on the timescale of seconds or minutes, all while running on a consumer-grade CPU. As we demonstrate, this opens the door to realizing thermal emitters with varying levels of spectral complexity. For example, realizing arbitrarily positioned single- and multi-peak thermal emission spectra, which can accurately match to the IR absorption spectra of greenhouse gases such as CO₂ and N₂O. We are also able to achieve quality-factors that far-exceed conventional plasmonic devices (Q > 300 for designed films), and control the full TP-dispersion and therefore spatial coherence of the thermal emission, all while maintaining a simple, planar structure. Therefore, the design principles used here outline a highly-tunable and potentially scalable platform for realizing applications such as filter-less non-dispersive infrared gas sensing and free-space communications.

This talk will discuss two recent developments related to engineering light emission. First, I will demonstrate a passive down-conversion imaging system that converts broadband ultraviolet light to narrow-band green light using perovskide nanocrystals, while preserving the directionality of rays, and thus enabling direct down-conversion imaging. The system preserves high transparency in the visible, enabling superimposed visible and ultraviolet imaging.

Then, I will discuss our design of a nanoscale light extractor (NLE) that enables efficient outcoupling and beaming of broadband light emitted by nitrogen-vacancy centers in diamond. The NLE consists of a patterned silicon layer on diamond and requires no etching of the diamond surface. Our design process is based on adjoint optimization using broadband time-domain simulations and yields structures that are inherently robust to positioning and fabrication errors.

Discovering novel, unconventional optical designs in combination with advanced machine-learning assisted data analysis techniques can uniquely enable new phenomena and breakthrough advances in many areas including on-chip circuitry, imaging, sensing, energy, and quantum information technology. Topology optimization, which has previously revolutionized aerospace and mechanical engineering by providing non-intuitive solutions to highly constrained material distribution problems, has recently emerged as a powerful architect for advanced photonic design[1]. Compared to other inverse-design approaches that require extreme computation power to undertake a comprehensive search within a large parameter space, topology optimization can expand the design space while improving the computational efficiency. This talk will highlight our most recent findings on 1) merging topology optimization with artificial-intelligence-assisted algorithms and 2) integrating machine-learning based analysis with photonic design and quantum optical measurements. Particularly, we will discuss our studies on implementing deep-learning assisted topology optimization for advanced metasurface design development[2-4]. Specifically, we will cover different generative network-based optimization strategies, covering local and global optimization frameworks. We will summarize our research on merging topology optimization technique with quantum device design for achieving ultrafast single-photon source that offers efficient on-chip integration. Finally, we will also describe our recent works on implementing a novel convolutional neural network-based technique for real-time material defect metrology at the quantum level that outperforms all existing approaches in terms of speed and fidelity. This new method rapidly extracts the values of the single-photon autocorrelation function at zero delays from sparse data and ensures one order speed up on solving “bad”/”good” emitter classification problems in comparison with conventional techniques[5]. Along with emitter classification, we will cover the ML-based regression schemes, which open up a way for the realization of rapid antibunching-induced super-resolution imaging applications.
References:
[1] Sean Molesky, Zin Lin, Alexander Y. Piggott, Weiliang Jin, Jelena Vučković, Alejandro W. Rodriguez. Nature Photonics volume 12, pages 659–670 (2018)
[2] Wei Ma, Zhaocheng Liu, Zhaxylyk A. Kudyshev, Alexandra Boltasseva, Wenshan Cai & Yongmin Liu, Deep learning for the design of photonic structures, Nature Photonics (2020) DOI: https://doi.org/10.1038/s41566-020-0685-y
[3] Zhaxylyk A. Kudyshev, Alexander V. Kildishev, Vladimir M. Shalaev, and Alexandra Boltasseva, Machine-learning-assisted metasurface design for high-efficiency thermal emitter optimization, Applied Physics Reviews 7, 021407 (2020); https://doi.org/10.1063/1.5134792
[4] Zhaxylyk A. Kudyshev, Alexander V. Kildishev, Vladimir M. Shalaev, and Alexandra Boltasseva, Machine learning–assisted global optimization of photonic devices, Nanophotonics (2020), DOI: 10.1515/nanoph-2020-0376
[5] Zhaxylyk A. Kudyshev, Simeon I. Bogdanov, Theodor Isacsson, Alexander V. Kildishev, Alexandra Boltasseva, Vladimir M. Shalaev, Rapid Classification of Quantum Sources Enabled by Machine Learning, Advanced Quantum Technologies, Vol. 3, 10, 2020 DOI: https://doi.org/10.1002/qute.202000067

Metasurfaces of high-refractive-index materials, such as silicon, III-V compounds, and similar material groups, enable manipulation of light on a subwavelength scale and support well-defined Mie resonances. These ultra-thin optical nanostructures of high-refractive-index materials can be used at the subwavelength scale for amplitude and phase modulation. Hence, the metasurface supporting Mie resonances has a high potential in effective control of coherent light states. The integration of light-emitting devices, such as VCSELs and VeCSELs, and high-refractive-index metasurfaces is proven to be an efficient method for addressing light-emitting beam-shaping problems.

The monolithic integration approach is a new direction and offers many opportunities in designing beam-shaping VCSELs. The III-V compound nanopillar meta-atoms can be used in polarization-insensitive metasurface at the back-side surface of a bare VCSEL [1]. In this design, each nanopillar functions as an independent resonator with a low-quality factor. The large refractive index of the metasurface elements enables the large transmitted amplitude. Instead of modifying the laser cavity, the integrated III-V compound metasurface acts as a passive element that forms the laser beam profile on the emission surface.

We aim at designing metasurface with the desired phase modulation, and we study properties of III-V compound nanopillars of various dimensions. We study periodic nanopillar arrays on either high-index or low-index substrate with an intermediate layer in the visible and near-infrared frequencies [2]. Arrays of various periodicity, with different heights and radii of nanopillars, as well as thicknesses of the intermediate layer are analyzed. Numerical simulations of III-V compound metasurfaces are carried out for arrays of different arrangements for reflection, transmission, and absorption spectral profiles.

The metasurface supports Mie type and other nanostructure resonances, and with an increase of intermediate layer thickness, the spectral positions of the nanostructure resonances move towards longer wavelengths. We report on the metasurface scattering characteristics to modulate the arbitrary beam shapes employing a phase-matching technique in the least-squares sense. We show the possibility of engineering the VCSEL’s output beam shape by introducing nanopillars of specific arrangement and adding the intermediate high-index layer.

[1] Yi-Yang Xie, et al. Nat. Nanotechnol 15, 125 (2020).
[2] V. Karimi, V. E. Babicheva, Proc. SPIE 11460, Metamaterials, Metadevices, and Metasystems 2020, 114601F (2020).

Engineering coherent systems is a central goal of quantum science and quantum information processing. Point defects in diamond known as color centers are a promising physical platform. As atom-like systems, they can exhibit excellent spin coherence and can be manipulated with light. As solid-state defects, they can be produced at high densities and incorporated into scalable devices. Diamond is a uniquely excellent host: it has a large band gap, can be synthesized with sub-ppb impurity concentrations, and can be isotopically purified to eliminate magnetic noise from nuclear spins. Currently-known color centers either exhibit long spin coherence times or efficient, coherent optical transitions, but not both. We have developed new methods to control the diamond Fermi level in order to stabilize a new color center, the neutral charge state of the silicon vacancy (SiV) center. This center exhibits both the excellent optical properties of the negatively charged SiV center and the long spin coherence times of the NV center, making it a promising candidate for applications as a single atom quantum memory for long distance quantum communication. We have recently discovered bound exciton transitions associated with SiV⁰, which enable efficient optical spin polarization and optically detected magnetic resonance. Finally, I will describe our efforts to integrate SiV⁰ centers in nanophotonic devices, specifically in heterogeneously integrated III-V/diamond nanophotonic platforms designed to enhance the atom-photon interaction and achieve quantum frequency conversion to the telecom band.

A grand challenge for nanophotonics is the realization of comprehensively tunable metasurface nanoantenna arrays enabling dynamic, active control of the key constitutive properties of light – amplitude, phase, wavevector and polarization. Achieving this will open new photonics applications in phased-array optical beam steering, visible light modulation for communication and thermal radiation management. I will discuss the status and outlook for electronically tunable and reconfigurable plasmonic, excitonic and dielectric metasurfaces whose elements are arbitrarily reprogrammable, enabling a wide array of functions, including steering, focusing, and frequency multiplexing of scattered radiation.

Locally enhanced electric fields in nanoplasmonic systems play an important role in a range of sensing and detection applications, including surface enhanced spectroscopies, fluorescence measurements, and refractive index-based biosensing. As these technologies move increasingly towards point-of-care applications, it is critical that sensor substrates maintain high-performance while being low profile, inexpensive, and easily fabricated. Here, we demonstrate tunable field enhancement in a self-assembled plasmonic metasurface via coupling to a stimuli-responsive photonic cavity. Our structure consists of a near-percolation plasmonic gold film on a polymer spacer above a gold mirror, creating a Fabry–Perot nanocavity. Simulations of the as-fabricated nanostructures reveal that the strongest field enhancement occurs when the cavity mode overlaps with the plasmonic top film, resulting in nearly perfect light absorption/trapping by the system. Compared to the nanoislands alone, the cavity-coupled system yields a ~4x increase in field enhancement. The random size and distribution of the plasmonic nanoislands contribute to its strong, broadband absorption >95% and corresponding field enhancement over 400-nm bandwidths. By varying the cavity thickness, the cavity mode overlap and resulting absorption-based field enhancement can be tuned to a spectral range of interest. Using a vapor-responsive polymer spacer, we demonstrate real-time, reversible tuning of these absorption bands across >100-nm ranges in wavelength in the presence of organic solvents. This structure was readily fabricated using standard wafer-scale thin-film deposition processes. The plasmonic top film can also be formed using other bottom-up techniques, including dispersion or printing of colloidal nanoparticles and seed-mediated particle growth, extending the range of potential applications and further simplifying the fabrication process. This platform forms the basis for a simple, dynamically tunable substrate which may be applied in a variety of sensing contexts, with potential application to other absorption-based processes such as energy harvesting and photodetection.

Wavefront shaping and control is essential for advancements in optical technologies including LiDAR, AR/VR, and LiFi systems. Metasurfaces offer a promising route for these technologies, as they allow for precise control of the amplitude, phase, and polarization of light in a sub-wavelength-thick footprint. However, most current metasurface designs are static, relying on fixed geometric patterning and lacking tunability once fabricated. Here, we design high quality factor (high-Q) metasurfaces that enable electro-optically tunable beam-steering through dynamic phase and amplitude control of the constituent nanoantennas. Our metasurface consists of a uniform array of subwavelength nanobars of Silicon, each 500 nm wide and 220 nm tall, atop 100 nm thick lithium niobate. By introducing subtle geometric perturbations into each constituent nanobar, we excite guided mode resonances with normally-incident light. Using full-field simulations, we show how these guided mode resonances excite strongly enhanced fields localized in the lithium niobate, with mode quality factors exceeding 20,000. Applying a voltage across individual nanoantennas shifts the spectral position of the high-Q resonance, modifying the antenna phase and amplitude. In reflection, we achieve a full 2 pi phase variation with applied bias, with a reflectance efficiency above 93%. Using these results, we computationally design a fully-reconfigurable, solid-state beam steerer: without bias light is directly reflected from the metasurface, whereas an applied biases of +9.2 V, 0 V, and -9.2 V across three constituent antennas diffracts the light to 30 degrees with 65% efficiency. Biases of 13 V, 2 V, -2 V, and -13 V across four constituent antennas diffracts the reflected light to 22 degrees. Near-continuous beam-steering can be achieved by modifying the bias across each antenna. We also show how this platform can enable dynamic modulation of other optical transfer functions, including beam-splitting and lensing. Our high-Q electro-optic metasurfaces provide a foundation for light-weight, fully reconfigurable, solid-state classical and quantum information processing spanning LiDAR, LiFi, AR/VR, and quantum communications.

Realization of dynamic beam steering for free-space data communication and light detection and ranging (LiDAR) technology, demands a class of high-performance devices with capability of generating fast-scanning and low-divergence beams. Optical phased arrays (OPAs), consisting of several typically identical optical nanoantennas, hold a great promise for achieving real-time beam steering, by imparting local, space-variant phase gradients on the incident light. Integration of dynamically tunable electro-optical materials into the constituent building blocks of OPAs facilitates active control over the intrinsic properties of subwavelength antenna elements with high speed and offers post-fabrication modifications to their optical response. The phase tuning mechanism of the active OPAs has thus far relied on the modulation of the resonant modes between the over-coupled and under-coupled regimes by applying external DC bias voltages to nanoantennas. This leads to high variations of amplitude during phase modulation and restricts the maximum achievable phase span into a narrowband critical coupling regime. Moreover, the phase modulations in such so-called quasi-static OPAs is typically less than 2π. Introducing time into OPAs, as an additional dimensionality, offers a way out to surmount these obstacles through generation of sideband signals and translating the spatial diversity of scattering into the spectral diversity. In this work, a comparative study on active beam steering performance of quasi-static and time-modulated reflective OPAs operating at near-infrared (NIR) spectral regime is presented. Plasmonic strip nanoantenna in a metal-insulator-metal (MIM) configuration integrated with indium-tin-oxide (ITO) is considered as the building block of the active OPA. First, the beamforming performance of the quasi-static OPA is analytically studied to highlight its main shortcomings including narrow bandwidth, limited angle-of-view, and large sidelobe level (SLL) as results of strong resonant dispersion of phase, limited dynamic phase span, and dramatic variations in amplitude. Moreover, a multiobjective evolutionary optimization is adopted to identify the non-intuitive beamforming tradeoffs between SLL and gain in the quasi-static OPA due to covarying amplitude and phase responses. Then, beam steering performance of a time-modulated OPA under the application of radio frequency biasing signals is investigated. Modulation waveform is optimized to achieve beam steering with uniform amplitude via non-resonant and dispersionless phase shift induced by modulation phase delay at the first order sideband. This dispersionless phase elevates time-modulated OPAs beyond their quasi-static counterparts in that it increases the functionality bandwidth, expands the angle-of-view, and minimizes the power coupled into undesired sidelobes by providing access to the full phase span (2π) with uniform amplitude. In addition, Taylor one-parameter distribution is employed to implement amplitude tapering for engineering of the beamforming tradeoffs between SLL and gain.

The two-dimensional designer metasurfaces have been established as a new class of versatile and powerful optical solution for controlling the classical light in various degrees of freedom such as phase, amplitudes, polarization and angular momentum. Expanding the control capability of metasurface from classical light to quantum state of single photons is an emerging direction that can lead to a new regime of light-matter interaction and applications for quantum technology. In
this talk, we will present our proposal and experimental demonstration of manipulating photonic quantum states. The demonstrated control over the effective quantum interaction between single photons that is impossible by traditional optics has been enabled by an unprecedented design of metasurface. Our work greatly empowers the operations and functionalities of optical quantum technologies.

