Symposium EQ02—Emerging Materials for Light-Based Quasiparticles, Mie-tronics and Metasurfaces

We have typically sought out “perfect” materials is to produce the best emitters for nano-photonic devices. Recently, however, defect states in wide bandgap semiconductors are defining a new frontier for nano-photonics. This talk will focus on a particular defect state: silicon vacancies in 4H-SiC. Integration of such defects within nanobeam photonic crystal cavities have shown an 80-fold optical enhancement of a Si-vacancy transition with emission at about 860 nanometers. The emitter-cavity coupling produces significant photonic signal amplification, and can serve as a building block, quantum repeater for larger-scale quantum information networks.
The photonic crystal cavities can also serve as “nanoscopes” that allow us to better understand the local environment of the silicon vacancies, the other defects that they encounter, the possible interactions with those defects and pathways to better processing and control of the defects. We will discuss insights provided by the cavities as nanoscopes. The accompanying spin signatures of the silicon vacancies, and the correlations between photon emission and spin modulation provide insights into further applications of these new nanophotonic emitters.

Exciton polaritons, the part-light and part-matter quasiparticles in semiconductor optical cavities, are promising for exploring Bose–Einstein condensation, non-equilibrium many-body physics and analogue simulation at elevated temperatures. However, a room-temperature polaritonic platform on par with the GaAs quantum wells grown by molecular beam epitaxy at low temperatures remains elusive. Here, we address this challenge by adopting a method based on the solution synthesis of excitonic halide perovskites grown under nanoconfinement. Such nanoconfinement growth facilitates the synthesis of smooth and homogeneous single-crystalline large crystals enabling the demonstration of XY Hamiltonian lattices with large sizes. Next, we report the first observation of polariton superfluidity in a Wannier-Mott exciton system at room temperature. Specifically, we demonstrate transitions from a normal fluid to superfluidity and supersonic fluid in both two- and one-dimensional cases in halide perovskite microcavities. In the two-dimensional case, we demonstrated a landmark zero-viscosity superfluid and a supersonic Čerenkov wave pattern at room temperature, on a par with low-temperature GaAs cases for the first time. In the one-dimensional case, we show that the back-reflection can be fully suppressed outside the superfluidity healing length when the polariton superfluid hits a hard potential wall under the critical velocity. With these demonstrations, we further establish perovskites as a promising platform for room temperature polaritonic physics and pave the way for the realization of robust mode-disorder-free polaritonic devices at room temperature.

Nature Materials (2022)
Manuscripts (2022) under review

Viscous Hall electron fluid in graphene has emerged at the forefront of many-body interacting electron systems due to its low dimensionality and impurity density. It is the first candidate for a non-local topological electromagnetic phase of matter. This 2D topological viscous Hall insulator is characterized by an optical N invariant, fundamentally different from the Chern and quantum spin Hall insulators. Here, we show that this viscous Hall fluid is in a topological electromagnetic phase arising from the repulsive nature of Hall viscosity. This feature is evident by studying the edge magneto-plasmons, which close the low-frequency electromagnetic bandgap for graphene. Based on the unidirectional topological edge plasmons, we design and simulate a topological circulator, which is a chiral quantum radio-frequency (RF) circuit component crucial for information routing and interfacing quantum-classical computing systems. Our work opens practical applications of graphene's viscous Hall fluid and simultaneously provides an experimental platform for studying topological hydrodynamics of light.

In the past decade, symmetry-protected bound states in the continuum (BICs) have proven to be an important design principle for creating and enhancing devices reliant upon states with high-quality (Q) factors, such as sensors, lasers, and those for harmonic generation. However, as I will discuss, it is possible to prove using representation theory that current implementations of symmetry-protected BICs in photonic crystal slabs can only yield BICs at the center of the Brillouin zone and below the Bragg diffraction limit, which fundamentally restricts their use to single- or few-frequency applications. Instead, this limitation must be overcome by altering the system’s radiative environment. In this talk I will discuss two separate methods for creating lines of BICs by altering the radiative environment surrounding a photonic slab, i.e., by using a superstructure: First, by using the symmetry of the environment to create a symmetry bandgap in the environment’s radiative channels in which slab states with un-represented symmetries are necessarily BICs [1]. Second, by using the environment to reduce the number of radiative channels, enabling the formation of lines of BICs accidentally [2]. The ability to create continuous lines of BIC may have ramifications for enabling multi-frequency and multi–wave vector applications in technologies that leverage these high-Q states.
[1] A. Cerjan et al., Sci. Adv., vol. 7, p. eabk1117, (2021).
[2] A. Cerjan et al., Phys. Rev. Lett., vol. 123, p. 023902, (2019).

Topological insulators (TIs) are materials with an electronic bulk bandgap crossed by surface states that exhibit linear dispersion and spin-momentum locking. Electrons occupying these surface states have low mass and large Fermi velocity due to the linearity of the band structure. The spin-momentum locking of the surface states leads to a reduction in electron scattering, since a change in momentum requires a spin flip. Among other interesting properties, two-dimensional Dirac plasmon polaritons (DPPs) can be excited from these surface electrons. These plasmon polaritons exhibit resonances in the terahertz (THz), a frequency range of interest for chemical identification, imaging, and communication. The TI plasmons inherit the unusual properties of their constituent electrons: the excitations are two-dimensional and massless, similar to those found in graphene. Unlike graphene, TI plasmons should also exhibit spin-momentum locking, leading to a reduction in scattering and an increased plasmon lifetime.
In this talk, I will summarize our past work on exciting DPPs in TI thin films, demonstrating record-high mode indices and long plasmon lifetimes. I will then discuss more recent work on coupling DPPs in adjacent microribbons to tune their frequency, exploring how the DPPs evolve as the TI film thickness is tuned through the topological transition, and synthesizing TI/normal insulator superlattices that house complex DPP modes. I will close by showing evidence for a transition to a THz Dirac hyperbolic metamaterial in multilayer stacks of TIs and normal insulators. These materials have the potential to be powerful components in future THz photonic devices.

Polaritons in van der Waals crystals possess a high degree of field confinement, allowing for the strong light-matter interaction and light manipulation at nanoscale. At the same time, when a polaritonic van der Waals crystal is placed in proximity to a highly conductive metal, the coupling between the collective charge oscillations and their images in the metal results in a manifestation of an even more compressed low-dimensional mode – the image polariton.
We use the near-field optical microscopy and the