Faced with the ability to arbitrarily engineer the interaction of electromagnetic energy with a medium, the optics community is enchanted by the innumerable, though fixed, configurations of passive metamaterials. Looking forward, infusing such structures with dynamic tunability may help realize both practical applications and novel functionality. While many phase change materials are capable of wild property modulations induced by heat, electrochemical intercalation presents opportunities for complete rearrangement of atomic structure and composition in a reversible manner, resulting in potentially dramatic permittivity changes. In the age of the lithium ion battery, rapid materials innovation and research has uncovered numerous compounds which accommodate lithium, though the resulting changes in optical properties lack clarity. We explore the properties of titanium dioxide (TiO₂), which is both a high-index dielectric material with negligible absorption in the visible spectrum, making it popular for all-dielectric metasurfaces, as well as an established lithium anode material known for minimal volume expansion (<1%) and good cyclability. Upon lithiation to Li_0.5TiO₂, DFT calculations predict significant changes in dielectric constants in the visible and near-IR regions of the electromagnetic spectrum. These calculations are verified by air-free ellipsometry measurements on TiO₂ thin films deposited via atomic layer deposition and electrochemically lithiated, thereby reducing the refractive index by Δn = 0.5 at ~650 nm. When incorporated into metal-insulator-metal type metasurfaces, the resulting devices demonstrate sweeping spectral shifts across the visible regime with applied voltages < 2 V. Through unraveling the optical properties of this abundant, non-toxic battery material, our findings open the door to low-power, multistable, active metasurfaces for color modulation and tunable filters.

Bragg reflectors from layer by layer (LBL) assembly of Slide Ring elastomer and Zirconia Nanoparticles have been shown to offer desirable elasticity and strain tun-able optical properties. With a high loading of zirconia nanoparticles of nearly 80 wt% in the photonic crystal results in a refractive index contrast of 0.18 between the filled and unfilled layers of 6 periods. Further, we study an integration of Au meta-surfaces by utilising soft-imprinting lithography of 400 nm spaced rectangular pillars and other modulated nanofeatures on the Bragg Reflectors and harness the enhanced tuning ability. The nanoimprinting ink is composed of a bi-layer polymer resist, among which only the top layer renders lower viscosity that fills the stamp via capillary pressure. The resulting patterns are metallised with 50 nm of Au using e-beam evaporating technique and followed by transferring the metal features on the Bragg reflectors. By stretching the resulting photonic crystal over 40% strain we demonstrate the mechano-chromic sensing abilities in a span of wavelengths.

Metallic plasmonic hybrid nanostructures have attracted enormous research interests due to the combined physical properties coming from different material components and the broad range of applications in nanophotonic and electronic devices. However, the high loss and narrow range of property tunability of the metallic hybrid materials has limited their practical applications. Here, a metallic alloy-based self-assembled plasmonic hybrid nanostructure, BaTiO₃-Au_xAg_1-x vertically aligned nanocomposite, has been integrated by a templated growth method for low-loss plasmonic systems. Comprehensive microstructural characterizations including (HR)STEM, EDS and 3D electron tomography demonstrate the formation of an ordered “nano-domino-like” morphology with the Au_0.4Ag_0.6 nanopillars as the round cores, and the BTO as the squared shells. By comparing with the BTO-Au hybrid thin film, the BTO-Au_0.4Ag_0.6 alloyed film exhibits much broader plasmon resonance, hyperbolic dispersion, low-loss, and thermally robust features in UV-Vis-NIR wavelength region. This study provides a feasible platform for complex alloyed plasmonic hybrid material design with low-loss and highly tunable optical properties towards all optical integrated devices.

Metasurfaces are planar optics composed of artificially fabricated subwavelength meta-atoms with unique optical scattering characteristics. Thanks to their outstanding abilities to modulate electromagnetic waves, metasurfaces have been enthusiastically researched. The research on metasurfaces especially focuses on wavefront modulation and many approaches have been proposed to implement novel optical devices with versatile functionality. It has been demonstrated that metasurfaces can implement high-quality holographic light reconstruction with both amplitude and phase information at sub-wavelength scale spatial resolution, which is expected to be applied to next-generation imaging technologies such as three-dimensional holographic imaging and optical data storage. In addition, metasurface lenses, called meta-lenses, have powerful features such as flatness, high numerical aperture, and versatility that cannot be found in conventional optical lenses. Based on these meta-optics, recent advances in metasurfaces have led to the development of various optical systems, and several studies have begun to be reported in recent years. In this talk, we will introduce our representative works that achieve significant progress of optical systems through utilizing elaborately designed multifunctional metasurfaces. The first part will introduce the passive and active type metasurfaces exhibiting enlarged light modulation capabilities with their practical applications. The passive type metasurface is designed to exhibit different scattering characteristics for two orthogonal polarizations of incidence light with their full-space controllability. Through exploiting multifunctionlaity, the polarizing beam-splitter and full-space holographic image generation are demonstrated. And, the active type metasurface is engineered to exhibit enlarged switching levels through exploiting optical-thermal analysis. To utilize the expanded switching level at best, the metasurface is designed to generate high-contrast holographic images according to the state change of metasurface. Thanks to the thermo-optical complexity of our active metasurface, the visual cryptosystem is demonstrated, providing an extremely high-security level. In the second part, we will introduce the novel concept of metalens, called see-through metalens for augmented reality (AR) device application. The see-through meta-lens effectively combines the real and virtual images in a compact manner thanks to the flatness and high-numerical-aperture of the proposed metalens. The proposed geometric-phase metalens can distinguish the two lights – light from outside object and one from an image projector based on polarization orthogonality. Especially, our study is highlighted for dramatically improving the field-of-view compared to previous AR imaging devices. Lastly, our perspective on the possibilities and challenges of metasurface hologram, metasurface multifunctional diffraction devices and metalens will be discussed.

Computer-generated holography (CHG) involves iterative numerical algorithms to obtain the phase and/or amplitude profiles needed to physically realize holograms. Metasurfaces consist of arrays of subwavelength nanoresonators that can control the wavefront of light in a desired way. They recently proved themselves to be an effective platform for CGH by surpassing the quality of traditional holograms in terms of image resolution and field-of-view. Those metasurface holograms showed prospects not only in imaging and display but also in security applications [1]. In particular, applying metaholograms to anticounterfeiting applications requires not only the technology of encoding multiple pieces of information, but also the manufacturability of highly efficient devices. To meet these complex needs, we have implemented high-efficiency metaholograms based on hydrogenated amorphous silicon (a-Si:H), which realize pragmatic images holograms working under unpolarized light (e.g. sunlight or flashlight of cellphone) and spin/direction-multiplexed metaholograms [2-4]. However, ‘real-time’ active operations of those flat optical devices have remained unresolved yet.
In this abstract, I will discuss our efforts in realizing dynamic metaholograms by leveraging specifically-designed (‘designer’) liquid crystals that can respond to target external stimuli. First, I will present high-efficiency interactive holographic displays, which can switch holographic images according to external stimuli like voltage, heat and touch sensing [5]. For examples, the voltage-responsive metahologram is able to switch the holographic images within few milliseconds promising for real-time video holographic displays demanding 60 ~ 120 frames/s. Also, the heat or touch-responsive metaholograms can monitor external temperature and impact by visualizing different hologram images according to the pre-programmed external stimuli standart. Such demonstrated systems may permit a diverse range of smart sensing and display applications such as smart hologram labels monitoring temperature/pressure/touch changes and interactive holographic displays recognizing haptic motions. Secondly, I will propose a compact gas sensor platform to autonomously sense the existence of a toxic volatile gas and provide an immediate visual holographic alarm [6]. By combining the advantage of the rapid responses to gases realized by liquid crystals with the compactness of holographic metasurfaces, we develop ultra-compact gas sensors without the requirement of additional complex instruments or machinery to report the visual information of gas detection. It is expected that such a holographic metasurface gas sensor platform will provide a path to ubiquitous, compact, and smart unconventional photonic sensing applications that quickly alert users about harmful gases or biochemical leaks.

[1] I. Kim et al., ACS Photonics 5, 3876-3895 (2018)
[2] I. Kim* et al., ACS Nano 11, 9382-9389 (2017)
[3] I. Kim* et al., Laser and Photonics Reviews 13, 1900065 (2019)
[4] I. Kim* et al., Nanoscale Horizons 5, 57-64 (2020)
[5] I. Kim et al., Advanced Materials (2020) (in press)
[6] I. Kim et al., Science Advances (2020) (in review)

We develop a theory for collective near-field interactions and associated ElectroMagnetic (EM) response of planar, closely packed, periodically aligned Single-Wall Carbon Nanotube (SWCN) arrays embedded in finite-thickness ultrathin dielectric layers. The features that make this system interesting are the periodic CN alignment and the spatially periodic anisotropy associated with it. Additionally, the vertical confinement in dense ultrathin planar systems of finite thickness leads to the effective dimensionality reduction from 3D to 2D while still retaining the thickness as a parameter to represent the vertical size. This is the transdimensional regime [1] — neither 3D nor 2D but something in-between — turning into 2D as the thickness tends to zero, challenging to study what the 3D-to-2D continuous transition has to offer for new material functionalities [2-4]. The spatial anisotropy, periodic in-plane transverse inhomogeneity and vertical quantum confinement make the ultrathin array near-fields strong and anisotropically nonlocal, adding both extra challenges in developing the problem theoretically [3] and extra flexibility in designing CN films with desired EM properties experimentally [5]. Here, we derive the SWCN array EM response tensor in terms of the individual SWCN conductivity, plasma frequency and the volume fraction of CNs in the dielectric film. In the CN alignment direction the real part of the EM response has a sufficiently wide negative refraction band, indicating that the CN film behaves as a hyperbolic metamaterial [4]. Inhomogeneous and thermal broadening of the exciton and plasmon resonances lead to their overlap, making the exciton-plasmon hybridization possible. Being very stable and highly sensitive to the vertical size/CN-density/diameter/chirality variations, the ultrathin periodically aligned SWCN arrays and films show great potential to serve as a new flexible multifunctional nanomaterial platform for single-molecule detection and manipulation, including the near-field control of photoluminescence rate/directionality, chemical reactivity, and Casimir-Polder interactions [6,7].
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Funding Acknowledgement: NSF DMR-1830874 (I.V.B.)
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[1] A.Boltasseva and V.M.Shalaev, Transdimentional Photonics, ACS Photon. 6, 1 (2019)
[2] I.V.Bondarev, H.Mousavi, and V.M.Shalaev, Transdimensional Epsilon-near-zero Modes in Planar Plasmonic Nanostructures, Phys. Rev. Research 2, 013070 (2020); MRS Commun. 8, 1092 (2018)
[3] I.V.Bondarev, Finite-Thickness Effects in Plasmonic Films with Periodic Cylindrical Anisotropy [Invited], Opt. Mater. Express 9, 285 (2019)
[4] C.M.Adhikari and I.V.Bondarev, Optical Response of Ultrathin Periodically Aligned Single-Wall Carbon Nanotube Films, MRS Advances, DOI: 10.1557/adv.2020.234; see also arXiv:2010.00139
[5] J.A.Roberts, S.-J.Yu, P.-H.Ho, S.Schoeche, A.L.Falk, and J.A.Fan, Tunable Hyperbolic Metamaterials Based on Self-Assembled Carbon Nanotubes, Nano Lett. 19, 3131 (2019)
[6] I.V.Bondarev and Ph.Lambin, Near-field electrodynamics of atomically doped carbon nanotubes, In: Trends in Nanotubes Research (Nova Publishers, NY 2006). Ch.6, pp.139-183
[7] J.Galego, F.J.Garcia-Vidal, and J.Feist, Suppressing photochemical reactions with quantized light fields, Nature Commun. 7, 13841 (2016)

Plasmonic nanocavities, consisting of metallic nanoparticles closely separated from a thin metal film by nanometric dielectric gaps, support hybridized plasmon modes with extremely small mode volumes and strongly enhanced local fields. They constitute a versatile nanophotonic platform for exploring many exotic light-matter interaction phenomena at the nanoscale, such as plasmon-enhanced nonlinear optics and quantum plasmonics.

This talk will review our recent studies on the linear and nonlinear plasmonic properties of metallic particle-on-film nanocavities (PoFNs), including plasmon resonance hybridization and decomposition [1-3], plasmon-enhanced polarized photoluminescence [4], and light-induced symmetry breaking for enhancing second-harmonic generation (SHG) [5]. I will present a new polarization-resolved spectral decomposition and color decoding approach for understanding the rich scattering radiation properties of two types of gold PoFNs. Our results reveal an unusual plasmon resonance in a gold nanosphere dimer-on-film nanocavity (absent in a nanosphere monomer counterpart), having strong radiation yet a large quality factor, which turns out to arise from the metal substrate-mediated dipole-quadrupole hybridization. Following this, I will show this strong yet narrow hybrid resonance gives rise to highly polarized photoluminescence emission from gold, with a 200-fold intensity enhancement and 5-fold linewidth reduction compared to a dimer of similar size on silica.

Finally, I will discuss a new mechanism for SHG enhancement in such plasmonic PoFNs, that is, the light-induced electromagnetic asymmetry. Our results show that such symmetry breaking efficiently suppresses the cancelling of locally generated SH fields and is further amplified in the SHG-induced plasmonic excitation through preferential coupling to the bright, bonding dipolar resonance mode of the nanocavity, leading to a record high far-field SHG efficiency of up to 3.56×10^-7 W^-1 in plasmonic nanostructures.

This work was supported in part by the Research Grants Council of Hong Kong (grant number: 15303417) and the National Natural Science Foundation of China (grant number: 62022001).

References:
G.-C. Li, Y.-L. Zhang, and D. Y. Lei, “Hybrid plasmonic gap modes in metal film-coupled dimers and their physical origins revealed by polarization-resolved dark-field spectroscopy”, Nanoscale 2016, 8, 7199-7126.
Q. Zhang, G.-C. Li, T. W. Lo and D. Y. Lei, “Polarization-resolved optical response of plasmonic particle-on-film nanocavities”, Journal of Optics, 2018, 20, 024010.
G.-C. Li, Q. Zhang, S. A. Maier, D. Y. Lei, “Plasmonic particle-on-film nanocavities: A versatile platform for plasmon-enhanced spectroscopy and photochemistry”, Nanophotonics 2018, 7, 1865-1889.
G.-C. Li, Y.-L. Zhang, J. Jiang, Y. Luo, D. Y. Lei, “Metal substrate mediated plasmon hybridization in a nanoparticle dimer for photoluminescence linewidth shrinking and intensity enhancement”, ACS Nano 2017, 11, 3067-3080.
G.-C. Li, M. Qiu, W. Jin, A. Zayats, D. Y. Lei, “Light-induced electromagnetic asymmetry for enhancing second-harmonic generation from an untrathin plasmonic nanocavity”, manuscript under review (2021).

In this presentation, we will review our recent results on the realization of vectorial holograms. We will also introduce a specific computational imaging technique, dubbed Ptychography, to reconstruct the Jones matrix map of the metasurfaces at microscopic resolution, including all amplitude, phase and polarization effects. We also report on the realization of broadband polarization maintaining metasurfaces.
In this contribution, we will present a general method that enables wavefront shaping with arbitrary output polarization by encoding both phase and polarization information into pixelated metasurfaces[1]. We apply this concept to convert an input plane wave with linear polarization to a holographic image with arbitrary spatial output polarization. Our approach relies on pixelated metasurfaces, in which each pixel acts as a deflector able to encode both the polarization and the holographic phase information, resulting in a holographic image in a specific angle with arbitrary polarization.
Vectorial ptychography technique is introduced for mapping the Jones matrix to monitor the reconstructed metasurface output field and to compute the full polarization properties of the vectorial far field patterns, confirming that pixelated interfaces can deflect vectorial images to desired directions for accurate targeting and wavefront shaping.
Exploring further this concept of polarization addressing metasurface, we propose a general method for polarization-maintaining and angular nondispersive wavefront shaping with, essentially, unlimited bandwidth[2]. Our results show that we can eliminate the chromatic dispersion of material by properly designing the geometry of the meta-structures, resulting in perfect nondispersive optical properties. As a proof of concept we realize white-light broadband image projection, in which we control both the polarization and the projection angle over an unlimited spectral range.
Our work solves the problem of polarization mode dispersion and demonstrates that classical Pantcharatnam broadband metasurfaces, such as those published recently by several groups and that address broadband angular dispersion relying exclusively on the nanostructure dispersive properties, are unable to control the polarization over broadband frequency range. To really prove the performance of our approach, we have adopted a metasurface doublet configuration to address both angular and polarization dispersion, thus demonstrating the first broadband polarization-maintaining and dispersive-less metasurface.
Our experimental results prove that metasurfaces, designed and realized using precise nanofabrication methods could efficiently tailor the wavefront and control the polarization in the entire visible range, promising various applications in polarization imaging, augmented/virtual reality displays, full color display and broadband-polarimetry.
References
[1] Ptychography retrieval of fully polarized holograms from geometric-phase metasurfaces, Q Song, A Baroni, R Sawant, P Ni, V Brandli, S Chenot, S Vézian, B. Damilano, P. de Mierry, S. Khadir, P. Ferrand and P. Genevet, Nature communications 11 (1), 1-8 (2020)
[2] Bandwidth Unlimited Polarization-Maintaining Metasurfaces, Q. Song, S. Khadir, S. Vézian, B. Damilano, P. de Mierry, S. Chenot, V. Brandli and P. Genevet, Science Advances, in press (2020)

The high dielectric optical contrast between the amorphous and crystalline structural phases of non-volatile phase-change materials (PCMs) provides a promising route towards tuneable nanophotonic devices [1-3]. Here [4], we employ the next-generation PCM In₃SbTe₂ (IST) whose optical properties change from dielectric to metallic upon crystallization in the whole infrared spectral range. This distinguishes IST as a switchable infrared plasmonic PCM and enables a new programmable nanophotonics material platform. We show how resonant metallic nanostructures can be directly written, modified and erased on and below the meta-atom level in an IST thin film by a pulsed switching laser, facilitating direct laser writing lithography without need for cumbersome multi-step nanofabrication. With this new technology, we demonstrate large resonance shifts of nanoantennas of more than 4 µm, a tuneable mid-infrared absorber with nearly 90% absorptance as well as screening and nanoscale “soldering” of metallic nanoantennas. Our novel concepts will empower new and improved designs of programmable nanophotonic devices for telecommunications, (bio)sensing and infrared optics, e.g. programmable infrared detectors, emitters and reconfigurable holograms.
[1] M. Wuttig, H. Bhaskaran and T. Taubner. Phase-change materials for non-volatile photonic applications. Nature Photonics 11, 465-476 (2017)
[2] A.-K. U. Michel, A. Heßler, S. Meyer et al.. Advanced optical programming of individual meta-atoms beyond the effective medium approach. Advanced Materials 31, 1901933 (2019)
[3] A. Leitis, A. Heßler, S. Wahl et al.. All-dielectric programmable Huygens’ metasurfaces. Advanced Functional Materials 30, 1910259 (2020)
[4] A. Heßler, S. Wahl, T. Leuteritz et al.. In₃SbTe₂ as a programmable nanophotonics material platform for the infrared. in submission (2020)

Nanophotonic metasurfaces employ dense arrays of optically-resonant nanostructures to manipulate the properties of light in nm-thick optical coatings. By harnessing plasmonic or Mie resonances in metallic or dielectric nanoparticles, the phase and amplitude of the scattered light can be controlled at the nanoscale. Based on rapid advances in metasurface design, metasurfaces are now widely applied in flat optical elements for beam steering, lensing, and holography. However, novel applications in dynamic holography and augmented reality require metasurfaces and metadevices with actively-tunable functionality. So far, the use of plasmonic and Mie-resonances in dynamic metasurfaces is limited as their optical resonances are difficult to tune dynamically.

Monolayer transition metal dichalcogenides like WS₂ exhibit strong exciton resonances in the visible spectral range that dominate their optical response. The excitonic light-matter interaction in these 2D quantum materials is inherently very strong and highly tunable, which can be leveraged to realize mutable flat optical elements. To unleash the full potential exciton-enhanced light scattering in atomically-thin metasurface elements, it is essential to first achieve detailed understanding of the role of the exciton’s quantum mechanical properties in passive nanophotonic wavefront shaping.

Here, we employ atomically-thin metasurface lenses carved out of a monolayer of WS₂ to directly study the influence of exciton decay and dephasing on the metasurface functionality and spectral line shape. We fabricate 500 μm diameter zone plate lenses using electron-beam lithography and reactive-ion etching. At resonance, excitonic light scattering strongly enhances the focal intensity. We systematically characterize the focal shape and focusing efficiency as a function of wavelength using confocal microscopy. To study the influence of exciton-phonon scattering and dephasing on the optical functionality of the lens, we then characterize the efficiency spectrum as a function of temperature.

At ambient conditions, the spectrum shows a strong asymmetric line shape revealing that the scattered light fields are directly governed by the monolayer susceptibility. This enables an almost background-free measurement of the optical properties of the monolayer, and thereby the excitonic light-matter interaction. Careful analysis of the line shape shows that the relative contribution from resonant excitonic light scattering is comparable to the non-resonant monolayer scattering. For decreasing temperatures on the other hand, the exciton’s non-radiative decay and dephasing are suppressed and the exciton becomes fully radiative. As a result, the asymmetric line shape not only narrows and increases in amplitude, but also transitions into a symmetric line shape. This directly shows the increasing prevalence of the exciton resonance in the focusing efficiency. By comparing the results to numerical simulations and an analytical model, we show that the efficiency of the metasurface lens directly scales with the excitonic oscillator strength.

The results give direct insight in the role of exciton dynamics in optical wavefront shaping using atomically-thin metasurfaces. A full understanding of the role of exciton resonances in metasurfaces paves the way for dynamic components, combining tunable effects in quantum materials with classical metasurface optics.

Surface arrangements of subwavelength formations are extensively discussed in the context of photocurrent enhancement for photovoltaic (PV) applications. Recently, the potential contribution of such arrangements toward photovoltage augmentation is suggested. This study numerically demonstrates the potential for photovoltage management based on arrays composed of inverted silicon cones, referred to as light funnel (LF) arrays. The transition from an optimized nanopillar (NP) array into an LF array is examined. It is shown that a decrease in NP bottom diameter (Db) is accompanied by an increase in open-circuit voltage (Voc). The highest photovoltage enhancement is recorded for the smallest considered Db ¼ 50 nm with a Voc increase of 75 mV and reflects a 22% Voc enhancement compared with the NP Voc. It is shown that this Voc increase is due to 250% increase in the excitation level, and that the spatially resolved excitation level of the array-nested LFs is more than two orders of magnitude higher than the highest spatially resolved excitation level in the array-nested NPs. Finally, it is shown that the suggested photovoltage management entails almost a factor of 2 increase in the nominal power conversion efficiency upon the transition from an NP PV cell into LF PV cell.

Acoustoelectric charge patterning of graphene is leveraged to create dynamically tunable infrared filters having expanded capability by not only modulating the properties of the plasmonic medium but the metasurface itself. While active spectral tuning within a graphene-based platform is well established, plasmon excitement has overwhelmingly been facilitated through a patterned metasurface whose properties derive from the physical shape of the material(s) involved thereby fixing its response at fabrication. The metasurface does not provide any tunability in itself but only excites the tunable plasmon. Here, in contrast, acoustoelectric induced charge aggregation of graphene permits a means to arbitrarily pattern charge distributions that can be modified in real time thus changing both the plasmonic dispersion and where on this dispersion the plasmon is excited.

To show the viability of a reconfigurable graphene metasurface controlled by a surface acoustic wave (SAW), a combined numerical and experimental methodology was employed. The interaction between the acoustic wave and graphene is obtained by solving for the coupling between the displacement field of the SAW and mobile charges at the graphene interface. In doing so, the periodic profile of the SAW-driven fields creates charge stripes in the graphene via the acoustoelectric effect, which in turn spatially modulates the two-dimensional material’s optical properties. Numerically solving for the full-field spectral optical response under normal mid-infrared illumination at various acoustoelectric powers reveals a dynamic resonance behavior that tunes in both energy and depth. In particular, the plasmonic response can be switched on or off by simply enabling/disabling the SAW. Through careful consideration of both the optical and acoustoelectric design spaces, a single system with compelling spectroscopic performance is obtained. Experimentally, charge stripes in graphene resting on top of a periodically poled substrate are observed via Raman imaging that confirms the achievable aggregation as well as the ability to spatially pattern graphene’s optical properties. Subsequently, the effect is extended to acoustoelectric devices leveraging the SAW to induce charge aggregation in graphene. Future directions are highlighted, where the interdisciplinary acoustoelectric-optoelectronic interaction can further customize the graphene plasmon coupling mechanism and hence the device’s optical response.

Acknowledgements: Sandia National Laboratories is a multi-mission laboratory managed and operated by the National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract No. DE-NA0003525. We thank Zachary Piontkowski for technical review of this abstract.

2D materials have a number of intriguing value propositions that could be harnessed for compact, tunable, high-performance optoelectronic devices when heterogeneously integrated in photonic circuits. Here I review our latest work including; (1) tunable TMD-based microring resonator with engineered critical-coupling condition, (2) a broadband graphene plasmon-slot detector (R=0.7A/W), (3) a bandgap-shifted strain-engineered absorption-enhanced MoTe2 photodetector at 1.55um (R=0.5A/W, low-dark-current <10nA@-1V), (4) a record-high responsivity (R=1.4A/W) slot-plasmon exciton-modulated MoTe2 detector, all enabled by our recently developed method of cross-contamination-free yet deterministic dry transfer 2D material ‘printer’ mimicking a 3D printer for enabling rapid prototyping. These devices are based on heterogeneous integration of 2D materials into Silicon and SiN photonics, with the latter used for on-exciton modulation or exciton absorption.

Van der Waals materials stacked with a relative twist angle are attracting tremendous interest in physics and have been intensively investigated to tune the electronic, magnetic, and optical properties of materials (known as“twistronics”). However, previous discoveries are based on the formation of peculiar moiré superlattices at small and specific twist angles. Here we report configurable nanoscale light–matter waves—phonon polaritons—by twisting stacked α-phase molybdenum trioxide (α-MoO₃) slabs over a broad range of twist angles from 0° to 90°. Our combined experimental and theoretical results reveal a variety of polariton wavefront geometries and topological transitions as a function of the twist angle, extending twistronics and moiré physics to nanophotonics and polaritonics. The origin of the polariton twisting configuration is attributed to the electromagnetic interaction of highly anisotropic hyperbolic polaritons in stacked α-MoO₃ slabs.

Polaritons are half-light-half-matter electromagnetic waves confined in materials. These nanoscale light-matter waves offer the access to optical properties and energy at the subwavelength scale. In this work, we studied the effect of environmental permittivity on polaritons in a prototype van der Waals materials of α-MoO₃. Different than majority polariton waves that spread towards all directions, polaritons in α-MoO₃ are highly anisotropic in the basal plane. Our nano-infrared imaging augmented with electromagnetics simulations reveal distinct effects of the sample suspension on the polariton parameters including wavelength, damping and dispersion, at various infrared frequencies. Our results are expected to offer guidance on engineering nano-polaritons propagation properties for desired nanophotonic functionalities.

Mingze He¹, Sami I. Halimi², Thomas G. Folland^{1, 6}, Sai S. Sunku^3,5, Song Liu⁴, James H. Edgar⁴, Dmitri N. Basov³, Sharon M. Weiss², Joshua D. Caldwell^1,2,
1, Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37212, USA. 2, Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37212, USA. 3, Department of Physics, Columbia University, New York NY 10027, USA. 4, Tim Taylor Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506, USA. 5, Department of Applied Physics and Applied Mathematics, Columbia University, New York NY 10027, USA. 6, Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa, 52242, USA
Silicon waveguides are the most indispensable building block in on-chip photonics. With demand for continuous increases in operational bandwidth, there is a desire to expand the operating frequency regime, as multiplexing of near- and mid-infrared (mid-IR) signals could provide unique applications for both signal processing and chemical sensing. However, this is challenging with integrated silicon photonics, since accommodating longer wavelength modes requires expanding the size of the silicon waveguide, which in turn would cause severe modal dispersion in the near-IR. Thus, an architecture that maintains the silicon waveguide's performance in the near-IR, while still providing confinement and propagation of mid-IR light would be highly beneficial.
Hyperbolic phonon polaritons (HPhPs) can compress long-wavelength free-space light to deeply sub-diffractional volumes and overcome the length-scale mismatch with structures designed for operation in the near-IR. Such HPhPs are supported within highly anisotropic media where the permittivity along orthogonal axes are opposite in sign, with hyperbolicity being demonstrated in an expanding list of natural materials, such as hexagonal boron nitride (hBN). Guiding of HPhPs in hBN has been studied previously using patterned strips and by introducing in-plane polaritonic refraction of HPhPs. However, previous research suffers from either unavoidable material damage of hBN or detrimental substrate absorption in the near-IR, which precludes the waveguide application at dual frequency domains. Thus, an architecture for frequency multiplexing of mid-IR and near-IR is yet to be demonstrated.
Here we address this challenge by realizing guided mid-IR and near-IR light within a hybrid hyperbolic-material/silicon waveguide heterostructure. By exploiting substrate-induced changes in polariton wavelength and thus in-plane polaritonic refraction, we demonstrate that the HPhPs supported by hBN can be confined laterally and guided by the underlying silicon waveguide structure, without the need for fabrication of the hyperbolic medium. Furthermore, we experimentally demonstrate that the hBN slab results in a negligible influence on waveguiding of near-IR light in the underlying Si waveguide. Thus, the prototype hybrid waveguide can operate in both the near- and mid-IR simultaneously. Such integration offers a generalizable approach for multiplexing dramatically different free-space wavelength light within a compact and on-chip footprint, offering a new toolset for nanophotonic design that maintains the low-loss properties of naturally hyperbolic materials.

I will discuss recent experiments aimed at creating plasmonic resonances in graphene in the short-wavelength infrared (SWIR). The plasmonic dispersion of graphene dictates that reaching such short wavelengths requires fabricating increasingly smaller nanostructures in the graphene surface, with lengthscales < 10nm. While conventional electron beam lithography has been shown to be effective at creating structures as small as 15nm, measurements of those systems revealed only mid-IR resonances, with wavelengths as short as 3.5um. We push to smaller lengthscales by introducing a new, bottom-up block co-polymer lithography method, which can easily pattern cm-scale sheets of graphene into 12nm nanoribbon arrays. Subsequent infrared absorption measurements from these samples reveals that they exhibit resonant behavior at frequencies much higher than predicted, reaching free-space wavelengths as short as 2.2um, well into the SWIR. We show that this behavior is due to strong non-local effects in the graphene, which effectitvely blue shifts the graphene plasmons at short wavelengths. This allows optoelectronic devices based on graphene plasmons to operate over a larger range than commonly assumed, possibly reaching the near infrared or visible. Another significant implication of this finding is that light-matter interactions driven by graphene plasmons are smaller than what is expected from first order calculations; for some spontaneous emission processes, these weaker interactions will create Purcell enhancements that are orders of magnitude smaller than expected. Finally, I will discuss recent attampts at integrating graphene nanoribbons derived using block co-polymer lithography with metal and diectric metasurfaces, with an aim of creating an enhanced optoelectronic response.

Inverse design algorithms enable metasurfaces and other nanophotonic devices to achieve new functionalities and high efficiencies. Inverse algorithms based on global topology optimization networks (GLOnets), in which global optimization is framed as the training of a generative neural network, are a promising approach towards globally searching a non-convex design landscape. However, the relationship between the GLOnets algorithm and network architecture is not clear and the selection of a proper architecture is required to ensure GLOnets stability. We will discuss the role of network architecture in the GLOnets algorithm and show that, counterintuitively, relatively shallow networks are sufficient to performing stable global optimization. With select problems based on benchmark mathematical functions and diffractive nanophotonic systems, we discuss GLOnets in the context of problem-dependent architectures. We will also discuss viable and effective pathways towards conditional GLOnets, in which ensembles of related devices are co-designed.

We experimentally demonstrate an array-level inverse design approach that accounts for the nonideal optical response to optimize the spatial array phase and amplitude profile of active metasurfaces for desired target functions [1]. Optical phase control in planar metasurfaces has recently been the subject of intensive worldwide research. While significant prior effort has addressed phase control in passive metasurfaces, active metasurfaces comprising of arrays of programmable nanoantennas have garnered widespread interest owing to their power for versatile wavefront shaping in real-time [2,3]. Notably, modulation of active metasurfaces generally relies on permittivity tuning near optical resonances in nanostructured antennas. This gives rise to an inherently nonideal optical response that creates strong interdependence between the scattered light phase and amplitude, and limits the accessible phase modulation range. As a consequence, conventional forward design of active metasurface phase profiles for beam steering results in significant losses of power coupling into undesired sidelobes, motivating our inverse design method, which enhances the performance of active metasurfaces with nonideal antenna components.

In contrast to passive metasurface design via shape optimization of individual antennas, active metasurfaces can be ‘designed’ by reconfiguration of independently gated, geometrically identical nanoantennas. This enables an array-level optimization of phase and amplitude for arbitrary user-defined metasurface functions, such as high-directivity dynamic beam steering. By performing optimization, we investigated the impact of phase and amplitude modulation on the beam steering performance of an active metasurface array of 100 non-interacting subwavelength antennas. We then examined the optical response of a nonideal plasmonic, electro-optically tunable metasurface using indium tin oxide as an active layer. The reflected light measurements indicate a limited phase modulation range of ~220° as well as covarying, non-unity amplitude at an operating wavelength of λ = 1548 nm. We find that nonintuitive array phase and amplitude profiles generate high performance despite a nonideal antenna optical response. Using inverse design, we demonstrate a reduction in the sidelobe radiation, thus increasing beam directivity compared to forward designs. We further used the array-level inverse design algorithm to experimentally validate high-directivity continuous beam scanning with a broad field-of-view of nearly 50° as well as simultaneous steering of beams in multiple different directions. Finally, we consider the impact of metasurface inter-antenna coupling on the beam steering performance. These findings highlight the power of array-level inverse design as a general approach for active metasurfaces, including all-dielectric metasurfaces. Inverse design thus has the potential to ultimately usher in a modern era of hierarchical co-design of nanophotonic materials, devices, and systems to realize highly efficient optical elements for complete space-time control of the scattered light wavefront.


[1] Thureja et al. ACS Nano (2020).
[2] Kafaie Shirmanesh et al. ACS Nano (2020).
[3] Shaltout et al. Science (2019).

The confinement of electromagnetic waves to nanometer-scale metal structures amplifies local fields that can be harnessed for application in information processing, energy harvesting, sensing and catalysis. Metal nanostructures offer the opportunity to manipulate the phases and amplitudes of electromagnetic waves to enable flat optics, subwavelength resolution imaging and patterning. However, the controlled fabrication of high-definition single-crystal subwavelength metal nanostructures has remained a significant hurdle due to the tendency for polycrystalline metal growth using conventional physical vapor deposition methods, and the challenges associated with placing solution-grown nanocrystals in desired orientations and locations on a surface to manufacture functional devices. Here, we introduce a scalable and green, wet chemical approach to monocrystalline noble metal thin films and nanostructures. The method enables the fabrication of ultrasmooth, epitaxial, single-crystal films of controllable thickness that are ideal for the subtractive manufacture of nanostructure through ion beam milling, and additive crystalline nanostructure via lithographic patterning for large area, single-crystal metamaterials and high aspect ratio nanowires. Our single-crystal nanostructures demonstrate improved feature quality, pattern transfer yield, reduced optical and resistive losses, and tailored local fields to yield greater optical response and improved stability compared to polycrystalline structures, supporting greater local field enhancements and enabling practical advances at the nanoscale.

Light-matter interaction has been the heart of electromagnetic and optics for many years and thus an immense effort has been put into introducing different platforms and materials that can effectively control the flow of light in the desired fashion. Spatiotemporally modulated systems have gained the attention of the scientific community in recent years due to their extended dimensionality and enabling several exotic space-time phenomena. In particular, it has been established that the temporal frequency conversion of an optical mode in such systems is accompanied by a correlated change in its spatial frequency [1]. This phenomenon, which is known as space-time photonic transition, not only enables numerous exotic functionalities such as generating an effective magnetic field to control the flow of photons but also is the underlying mechanism for constructing optical isolators in space-time varying platforms [2]. Recently, it has been demonstrated that time-modulated metasurfaces (TMMs), which are planar structures whose subwavelength unit cells are tuned periodically in time via an external stimulus, can also enable nonreciprocal wavefront engineering and extend the degree of light manipulation through space-time photonic transitions. In addition to the nonreciprocal responses, TMMs have been shown to enable a wide range of other novel physical phenomena including wavefront engineering and signal camouflaging [3,4].
In this work, we introduce a new type of space-time photonic transition of light, dubbed as topological space-time photonic transition, which occurs upon the scattering of light from an angular-momentum-biased metasurface and gives rise to the generation of a twisted light beam that carries an orbital angular momentum (OAM) with proportional topological charge and shift in the temporal frequency. An angular-momentum-biased metasurface renders a virtually rotating TMM which yields a superposition of OAM-carrying beams at distinct frequencies, whose topological charges can be controlled via varying the applied phase delay modulation. The competency of topological space-time photonic transitions in the active tuning of OAM states with high mode-purity is analyzed, which yields minimal cross-talk between OAM channels in a mode-multiplexed communication system. Moreover, the role of the spatiotemporal modulation profile of the metasurface on the spatial and spectral diversity of OAM states is explored in detail. It is also demonstrated that the established concept can pave the way for hybridized mode-division and wavelength-division multiple access through the generation of distinct OAM states at different frequency harmonics. The nonreciprocity and broken time-reversal symmetry in topological space-time photonic transitions across the temporal frequency domain and Hilbert space of OAM states is also investigated giving rise to distinct twisted light channels in up- and down-links. To realize a time-modulated metasurface with angular-momentum biasing, a reflective dielectric metasurface is considered consisting of silicon nanodisk heterostructures integrated with indium-tin-oxide and gate dielectrics placed on a back mirror forming a dual-gated field-effect modulator. The metasurface is azimuthally divided into several sections where the nanodisk heterostructures are interconnected via biasing lines. This configuration enables addressing each section independently via a radio-frequency biasing signal and with a different modulation phase delay, thus enabling the flexible implementation of different spatiotemporal modulation profiles.
References
[1] Winn, Joshua N., et al. Physical Review B 59.3 (1999): 1551.
[2] Fang, Kejie, et al. Nature photonics 6.11 (2012): 782-787.
[3] Salary, Mohammad Mahdi, et al. New Journal of Physics 20.12 (2018): 123023.
[4] Liu, Mingkai, et al. Physical Review Applied 12.5 (2019): 054052.

Over the last decades, photonic metamaterials with tailored -i.e. with deliberately introduced- structural disorder have attracted considerable interest in various optical applications due to their extended spectral and angular range of effectiveness¹. However, millions of years of evolution in the biological world has developed a plethora of micro- and nanoscopic photonic structures with the deliberately introduced disorder in their respective geometries and compositions². In this talk, I will discuss how the development of metaphotonic devices harnessing bioinspired attributes can provide novel yet highly practical solutions for the global health sector. I will present our studies on biophotonic nanostructures found in butterfly wings that show unique optical properties with the tailored structural disorder, and their successful replication in laboratories using self-assembly based scalable nanofabrication techniques^3,4,5. We utilize this approach to pattern Si₃N₄-based metasurfaces onto a Fabry-Perot-resonator-based intraocular pressure (IOP) sensor for glaucoma management⁴. The metasurface integration onto the IOP sensor led to a 2.5-fold improvement in readout angle allowing easy handheld monitoring and in a one-month in vivo study conducted in rabbits, showed a 3-fold reduction in IOP error and a 12-fold reduction in tissue encapsulation and inflammation, compared to an IOP sensor without nanostructures. We will further discuss our second-generation metaphotonic IOP sensor that is made of only flexible nanostructured foil with a 3D hybrid periodic and amorphous photonic crystal using a newly developed colloidal lithography⁵. I will conclude the talk showing our recent progress in developing an ultra-compact optical spectrometer for smartphones using bioinspired tricks with high angular tolerance, resolution, and throughput, suitable for the realization of high-performance point-of-care biosensing⁶.

References
1. D.S. Wiersma, Nature Photonics 7(3), 2013.
2. V. E. Johansen, O.D. Onelli, L. M. Steiner, S. Vignolini. Photonics in nature: from order to disorder, In Functional surfaces in biology III, Springer, 2017.
3. R.H. Siddique*, Y.J. Donie*, G. Gomard, S. Yalamanchili, T. Merdzhanova, U. Lemmer, H. Hölscher, Science Advances 3(10), 2017.
4. V. Narasimhan*, R.H. Siddique*, J.O. Lee, S. Kumar, B. Ndjamen, J. Du, N. Hong, D. Sretavan, H. Choo, Nature Nanotechnology 13(6), 2018.
5. R. H. Siddique, L. Liedtke, H. Park, S. Y. Lee, H. Raniwala, D. Y. Park, D. H. Lim, H. Choo, IEDM 2020.
6. R.H. Siddique*, D. Assumpcao*, H. Choo, in preparation, 2021.

Materials whose dielectric functions approach zero have many unique properties ranging from enhanced non-linear behavior to suppression of quantum electromagnetic fluctuations. Beyond these interesting physical phenomena, epsilon-near-zero (ENZ) concepts can be applied to devices to exploit these effects. In this talk, I will discuss our recent work on the development of devices that take advantage of these effects to create super-absorbing optical films for novel photodetectors and hydrogen sensors, as well as quantum effects that can be exploited using ENZ materials. Two examples of the later are the suppression of spontaneous emission and modifications to the Casimir force, a force that it purely quantum in nature and is related to the electromagnetic boundary conditions placed on vacuum fluctuations. We will conclude with an outlook for this emerging area of research.

Phonon polaritons are quasiparticles comprising a photon and a coherently oscillating charge on a polar lattice, which are supported in the form of propagating (SPhP) and localized surface phonon polaritons (LSPhP). The promising properties of LSPhP modes are exceptionally high predicted Purcell enhancements and narrow resonance linewidths, with the potential for near-unity absorption (emissivity). However, one drawback is that as a highly localized mode, they offer no significant degree of spatial coherence (directionality) for thermal emission applications. Alternatively, high spatial coherence can be achieved using propagating SPhPs launched by grating elements. However, the non-localized nature of such propagating modes yields thermal emission into frequency-specific angles across the entire Reststrahlen band where such modes can be supported. The introduction of strong coupling between different polaritonic modes, therefore, provides us an opportunity to combine the virtues of the narrowband LSPhP resonances with the high spatial coherence associated with propagating SPhPs into a novel, mixed character polariton. Further, it has been proposed that strong coupling between LSPhPs with zone-folded longitudinal optic phonons (ZFLO) could provide a mechanism to use the longitudinal fields of an electrical bias to stimulate the transverse fields of SPhPs through Ohmic loss. Thus, we propose that through inducing strong coupling between LSPhPs, propagating SPhPs, and ZFLO phonons, that realization of a narrow-band, spatially coherent emitter amenable to electrically driven emission could be possible. Additionally, through coupling to such a ZFLO mode, the extremely narrow linewidths could be employed via strong coupling to further reduce the linewidths of the SPhP modes.
In this work, we report on three-oscillator strong coupling within a SPhP platform using nanopillar arrays fabricated into a 4H-SiC substrate. Here, we experimentally manipulate the dispersion relation of coupled SPhP modes by strongly coupling LSPhPs, propagating SPhPs, with the ZFLO. In the strong coupling regime, the formation of such hybrid modes with mixed character is expected. Furthermore, the strength of the interactions between such optical modes can be precisely controlled through the hybridization of three oscillators. We further report on the influence of such strong coupling upon thermal emission within the long-wave-IR (LWIR), demonstrating significant narrowing of the spectral and spatial dispersion of the individual modes within this strongly coupled regime. In our three-oscillator strong coupling platform, we simultaneously demonstrate a five-fold reduction in the angular spread of the thermally emitted light and a three-fold enhancement of the quality factor over that of the uncoupled LSPhP mode at the anti-crossing point where the splitting occurs. Furthermore, the high Q-factors (over 200) achieved are realized using traditional photolithography, enabling such devices to be produced at large-scale and reasonable costs. Our results demonstrate that by leveraging three-oscillator strong coupling that the spectral and spatial dispersion of thermal emission can be engineered for a variety of LWIR applications extending from spectroscopy, sensing, to free-space communications.

For all thermal absorbers and emitters in equilibrium, the amount of radiation absorbed from a blackbody at a specific wavelength and incident angle is re-emitted reciprocally from the absorber back to the blackbody. This phenomenon has been formalized as Kirchhoff’s law of thermal radiation (KLTR) and relies on the absorber/emitter obeying time-reversal symmetry.
In our experiments, we showWe report the first experimental observation of a reciprocity breaking of this reciprocal , behavior in theat mid-infrared wavelengths, using in a magneto-optical deviceheterostructure. The device consists of a low-loss, Si guided mode resonant (GMR) grating waveguide structure made of Si on a top of degenerately-dopeddegenerately doped InAs wafer, which in an applied magnetic field, acts serving as our magneto-optical material. The degenerately-dopeddegenerately doped InAs, behaves responds optically as a Drude metal conductor with non-zero off-diagonal permittivity values when ain an applied magnetic field is applied. We design the Si GMR grating to critically couple to light at wavelengths (~17 microns) in the epsilon-near-zero regime of for the InAs wafer, where the magneto-optical effect response is largest. Using this structure, we are able to break time-reversal symmetry by changing the direction of an in-plane magnetic field (~ 0.5 T) applied along the length of the grating. We are able toBy measuring te the polarization- and angle- dependent reflectivityce of the structure’sand absorption, we obtained dispersion relations for the non-reciprocal, time-asymmetric InAs reflectivity and compared the our results to theoretical predictions.¹ We do not observe a breaking of reciprocity for s-polarized infrared light but do see reciprocity a breaking of reciprocity for p-polarized light. This is, in agreement with the direction in which we apply ourpredicted field polarization required to break time-reversal symmetry. This work shows theconstitutes the first experimental observation of reciprocity ability to break ring, and was performed using thermal radiationeciprocity in under room ambient conditions, and using modest magnetic fields attainable with permanent magnets.
[1] B. Zhao et al., Optics Letters 44, 4203 (2019).

Above-equilibrium “hot” carrier generation in metals is a promising route to convert photons into electrical charge for efficient near-infrared optoelectronics. However, metals that offer both hot-carrier generation in the near-infrared and sufficient carrier lifetimes remain elusive. Alloys can offer emergent properties and new design strategies compared to pure metals. We will show that a noble-transition alloy, Au_xPd_1-x, outperforms its constituent metals concerning generation and lifetime of hot carriers when excited in the near-infrared. At optical fiber wavelengths (e.g., 1550 nm), Au₅₀Pd₅₀ provides a 20-fold increase in the number of ~0.8 eV hot holes, compared to Au, and a 3-fold increase in the carrier lifetime, compared to Pd. The discovery that noble-transition alloys can excel at hot-carrier generation reveals a new material platform for near-infrared optoelectronic devices.

While photonic crystal waveguides have an extensive history in nanophotonics, we introduce a photonic crystal concept - as an infrared photon (thermal) concentrator, designed for solar-driven thermal heterogeneous catalysis for carbon monoxide reduction. A key goal in reducing global CO₂ emissions is the development of a net-zero carbon synthesis route for renewable generation of chemical fuels to replace conventional fossil fuels. Solar driven processes under mild conditions present a sustainable alternative to large-scale industrial processes that typically operate under high temperatures (>200 ^oC) and elevated pressures (>10 atm). However, thermal radiation losses typically prevent a catalyst photothermally heated by unconcentrated sunlight from reaching the minimum operating temperatures for heterogeneous catalysis of carbon monoxide which is generated by photoelectrochemical CO₂ reduction, to fuel and chemical products by the Fischer-Tropsch reaction pathway under direct 1 sun (1000 W m^-2) illumination. While higher temperatures can be reached through the use of geometry solar concentrators, these systems are difficult to scale up due to their complexity and reliance on solar tracking systems to optimize concentration.
Instead, building upon the concept of photonic crystals as medium for light trapping in photovoltaics, the emission of a catalyst coupled to an infrared photonic crystal can be trapped to infrared photons generate by solar heating of a visible photoabsorber, thereby concentrating the solar-generated heat. A 3.5 micron thick lightly doped germanium is chosen as the photonic crystal waveguide material due to its high index, solar absorption absorption coefficient stability in the visible range, and nearly lossless infrared characteristic. While two-dimensional photonic crystal designs can involve rod or hole arrays, the benefits of a hole array in this application are twofold: (1) stable trapping across a wide range of wavelengths, and (2) the ability to flow a reactive gaseous easily through the structure. We have designed a holey germanium photonic crystal with thermal infrared photon trapping efficiency above 90% for wavelengths from 8-15 microns. This prototype indicates promise for a first photonic crystal thermal concentrator to address scaling challenges and temperature requirements for solar thermochemical reactions under ambient sunlight.

Metasurface based flat optics, an ultrathin layer of structured nano-antennas imparting local and space-variant abrupt phase changes, promises great potentials in shaping light's wavefronts in desirable manners. However, most of the reported metasurfaces are restrained by their design strategies and can only allow one or two physical parameters such as phase, polarization or amplitude to be controlled at the same time. Geometric metasurface provides dispersionless phases depending on the in-plane orientation angles of the meta-atoms. However, the polarization state of the incident light is restricted to circular polarization only. The combination of geometric phase with propagation phase was proposed to relieve the constraint and realize full control of the phase and polarization. However, this is achieved at the cost of increased thickness of the antennas to wavelength scale and concomitant massive computations for complicated meta-atom designs with intricate geometries at variant locations, meanwhile, the propagation phase is intrinsically dispersive, which restricts its ability to working at a specific wavelength. Here, we demonstrate a new dimerized metasurface design that can allow simultaneous control of the four basic parameters in one go as well as underpin variety of multi-functional optical elements. The proposed dimerized metasurface is consisting of two identical meta-atoms with exquisitely controlled in-cell displacements and orientations. Consequently, multi-functional holography that the amplitude, phase, polarization and color components of the diffracted beam can be completely controlled has been demonstrated. The proposed diatomic metasurfaces may extensively promote applications based on flat optics

Meta-lens can achieve diffraction-limited focusing with nanoantenna array in a compact size. Many flat optical devices have been demonstrated using meta-lens lately. For applications of full-color imaging and detections, the correction of chromatic aberration is a key issue. We introduced the integrated-resonant unit to incorporate with the geometric phase method to realize achromatic meta-lens. Broadband meta-lenses working over the near-infrared in reflection and entire visible spectrum in transmission are achieved. The full-color imaging is demonstrated by using GaN-based achromatic meta-lens. The high-dimensional light field imaging system was implemented by the nature-inspired 60 × 60 achromatic meta-lens array. The depth and velocity sensing of the imaging objects are achieved. A high-dimensional quantum entanglement light source is demonstrated by using a meta-lens array which composed of 10 by 10 meta-lenses. The meta-lens array can make multi-focusing spots into the nonlinear crystal simultaneously, and generate the multi entangled photon pairs. The entangled photon pairs are generated due to the spontaneous parametric down-conversion (SPDC) effect in the nonlinear crystal. We demonstrated the high-dimensional entanglement and 2-, 3- and 4-dimensional two-photon path-entanglement with different phases coded by the metalenses. It is a great progress for the quantum light source and quantum applications such as quantum computing, quantum cryptography, and quantum communication. We have developed optical meta-devices for beam deflection and reflection, polarization control and analysis, holography, second-harmonic generation, laser, tunability, imaging, absorption, color display, focusing of light, multiplex color routing, light-field sensing, and high-dimensional optical quantum source. The great advantages of meta-lens and meta-devices are their new properties, lighter weight, small size, high efficiency, better performance, broadband operation, lower energy consumption, and CMOS compatibility for mass production.

Spin Hall effect of light (SHEL) refers to a spin-dependent and transverse splitting of light at an optical interface. This phenomenon can be readily found in various interfaces, but has been often left out of consideration because the splitting is much smaller than the wavelength. Thus, increasing the amount of the shift by using artificially structured materials has attracted scientific interests recently. We demonstrate that vertical hyperbolic metamaterial, or nanotrench structure consisting of metal and dielectrics, can be served as a platform to enhance the SHEL [1]. Under the same conditions of material combinations and total thickness, the enhancement, which is incident angle-dependent, can be higher than 800-fold when the incident angle is 5°, and 5000-fold when the incident angle is 1°. The gigantic SHEL in a vertical hyperbolic metamaterial will enable helicity-dependent control of optical devices including filters, sensors, switches, and beam splitters.
This tendency of diverging shift as the incident angle decreases is not a unique characteristic of the vertical hyperbolic metamaterial, but is a universal feature that appears in many systems to increase SHEL. In other words, previous proposals to enhance the SHEL is limited to a small incident angle, on the order of milli-radians. Diffraction can be used to improve SHEL at a large incident angle [2]. We present the enhancement of SHEL in a dielectric grating on a metal film by maximizing the difference between the transmission coefficients of two linear polarizations. The metal film impedes light transmission except the p-polarized first-order diffracted mode that is coupled to surface plasmon polaritons. These polarization-dependent transmission coefficients lead to a large SHEL at a large incident angle.
Lastly, an approach to achieve a large SHEL with near-unity efficiency is proposed. Despite the remarkable enhancement of SHEL, the efficiency of the effect has been rarely discussed. The enhancement of SHEL in most of the previous proposals and demonstrations has been underpinned by small Fresnel coefficients, which in turn yield extremely low efficiency. In this last part, we present an approach using anisotropic impedance mismatching to attain a large SHEL with near-unity efficiency in the microwave spectrum [3]. A wire medium that has a near-unity transmission for one polarization and low transmission for the other is used to achieve high efficiency. The spin-dependent splitting is experimentally confirmed by measuring transmission coefficients and the spatial profile of Stokes parameters. The large SHEL with near-unity efficiency will enable highly efficient devices with spin-selective functionalities.

References
[1] M. Kim et al. ACS Photonics 6, 2530-2536, 2019
[2] M. Kim et al. APL Photonics 5, 066106, 2020
[3] M. Kim et al. Laser & Photonics Reviews (accepted)

Surface-enhanced Raman spectroscopy has been well investigated in the visible to the IR regime. In the visible to IR spectrum, several plasmonic materials have been given promised results like gold (Au) and silver (Ag). To explore the plasmonic properties in the Ultraviolet (UV) regime, aluminum (Al) has enormous potential to operate in the deep UV to the visible spectrum, so we present deep UV surface-enhanced resonance Raman Spectroscopy (SERRS) driven ultraviolet (UV) plasmonics through aluminum nanoholes. Therefore, the generation of deep UV range localized surface Plasmon resonance (LSPR) for electric field enhancement, the epitaxial aluminum film is grown by PA-MBE (plasma-assisted Molecular beam epitaxy) on sapphire substrate and CMOS compatible techniques are used to fabricate Al nanoholes. To assess, the deep UV SERRS nature of epitaxial Al film, we used the basis of nucleic acid and 2D materials as a benchmarked analyte. An ultra-thin layer ( ~1 nm) of Adenine, Thymine, Cytosine, and Guanine is sublimated on Al nanoholes and recorded the highest Raman intensity enhancement (~10⁶) with comparison to the non-plasmonic substrate. Here, we report the ultrasensitive detection limit of adenine and enhanced visualization of second-order Raman mode of MoS₂ using 266 nm wavelength Raman. This work thus enables, the sensitive detection of DNA and 2D materials.

The Particle-in-Cell (PIC) simulation is a widely used numerical method in plasma physics. We show that it can also be used to robustly describe plasmon resonances, similar to the conventional FDTD method, but with a unique emphasis on the motion of the electrons by tracking the individual conduction electrons in the time domain [1]. Our statistical studies of electron motions provide insight into the femtosecond time-scale dynamics of electrons in plasmons, including the plasmon dephasing, the contributions from different plasmon damping channels, and the electron kinetics during damping. An analysis of the time-resolved velocity distribution of the conduction electrons shows that only a small offset in this distribution constitutes the plasmon oscillation in each cycle. We describe the non-radiative damping through both electron-electron and electron-surface scatterings, where the latter is automatically and inherently included in the electron motions. Electron-surface scattering in PIC can be interpreted as a self-consistent interaction between the electrons and the enhanced local field at the surface. The presented framework will be particularly useful in future studies of plasmons in bimetallic nanostructures.

[1] Ding et al., “Particle Simulation of Plasmons”, Nanophotonics 2020, 9(10), 3303–3313
[2] Do et al., “Electron Dynamics in Plasmons”, (manuscript under review)

Broadband perfect absorbers in the visible have attracted a great deal of attention in many fields, especially for solar thermophotovoltaic (STPV) and energy harvesting systems. However, realizing light absorbers with high absorptivity, thermal stability and broad bandwidth remain a great challenge. In this work, we theoretically and experimentally demonstrate that a single-layer titanium nitride (TiN) metasurface absorber with a total thickness of 160 nm that exhibits broadband perfect absorption with an average absorption of more than 92% for a broad wavelength range from 400 nm to 750 nm in the visible. Unlike reported metamaterial absorbers based on the metal-insulator-metal (MIM) structure, we use a single-layer TiN metasurface, which simplifies the nanofabrication process and also maintains high absorption. The broadband perfect absorption of a single-layer TiN metasurface absorber is contributed by the combination of the intrinsic absorption of TiN and the broadband localized surface plasmon resonance (LSPR) of the lossy TiN nanodisk arrays. These results enable a new approach to realizing hot-carrier devices and solar-thermal energy conversion devices. In addition, we use the same concept of metasurface structure to design and fabricate the refractory color filters. We will also discuss the outlook for refractory metasurface in applications of the hot-carrier photodetection, photocatalysis, radiative cooling, and solar thermophotovoltaics.

Chiral plasmonic structures, which exist in two distinct non-superimposable mirror-image forms, are interesting for (bio)chemical sensing. Almost all biomolecules are chiral, and are of only one handedness. Typically their handedness is detected via differential interactions with circularly polarized light. These interactions are weak, because the molecular dimensions are much smaller than the wavelength of light. Larger plasmonic nanostructures can boost the chiral signal and hence possibly yield high sensitivity and selectivity as well as permit detection in ultra-low volumes. Chiral metasurfaces exploit this and their interaction with chiral molecules, including proteins, has been studied. However, a difficulty in all experiments on metasurfaces to date is that – depending on their orientation – even achiral structures were reported to give chiral signals. This is because the experimental geometry (light in, object, light out) may become handed. It is then not trivial to distinguish the true (inherent real chirality) of a nanostructure from effects that arise due to the geometry of the setup.
We have succeeded in devising a new scheme to measure the true chirality of a single nanostructure that is free from artefacts and that reports a new observable [1]. The polarization-sensitive light scattered off a single nanoparticle can be measured while it is freely suspended in solution. For this, we have engineered a complex-shaped metal nanoparticle with defined handedness [2]. A novel spectrometer is introduced that can measure for the first time an artefact-free circular dichroism spectrum of a (reorienting) single nanoparticle. We thereby show that only the average spectrum taken over isotropic orientations of a single particle gives the same information as the corresponding ensemble spectra measured with the traditional instruments. This is in accordance with the ergodic principle, which is demonstrated for the first time in the context of chiroptical spectroscopy. Finally, we show that by growing hybrid nanostructures that contain a magnetic moment, we can re-orient single chiral metamaterials by means of an external field and obtain their spectral response as a function of their orientation.

[1] Sachs, J., Günther, J., Mark, A.G. et al. Chiroptical spectroscopy of a freely diffusing single nanoparticle. Nat Commun 11, 4513 (2020). https://doi.org/10.1038/s41467-020-18166-5
[2] Mark, A., Gibbs, J., Lee, T. et al. Hybrid nanocolloids with programmed three-dimensional shape and material composition. Nature Mater 12, 802–807 (2013). https://doi.org/10.1038/nmat3685

To study the effect of surface lattice resonances (SLRs) on energy transfer, SLR-supporting square lattices of vertically layered Al-Al₂O₃-Al nanocylinders were fabricated onto a glass substrate. The substrate and lattices were coated with a film containing 800 mM (20 wt %) of the donor dye (P580) and 8 mM (0.16 wt %) of the acceptor (P650) dispersed in poly(methyl methacrylate) and the film's fluorescence spectra on the bare substrate and on the lattices were studied. In the absence of any SLR-supporting lattice, the fluorescence of the donor dye was less than that of the acceptor, with a donor-to-acceptor peak fluorescence ratio of 0.45, indicating that energy was readily transferred from donor to acceptor. In contrast, on a lattice that supports an SLR at 551 nm at k‖ = 0, coinciding with the donor dye emission peak at 550 nm, the fluorescence of the donor dye exceeded that of the acceptor, giving a donor-to-acceptor peak fluorescence ratio of 5.4. Additionally, the film exhibited a greater absolute donor fluorescence and a lesser absolute acceptor fluorescence on this lattice than on those that supported SLRs at other wavelengths. These results suggest that the SLR that coincides at k‖ = 0 with the donor dye's emission peak enhances the radiative decay of the donor at the expense of energy transfer to the acceptor. Notably, the SLRs that coincided with the donor emission peak at larger values of k‖ (at angles of emission of ca. 20° or 50°) did not have this effect.

Through development of an electron-based imaging technique, the electromagnetic field profiles emanating from the edges of an atomically thin MoS₂ flake buried between Al₂O₃ and SiO2 were characterized using photoemission electron microscopy excited by deep-ultraviolet light (DUV-PEEM). The photoemission yield is proportional to the square of the electric field for the single-photon process excited by the UV-light, thereby providing a pathway to the imaging of nanophotonic phenomena using emitted electrons. Because electrons are the sensing entity, the resulting images exhibit resolutions below the photon wavelength. To validate this concept, the dependence of photoemission yield on the wavelength and polarization of the exciting light was first measured and then compared to simulations of the optical response and the electric field quantified with classical optical theory. Close correlation between experiment and theory indicates that photoemission probes the optical interaction of the UV-light with the material stack directly. Utility is then demonstrated by employing the polarization dependence of photoemission to observe fringes indicative of the electromagnetic field profiles resulting from the interaction of the exciting UV-light with the MoS₂. Taken together, this “electron-based ellipsometric imaging” offers an analytical approach through which to visualize the electromagnetic field distributions central to many nanophotonic phenomena while simultaneously mapping optical property variation at sub-wavelength scales.

Acknowledgements: This work was performed under the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories and undertaken, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

We propose bi-functional metasurface which contains structurally colored print and vectorial holograms with eight polarization channels towards advanced encryption applications. The encoded structural color prints can be observed under white light and the fully polarized holograms can be reconstructed using coherent laser source with combination of output polarizer/retarder. To encode multiple hologram images having different polarization states, a pixelated metasurface is adopted thereby digitalizing sets of phase distribution retrieved from the images into single metasurface. Such superpixel consists of four phase-gradient metaatom groups: meta-atom group rotated either clockwise or counterclockwise. Depending on the combination of clockwise and counterclockwise rotating meta-atom group, the polarization states of the reconstructed images are determined. The metaatom contains specifically designed geometric and propagation phase, and reflection spectrum at each spatial location. As a proof-of-concept, we devise electrically tunable optical security platform using our multifunction metasurface incorporated with liquid crystal. The optical security platform is double encrypted: Color printing image that can be decrypted by camera scanning provides first key and corresponding information will be used to fully unlock the double-encrypted information via projected vectorial hologram images. Such an electrically tunable optical security platform will provide a new route towards internet-of-things sensors for security and anticounterfeiting applications.

All inorganic CsPbX₃ perovskite quantum dots (PeQDs) have received much attention due to their excellent properties such as large absorption coefficients, tunable bandgap, and narrow band emission. However, due to their low upconversion efficiency under near-infrared light excitation, potential applications have been limited. Recently, upconversion nanoparticle (UCNP) sensitized PeQD has been reported as a solution, but the low quantum yield of UCNPs and low photoluminescence (PL) intensity of PeQDs film have still remained challenges. In this study, enormous luminescence enhancement of PeQDs was achieved under near-infrared (NIR) excitation through sensitization by UCNPs and plasmonic coupling. To overcome the low luminescent quantum yield of UCNPs, gap plasmonic mode was integrated into the UCNP-perovskite film via metal-insulator-metal (MIM) configuration consisting of gold nanorods (AuNRs) and Ag thin film. The AuNRs-UCNPs/PeQDs-Ag film (MUPM) configuration is reported, for the first time, using UCNPs and PeQDs as an insulator layer, which provides high interfacial stability of perovskites-metallic nanostructures. Despite a thin active layer, a dominant green emission of PeQDs is observed under NIR excitation with high energy transfer efficiency by using similarly sized UCNPs and PeQDs. Furthermore, by capping AuNRs with the amphiphilic diblock copolymer, PL quenching and morphology deformation were suppressed. Therefore, an overall 29-fold upconversion enhancement was achieved for the green emission in the MUPM configuration owing to the strong localized electric field and the coupling of longitudinal localized surface plasmon resonance band of AuNRs with excitation of UCNPs. The present study provides a novel route to prepare highly efficient and effective emissive devices based on MIM configurations using an insulator layer composed of UCNPs and PeQDs, which can be expanded to serve a generalized platform in a broad range of applications.

In situ measurements of trace gases in planetary environments are crucial for the understanding of atmospheric, geological, and possible biological processes. Currently, most planetary missions rely on large, heavy, high power instrumentation such as mass spectrometers for in situ chemical analysis. The size, weight and power (SWAP) of these payloads make the overall missions costly and challenging. In addition, some of the key species required for the origin of life as we know it, such as methane, ammonia, and water, are difficult to distinguish using mass spectrometry alone due to mass interference issues. To enable lower cost science missions, we have developed a highly miniaturized and compact multifunctional environmental sensor platform based on additive manufacturing techniques of low dimensional materials. The unique electrical and physical properties of nanomaterials coupled with the high surface area-to-volume ratio make them outstanding candidates as extremely sensitive gas detecting elements. In addition, our functionalized sensors are able to distinguish between key species of interest to astrobiology. The ability to print the sensor systems, heaters, interconnects and wireless antenna directly on the same substrate eliminates the need to integrate individually fabricated components. This makes the packaging significantly more robust and reduces the footprint of the overall instrument. In this talk, I will present the development and status of a unique instrument-on-a-chip for in situ planetary research, and potential upcoming mission opportunities.

Kirigami, the art of paper cutting, presents a powerful tool to create complex and reconfigurable three-dimensional (3D) geometries from simple 2D cut patterns, which can be scaled across many orders of magnitude to yield macro- to nanoscale structures. The ability to achieve out-of-plane buckling, designed 3D shape, the robustness of the patterns under cyclic reconfiguration and the compatibility to conventional fabrication processes of kirigami structures together promise untapped possibilities for the efficient modulation of optical beams. Here we show that kirigami optics affords real-time modulation of THz beams with polarization rotation and ellipticity angles as large as 80° and 40° over thousands of cycles, respectively. The unusually large amplitudes of polarization rotation and ellipticity angles exceeding all known THz modulators were enabled by double-scale patterns composed of microscale metallic stripes together with submillimeter-scale kirigami cuts. We measured terahertz circular dichorism (TCD) spectra of several representative biological samples using chiral kirigami metamaterials and found distinctive TCD peaks. Kirigami metamaterials will also play an indispensible role for other applications, such as biomedical imaging, biosensors, line-of-sight telecommunication, information encryption and space exploration.

The objective of this work is to understand the underlying molecular mechanisms of structural and mechanical properties of shape memory polymer (SMP) nanocomposites used for reconfigurable structures. In this work, we have performed coarse-grained molecular dynamics simulations and entanglement analysis of buckypaper (BP)/epoxy nanocomposites with a focus on their mechanical and shape memory performances, specifically on prediction of the Young’s modulus of the material as a function of carbon nanotube (CNT) loading. We found that the Young’s modulus linearly increases with CNT volume fraction below 0.16 (40 wt%) followed by a sharp growth of the modulus at higher loading where the onset of entanglements of nanotubes was determined. Additionally, we found a significantly greater increase of the modulus at T > T_g compared with the values below the glass transition temperature for all considered systems. The simulation suggests that incorporation of BP restricts relaxation of network strands of the polymer matrix and leads to resistance in the recovery process of composites. Computational results are compared with our experimental data on temperature controlled mechanical testing in tension of BP/epoxy nanocomposites.

A number of recent viral outbreaks, including the recent COVID-19 pandemic, have caused significant public health and economic concerns. Rapid and reliable point of care (POC) diagnostic technologies would provide a necessary first step for the proper treatment and management of these diseases [1]. State-of-the-art in vitro diagnostics (IVD) technologies including enzyme-linked immunosorbent assays (ELISA), and quantitative polymerase chain reaction (qPCR) are labor intensive, require cumbersome sample preparation, sophisticated equipment, and skilled operators. Here, using a novel nanophotonic approach, we present an ultrasensitive optofluidic diagnostic platform allowing detection of viral antigens with simple instrumentation at a concentration level five orders of magnitude lower than the most sensitive ELISA tests, and orders of magnitude more sensitive than the most advanced IVD technologies. In our tests with human serum samples, we experimentally demonstrated quantitative detection of viral antigens at attomolar sensitivities corresponding to 5-10 virus particles per 100 microliters. At this sensitivity level, our novel nanophotonic platform provided a compelling alternative to RNA-based quantitative-PCR tests for viral load detection.

[1] Cormac Sheridan, “Coronavirus and the race to distribute reliable diagnostics”, LNature Biotechnology 38, 382-384 (2020)

In order to continue improving the performance of transistors beyond the 5nm node, there is interest in optimizing the use of Si and Ge in unique geometries [1]. A novel diffusion mechanism of Ge along an oxidizing Si/SiO₂ interface was reported, which resulted in the formation of strained Si nanowires [2]. Taking advantage of this diffusion mechanism has the potential of forming nanowires, nano-dots, and arbitrary shapes based on lithographic patterning; however, there is no understanding of the diffusion mechanism that results in the formation of these structures. To investigate this mechanism, alternating 20nm thick layers of Si and Si_0.7Ge_0.3 were grown in a superlattice and patterned into fins via electron beam lithography. The final pattern contained multi-layered Si/SiGe fins with widths varying from 70 to 280nm with a single 100nm layer of Si at the base in which to observe the diffusion of Ge down the fin’s sidewall during high temperature oxidation.

This lateral diffusion was observed over a range of times and temperatures between 800 and 950°C. Cross-sectional TEM samples were prepared after each anneal and the rate of diffusion was measured through analysis of HAADF-STEM images. The diffusion constant for Ge’s lateral movement down the Si sidewall was measured to be approximately 1x10^-14cm²/s during 900°C oxidation. This is several orders of magnitude larger than Ge’s interdiffusion with Si, which helps to explain why the nanowires and quantum dots are formed within these materials [3], [4]. The correlation between the thickness of the thermally grown oxide, the width of the SiGe layer forming on the side of the fin, and the length of the Ge diffusing down the side of Si sidewall has also been studied, providing additional insight into the diffusion mechanism. This analysis has been carried out at multiple temperatures between 800 and 950°C, and the activation energy for Ge’s lateral diffusion will be presented. These experimental results will be accompanied by a model using the Florida object-oriented process and device simulator (FLOOXS), which has been using to assist in the extraction of diffusivities. Preliminary results using the process simulator’s model will be compared to the experimentally observed evolution of the nanostructures.
Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.
[1] T. David et al., “New strategies for producing defect free SiGe strained nanolayers,” Sci. Rep., vol. 8, no. 1, pp. 1–10, Feb. 2018, doi: 10.1038/s41598-018-21299-9.
[2] W. M. Brewer, Y. Xin, C. Hatem, D. Diercks, V. Q. Truong, and K. S. Jones, “Lateral Ge Diffusion During Oxidation of Si/SiGe Fins,” Nano Lett., vol. 17, no. 4, pp. 2159–2164, Apr. 2017, doi: 10.1021/acs.nanolett.6b04407.
[3] R. Kube et al., “Composition dependence of Si and Ge diffusion in relaxed Si1−xGex alloys,” J. Appl. Phys., vol. 107, no. 7, p. 073520, Apr. 2010, doi: 10.1063/1.3380853.
[4] Y. Dong, Y. Lin, S. Li, S. McCoy, and G. Xia, “A unified interdiffusivity model and model verification for tensile and relaxed SiGe interdiffusion over the full germanium content range,” J. Appl. Phys., vol. 111, no. 4, p. 044909, Feb. 2012, doi: 10.1063/1.3687923.

Thermophotonics (TPX) is a technology close to thermophotovoltaics (TPV), where a heated light- emitted diode (LED) is used as the active emitter of the system [1]. With the development of LEDs and the increase of their achievable quantum efficiency, TPX has come out as an attractive concept for both energy harvesting and refrigeration [2]. The many studies on near-field (NF) thermal radiation and their application into efficient NF TPV devices [3] highlight the possibility to extend the concept to near-field thermophotonics [4], where enhanced energy conversion is due to both the electric control and wave tunneling.
This contribution explores the theoretical capabilities of NF-TPX systems. Ideal cases are compared with more realistic structures, involving materials such as Si, GaAs and GaN. Based on the local absorption and emission distributions [5], the results include detailed IV characteristics of the LED and PV cell sides around the maximum power point, and highlight in particular the advantages in comparison to far-field TPX and NF TPV. A particular attention is drawn to the search of optimal surface properties for the LED and the PV cell, which could potentially be approached by using metasurfaces. The impact of the temperature difference between these two elements, their quantum efficiency and the thickness of the different components are amongst the studied parameters.

[1] N. P. Harder and M. A. Green, Semicond. Sci. Technol. 18, S270, 2003. [2] T. Sadi et al., Nat. Phot. 14, 205, 2020. [3] C. Lucchesi et al., arxiv: 1912.09394, 2019. [4] B. Zhao et al., Nano Lett. 18, 5224, 2018. [5] M. Francoeur et al., J. Quant. Spectr. Rad. Transf. 110, 2002, 2009.

We acknowledge the funding of EU H2020 FET Proactive (EIC) programme through project TPX-Power (GA 951976).

By exploiting computational backend, coupled with a designer meta-optics we demonstrate high-quality aberration free imaging using a single meta-optic in the visible wavelength range. The aperture is currently 0.5-1mm, but can be extended further. Several inverse design tools and end-to-end optimization are used to achieve such performance.

Engineering light absorption in GeSn structures is crucial to enhance their basic device performance for a variety of applications such as MIR photodetectors and solar cells. Surface texturing to reduce the reflectivity is a key strategy to tune the optical properties. Since Group IV semiconductors typically have large refractive indices compared to air (between 3.4 and 4.2), planar opto-electronic devices are plagued by this refractive index mismatch. A promising method to circumvent this limitation is the use of semiconductor nanowires arranged in arrays covering an area of macroscopic dimensions. Top-down etched GeSn nanowire (NW) arrays were microfabricated with varying geometrical configuration. Visible and near IR spectroscopic ellipsometry measurement was undertaken in the spectral range from 900 nm to 2500 nm to evaluate the complex optical constant (n and k) for a 10% GeSn material. Detailed finite difference time domain (FDTD) simulations were combined with experimental analyses to systematically investigate light-GeSn nanowire interactions to tailor and optimize the NW array geometrical parameters and the corresponding optical response. The diameter-dependent leaky mode resonance peaks are theoretically predicted and experimentally confirmed with a tunable wavelength from 1.5 to 2.2 μm. A three-fold enhancement in the absorption with respect to GeSn thin film at 2.1 µm was achieved using nanowires with a diameter of 325 nm. The coupling between the HE₁₁ and HE₁₂ resonant modes manifests at NW diameters above 325 nm, while at smaller NW diameters and longer wavelengths the HE₁₁ mode is guided into the underlying Ge layer. Additionally, the presence of tapering in NWs further extends the absorption range while minimizing reflection. The ability to manipulate light-matter interactions at the nanometer scale with GeSn is opening up new opportunities for spectral tunability in the extended short-wave infrared range.

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 metasurfaces, i.e. nanostructured optical coatings, has further accelerated the development of flat optics through the realization that resonant optical antenna elements can be utilized to facilitate local control over the light scattering amplitude and phase. At the same time, researchers have aimed to identify ways to dynamically tune the properties of resonant optical antennas. The two developments are now leading to the development of dynamic flat optics.
In this presentation, I will highlight recent efforts in our group to realize electrically-tunable metasurfaces employing nanomechanics, electrochemistry, microfluidics, phase change materials, and atomically-thin semiconductors. Such elements can find application in systems for optical beam steering and wavefront manipulation as well as dynamic holography. I will illustrate how the proposed optical elements can be fabricated by scalable fabrication technologies, opening the door to many commercial applications.

Quantum emitters strongly coupled to optical cavity modes create new hybrid states called polaritons, resulting in a vacuum Rabi splitting (Ω). Strikingly, the magnitude of this splitting correlates with modified emission properties and chemical reaction rates. However, active control of this coupling strength is difficult due to the fixed properties of the coupled oscillators (both, the quantum emitter and optical resonator). Here, we demonstrate active tuning of excitonic strong coupling in a system where organic dyes strongly couple to propagating surface plasmon polaritons (SPPs). After electropolymerization of a methylene blue (MB) film on a SPP-supporting Au surface, we demonstrated active control of coupling strength through reversible redox cycling of the MB film. Excitonic strong coupling was effectively cycled on and off with electrode potential either continuously tuned (transient) or held at a fixed value (static) and were quantitatively correlated with simultaneously measured electrochemical charge. Switching between reduced and oxidized forms of the dye resulted in Ω values tuned from ~0 to ~280 meV, i.e., ~14% of the transition energy. The ability to control coupling strengths in a given emitter-cavity coupled system is a key capability for utilizing polaritonic states for cavity-mediated chemical reactions or optical devices.

Hybrid states such as polaritons emerging from light-matter interaction are mostly observed in excitonic materials integrated in an external optical cavity. Two-dimensional (2D) hybrid organic/inorganic perovskites namely Ruddlesden-Popper (RP) perovskite have shown strong exciton confinement and oscillator strengths (12 (n=1) and 6 (n=2)).¹ Further, their optical constants suggest that the loss tangents in RP phase perovskites are among the highest for known excitonic semiconductor materials. These extraordinary optical properties can be exploited to create both the gain and cavity media in the semiconductor for strong light-matter interaction. Here, we show that intrinsic cavity mode formation in 2D perovskite flakes of intermediate thickness (~200 nm) on Au substrate leads to polariton state formation with Rabi splitting ~135 meV. In contrast, the thin flakes (15 nm) remain purely excitonic in nature. We confirm exciton-polariton formation from reflectance spectroscopy and associated emission spectroscopy at room temperature. Using transfer-matrix calculations, the experimentally observed exciton-polariton states can be modelled with two-coupled oscillators that corroborate well with the reflectance spectra. Further, temperature dependent photoluminescence and reflectance spectra shows the Rabi splitting and hybrid emission from exciton-polariton states present at room temperature, remains unperturbed down to 80 K. Tunable polariton formation from 520 nm to 610 nm by varying the perovskite composition will be presented. Rabi splitting increases with increase in oscillator strength of the perovskite as n is varied from 4 to 1. We believe that our work will open new avenues for exploring polariton in optoelectronic devices and photochemistry.

Keywords: Ruddlesden-Popper perovskites, Hybrid states, Polaritons, Rabi splitting, Intrinsic cavity mode


References:
1. Song, B. et al. Determination of Dielectric Functions and Exciton Oscillator Strength of Two-Dimensional Hybrid Perovskites. (2020) doi:https://arxiv.org/abs/2009.14812.

We present a holistic nanophotonic 2D material-based design strategy for ultrathin laser-driven lightsails that provides both high weight-constrained reflectance over a defined bandwidth and enhanced thermal emittance. Our investigation explores the use of hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS₂) arranged in an inverse-designed layered photonic crystal architecture. Both the optical and thermal properties of the design are shown to be viable, emphasizing the holistic merit of its functionality. Relative to other designs, our concept exhibits impressive spectral emissivity values and accelerations for its mass, as well as the potential for excellent mechanical compliance. We compare its performance against other designs and show reasonable benchmarks of emissivity necessary to produce given steady state temperatures.
The Breakthrough Starshot Initiative aims to send a nanocraft to Earth’s nearest habitable exoplanet, Proxima Centauri B, within 20 years of launch. To do so, the craft will use a reflective light sail accelerated by a ~100 GW phased array of lasers to speeds of 0.2c¹. This requires a sail that is highly reflective over a Doppler-broadened wavelength range starting from the laser wavelength, is highly emissive in the mid-wave and long-wave infrared to effectively dissipate heat during acceleration, is mechanically robust enough to not degrade under the large forces acting on it, is correctly shaped to provide beam riding stability, and has a mass totaling ~1 g with a similarly-sized payload. Previous work has demonstrated designs using conventional dielectric materials such as Si₃N₄², Si, and SiO₂^3,4. Given the stringent weight and mechanical requirements of the sail, 2D materials may be ideally suited to serve as key lightsail components; however, to date they have not been extensively explored for this application. In addition, while several studies have considered optical factors in order to minimize acceleration distances, few have co-optimized thermal properties in order to present a holistic assessment of the overall suitability of a sail design.
Here we present an optimized 2D material-based nanophotonic sail design that is optically suitable and able to effectively dissipate heat as thermal radiation to maintain reasonable in-flight temperatures. This sail consists of a 1.264-gram multi-layer photonic crystal slab composed of h-BN and MoS₂ layers and is shown, through full-field electromagnetic simulations, of being capable of accelerating an equivalent mass payload to 0.2c within 14 gigameters. The materials utilized have refractive indices ranging from ~2.13 for h-BN to 3.98-3.87 for MoS₂ over the doppler broadened range. We show the sail’s fabricability using currently available patterning and etching techniques, provide benchmark material emissivity values necessary to maintain a given steady-state sail temperature, and demonstrate how payload weight influences the optimal sail design. Additionally, we discuss thermal considerations such as vacuum evaporation of features and how maximum temperature limits of materials should be set to ensure sail viability. Finally, we discuss the need for high resolution measurements of 2 dimensional optical, thermal, mechanical, and coupled material constants to continue to improve simulation accuracy and true viability of future sail designs.

1) Atwater Nat. Mater. 17(2018)861
2) Jin, W. ACS Photonics 7(2020)9
3) Salary Laser Photonics Rev. 14(2020)1900311
4) Ilic Nano Lett. 18(2018)5583

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 Particularly, these van der Waals bonded semiconductors exhibit strong excitonic resonances and large optical dielectric constants as compared to bulk 3D semiconductors. .
First, I will focus on the subject of strong light-matter coupling in excitonic 2D semiconductors, namely chalcogenides of Mo and W. 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.^{2, 3} 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.⁴
Next, we will show the extension of these results to multilayers and superlattices of excitonic chalcogenides with alternating layers of boron nitride and aluminum oxide. These hybrid multilayers offer a unique opportunity to confine light in < 3 nm thick direct band gap absorbers over cm² scale areas. We will discuss the physics of strong light-matter coupling and applications of these multilayers. Finally, we will also present our recent and on-going works on tunable light-matter interactions in hybrid organic-inorganic perovskites⁵ where we observe exciton-polariton hybrid state emission at room temperatures in an external cavity-less geometry. Finally, I will also present our recent work on giant gate-tunability of optical constants in the telecom band in thin-films of high purity, semiconducting, carbon nanotubes.⁶ Our results highlight the vast opportunities available to tailor light-matter interactions in quantum confined materials in simple and practical designs enabling study of novel photonic phenomena and presenting avenues for practical technologies.
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.; Wong, J.; Atwater, H. A. ACS Photonics 2017, 4, 2692-2970.
3. Brar, V. W.; Sherrott, M. C.; Jariwala, D. Chemical Society Reviews 2018, 47, (17), 6824-6844.
4. Zhang, H.; Abhiraman, B.; Zhang, Q.; Miao, J.; Jo, K.; Roccasecca, S.; Knight, M. W.; Davoyan, A. R.; Jariwala, D. Nature Communications 2020, 11, (1), 3552.
5. Song, B.; Hou, J.; Wang, H.; Sidhik, S.; Miao, J.; Gu, H.; Zhang, H.; Liu, S.; Fakhraai, Z.; Even, J.; Blancon, J.-C.; Mohite, A. D.; Jariwala, D. arXiv preprint arXiv:2009.14812 2020.
6. Song, B.; Liu, F.; Wang, H.; Miao, J.; Chen, Y.; Kumar, P.; Zhang, H.; Liu, X.; Gu, H.; Stach, E. A.; Liang, X.; Liu, S.; Fakhraai, Z.; Jariwala, D. ACS Photonics 2020, 7, (10), 2896-2905.

High quality factor (“high Q”) cavities have revolutionized information processing, communications, sensing, and nonlinear optics by increasing photon storage times and significantly enhancing light-matter interactions. However, when the size of dielectric cavities is reduced to the nanoscale, resonant modes start to resemble point sources, scattering an incident wave in many different directions. While this scattering has been leveraged to create remarkable metasurfaces that precisely control the phase, amplitude, and polarization of light in an ultrathin footprint, metasurfaces generally exhibit high radiative loss rates and thus low Q-factors. Here, we present a general strategy for crafting high quality factor resonances in a phase gradient metasurface, and apply these results to achieve 1) multiplexed nucleic acid detection and 2) efficient electro-optic modulation. To creat a high-Q metasurface, we introduce subtle structural perturbations to individual resonators to weakly couple free-space light into otherwise bound modes. We experimentally demonstrate control over the quality factor and resonant wavelengths in this scheme, achieving record phase-gradient metasurface Q’s greater than 2500. We highlight this scheme’s general applicability by designing and fabricating high-Q metasurfaces that act as beamsteerers to different angles, beam splitters, and lenses. Next, we show how high-Q metasurfaces can enable multiplexed nucleic acid detection. Our high-Q metasurfaces are functionalized with single-stranded DNA to target specific RNA gene sequences, then illuminated with a laser diode; the scattered intensity provides a quantitative measure of the bound nucleic acid concentration. As a proof-of-concept, we focus on SARS-CoV-2 genetic sequences, including recent viral variants; through “multi-color” printing of the probe nucleic acid, we show multiplexed detection of viral variants within 15 minutes. Finally, we show how high-Q metasurfaces can be integrated with electro-optic materials for low-power metasurface modulation. Here, an array of uniform lithium niobate-on-Si antennas is individually addressed with an electrical bias, leading to a full 2 pi phase variation. We show how near-continuous beam-steering can be achieved by modifying the bias across each antenna, en-route to fully reconfigurable, solid-state information processing spanning LiDAR, LiFi, AR/VR, and quantum communications.

Magneto-optical coupling incorporates photon-induced change of magnetic polarization that can be adopted in ultrafast switching, optical isolators, mode convertors, and optical data storage components for advanced optical integrated circuits. However, integrating plasmonic, magnetic and dielectric properties in one single material system is challenging since one natural material can hardly possess multiple functionalities. We use a bottom-up self-assembling synthesis method to integrate multifunctional phases as a nanopillar-in-matrix thin film nanostructure that realizes epitaxial quality, sharp atomic interface and large throughput. Using titanium nitride (TiN) as a durable plasmonic matrix, a metal-free metamaterial platform with embedded nickel oxide (NiO) vertical nanorods that function as tunable ferromagnetic nanodomains has been demonstrated. Such a dissimilar ceramic-ceramic combination enables a strong hyperbolic dispersion in the visible and near infrared frequencies. More interestingly, when Au is introduced in TiN-NiO, a hybrid core-shell nanopillar array is formed where the two-monolayer Au shell serves to release the strain energy at the TiN/NiO interface. We demonstrate that a significantly enhanced long-range ordering of core-shell nanopillars can be achieved by using a template bottom-up growth process, which enables stronger Kerr anisotropy that is promising for building tunable and modulated all-optical nanodevices.

Plasmonics is an emerging field in the intersection between nanotechnology, photonics and electronics. Surface plasmons are coupled collective excitations of conduction electrons and an applied electromagnetic field. Plasmons hereby enable enhancement, confinement and manipulation of light at the nanoscale level. Noble metal-based nanostructures are the most widely studied plasmonic materials since they exhibit strong electromagnetic field enhancement in the visible spectral range. However, strong dissipation originating from interband electronic transitions and losses in noble metals, makes it imperative to investigate alternative building blocks with lower losses and more diverse properties. Metamaterials are new emerging building blocks in electronics and photonics. They are proposed as versatile and tunable platforms supporting various surface plasmon modes exhibiting exotic plasmonic phenomena.

This study will focus on the plasmonic behavior of novel 3D hybrid metamaterials using monochromated electron energy loss spectroscopy (Mono-EELS) in conjunction with scanning/transmission electron microscopy (S/TEM) and X-ray energy dispersive spectroscopy (XEDS) to identify and spatially resolve various surface plasmon resonances (SPRs) over a wide spectral range of NIR-Vis-UV. Metalattices as a subgroup of metamaterials, are nanostructured 3D ordered hybrid materials on the range of sub 100 nm (sub wave-length scale). The metalattice nanostructures are high pressure confined CVD (HPcCVD) synthesized nanostructures, compromised of a SiO₂ close-packed template infiltrated by either Ag or Si-Ge, forming periodic and long-range interconnected structures. A void-free infiltration of these 3D ordered frameworks can provide a versatile and tunable platform as integrated plasmonic interconnects for large optical confinement and long propagation distance applications. By spatially and spectrally resolving the plasmonic response, localized and delocalized effects are studied, and the implications of confinement, interconnectivity, substrate effect, presence of cavity arrays as potential tools for modulating and tuning SPRs are explored. This work also utilizes theoretical calculations to further support the experimental measurements.

Plasmonic metalattice as a periodic and long-range interconnected structure is a novel route to make highly tunable and cost-efficient plasmonic materials with enhanced photonic properties providing a significant control over ultra-local modification of surface plasmon resonances. This understanding is a crucial key for a more efficient electromagnetic energy storage, enhanced biological, chemical sensing and next generation of transparent and flexible optoelectronic/plasmonic devices.

Passive radiative cooling (PRC) of terrestrial objects is achieved by radiative heat loss into space through the long wavelength infrared (LWIR) atmospheric transmission window. Due to its passive nature and net cooling effect, it is a sustainable way to cool human environments. A major goal of radiative cooling research is to create designs with near-ideal spectral properties – i.e. selective emittance in the LWIR (λ~8-13 μm), and perfect reflectance elsewhere in the solar-thermal wavelengths (λ~0.2-40 μm). However, most PRC designs are non-ideal with regard to selectivity [1-2], or else need multiple materials and complex architectures [3].
This presentation proposes a bilayer porous ceramics as selectively LWIR emissive radiative coolers with near-ideal optical performance. The selective emittance arises from the Christiansen effect, while the high reflectance elsewhere arises from the porous structure (for λ < 8 μm) and Restrahlen reflection (for λ > 13 μm). It theoretically shows how this can lead to near-ideal spectral properties, and demonstrate real examples that can achieve this behaviour. Along with their robustness and resistance to weathering, this makes porous ceramics near-ideal materials for radiative cooling.

[1] J. Mandal et. al., Science 362, 315 (2018)
[2] Y. Zhai et. al. Science 355, 1062–1066 (2017)
[3] A. Raman et. al., Nature 515, 541 (2014)

Jyotirmoy Mandal is supported by Schmidt Science Fellows, in partnership with the Rhodes Trust.

Materials drive innovation across science and technology, and photonic applications are no exception. In the last decade, a wide range of new and emerging materials have been realized to improve nonlinear interactions, support single-photon emission, and explore new realms of light-matter interaction. Among them, two classes of materials have had a particular impact in the area of nanophotonics, transition metal nitrides and transparent conducting oxides. The former has provided a robust and CMOS-compatible metallic alternative to traditional elemental metals while the latter has unlocked a new field of dynamic tunability and enhanced nonlinearities through epsilon-near-zero properties. In this talk, we will highlight our work to extend the development of two key materials, TiN and Al:ZnO, using new realms of scalable, low-temperature, CMOS-compatible plasma-enhanced atomic layer deposition. In addition, we will discuss some recent applications of these materials to realize robust plasmonic security devices, and enhanced dynamic applications for nonlinear optics and integrated photonics.

Miniaturization is a main stream in modern technology, but reduction of conventional optical components accompanies performance degradation that limits the minimum feature size of optical devices. Metasurfaces that consist of ultrathin subwavelength antenna arrays can be a promising solution because metasurfaces provide an effective way of wavefront engineering without constraints on the device size. Electromagnetic responses of individual building blocks are determined by its geometric configurations, and many kinds of antennas have been explored to clarify the capability of metasurfaces; thereby, it has been verified that dielectric antennas can control amplitude, phase, and even both of them simultaneously.

The capability of wavefront engineering allows to realize versatile future applications such as holograms, lenses and color filters. Fundamental limitations of conventional holograms such as twin image and narrow viewing angle can be removed by metaholograms due to their sub-wavelength pixel size. Propagation phase of isotropic building blocks enables polarization-insensitive operation while geometric phase of anisotropic building blocks allows broadband operation of multifunctional metaholograms (i.e. image hologram, multiplexed metaholograms). Furthermore, both propagation phase and geometric phase can be considered in design of meta-atom, which enables a multicolor metahologram and complex-amplitude hologram. The recent advanced understanding of building blocks brings about an increase of the number of hologram encoded in the metasurface based on dispersion engineering and orbital-angular-momentum multiplexing. The metaholograms can also be extended to random point-cloud generation for application toward 3D object detection. The same design method described above can be applied to polarization independent broadband beam splitting and ultrathin light-focusing devices, i.e. metalenses.

Metasurfaces can engineer transmission/reflection spectrum in visible regime, i.e. sub-wavelength color printing. The building block to modulate scattering response is high-index dielectric Mie-scatterer which resonantly radiate light with fundamental modes when its size become comparable to wavelength of incident light. The metasurface, composed of arrays of Mie-scatters, transmit/reflect lights with the resonance modes, thus rendering structural colors which are changed according to the geometry of the scatterers. The metasurface which consists of asymmetric unit structure switch its colors depending on the polarization state of incident light, thus enabling application for optical cryptograpy. Adoption of phase change materials and stimuli-responsive materials enables active color filters.

Comprehensive metasurfaces that control both phase and amplitude have been realized by adjusting unit structures. The hologram resolution can be drastically improved by controlling complex amplitude using X-shaped antennas, and both functions of holography and color printing can be integrated in a single metasurface.

Recently, much metasurface research has aimed to embed nanoparticle-based hierarchy in building blocks to enhance the chirality²¹ and refractive index^16,22,23. Furthermore, actively tunable meta-holographic displays with designer liquid crystal modulators will enable interactive holographic displays and unconventional photonic sensor applications^24,25. In the future, metasurface research will be further expanded to a practical region by exploiting diverse light properties (e.g. orbital angular momentum) to realize real-time 3D holographic video displays or advanced optical security labels⁶.

Hydrogenated amorphous silicon has emerged as materials for dielectric metasurfaces, which are promising optical platforms since it has compatibility with mature complementary metal-oxide-semiconductor processes. However, the bandgap of hydrogenated amorphous silicon filters electromagnetic waves at the visible frequencies, preventing it from being optical devices at the visible. Here, we investigate structural disorders of silicon and hydrogenation in order to suppress the bandgap of it to produce visibly transparent hydrogenated amorphous silicon. The structural configuration of silicon-hydrogen bindings is varied by the chemical deposition equilibrium of plasma-enhanced chemical vapor deposition chambers. The chamber atmospheres are changed by manipulating substrate temperatures, chamber pressures, radio frequency power, and input gas ratio. Chemical deposition equilibrium affects the bonding configurations and we reveal that substrate temperature and chamber pressure affect bonding configuration, providing wide coverages of complex refractive index at the visible frequencies. The refractive index of hydrogenated amorphous silicon can be changed from 3.0 to 4.1; the extinction coefficient can be as low as 0.09 at the wavelength of 450 nm. We also reveal the bonding configuration to interpret the relationship between bandgap and composition of hydrogenated amorphous silicon. The X-ray diffraction and Raman were conducted to uncover the crystallinity and stretching vibration, respectively. Micro-crystallinity induces a low extinction coefficient at the visible, and the polyhydride bonding contributes to high transparency. This low-loss hydrogenated amorphous silicon achieves the lowest values of the extinction coefficient are 0.082, 0.017, and 0.009 at the wavelengths of 450, 532, and 635 nm, respectively. Low-loss hydrogenated amorphous silicon is confirmed with beam steering metasurfaces, which deviate the incident light with precisely designed angles. Demonstrated low-loss metasurfaces achieve modulation efficiencies of 64.7% at 450 nm, 90.9% at 532 nm, and 96.6% at 635 nm, which are compatible with conventional low-loss dielectrics such as titanium dioxide and gallium nitride. The metasurfaces steer incident light with 9.9, 12.7, and 11.3 degrees operating at 450, 532, and 635 nm, respectively, which highly coincide with 10.8, 12.8, and 11.4 degrees at each wavelength. Considering its low optical losses and a large coverage of the complex refractive index, our low-loss hydrogenated amorphous silicon will be the dominant platform material at the visible frequencies.

Passive radiative cooling is a concept where an object on the Earth can radiate heat into outer space through the atmospheric window (AW) in the mid-infrared spectrum (8-13 μm). As the objects radiate, they cool down. For effective energy-free radiative cooling, the object must have low absorption of energy from the atmosphere, high emission through the AW, and minimal heat exchange with its surroundings. Passive cooling of an object under direct sunlight in the daytime can be achieved by radiating more energy away than what is absorbed. For this process, absorption of the strong solar radiation is undesirable, so it must be minimized by achieving high reflectivity in the ultraviolet (UV) and near-infrared (NIR) regime (0.3-2.5 μm), while simultaneously maximizing the emission of energy through the AW.
In this work, we propose an approach for passive radiative cooling that uses silica-coated porous anodic aluminum oxide (AAO), which shows near perfect spectral emissivity in the AW. Nanoporous structures have the benefits of simple and low-cost fabrication, and compatibility with engineering to achieve selective emissivity in the AW. However, conventional AAO has a large extinction coefficient over 10 μm and by itself, does not produce the required near perfect spectral emissivity band over the entire AW. To compensate for this, thin layers of silicon dioxide (SiO₂) are coated on the porous AAO to achieve near perfect emissivity over the AW. The experimentally fabricated SiO₂-coated AAO membrane shows an average cooling flux of 65.6 W/m² during the daytime, and a maximum cooling of 6.1 °C below ambient temperature under direct sunlight.

Reference: Nano Energy 79, 105426 (2021)

Metallic nanoparticles such as gold (Au) and silver (Ag) shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices due to their spectrum tunability. We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and the semiconductor photoelectrode.[1]-[7] Previously, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[8],[9] However, for the monolayer of Au-NPs on the semiconductor the insufficient absorption limited its solar energy conversion efficiency.
Recently, we reported Au-NPs/TiO₂/Au-film photoanode with a modal strong coupling between Fabry–Pérot nanocavity (FPnanocavity) mode and LSPR of Au-NPscan enhance water splitting reaction.[10] In particular, it should be noted that in addition to the absorption increment, the internal quantum efficiency (IQE) is enhanced under strong coupling conditions.
Additionally, we investigated the efficiency of hot-electron transfer under modal strong coupling conditions by monitoring the photocurrent generated at a plasmonic photoanode. We explored the effect of the modal strong coupling on the incident photon-to-current conversion efficiency (IPCE) and IQE in the presence of triethanolamine (TEOA) as a sacrificial electron donor to accelerate the surface reaction enough.[11] The absorption spectrum showed distinct dual bands,which corresponded to the strong-coupling-induced splitting ofenergy levels into upper and lower branches. The IPCE was dramatically enhanced as the TEOA concentration increased, and finally, the IPCE reached a maximum of ca. 4%. Additionally, both hybrid modes formed by the modal strong coupling contributed to the hot-electron transfers and photocurrent generation in the presence of TEOA because the IPCE action spectracan be separated into two peaks. Furthermore, the integrated IQE, which was obtained for wavelengths from 500 to 800 nm, was enhanced by approximately 5 times upon the addition of 1 vol% of TEOA and reached 3%.

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Interfacial engineering via the artificially constructed structures of ultrathin thickness compared to the wavelength has enabled a plethora of advanced manipulations of light-matter interactions. I will report some of the most recent developments in my group as well as in the field of the interfacial engineering of manipulation of light-matter interactions, via the artificially nanostructured metasurfaces. Amongst various applications of metasurfaces, I will focus on how to design vortex metasurfaces¹ to generate and multiplex orbital angular momentums (OAMs), with other degrees of freedom of light such as polarization and frequency. Furthermore, we will show some more recent and exciting results about high-purity orbital angular momentum lasing by synergize the metasurfaces and cavities. It may provide an alternative paradigm toward an extremely compact and multifunctional nanodevices resorting to the OAM states of the light. The multiplexing and hybridization of OAM states with other properties of light open up new opportunities for the advanced flat-profile optics.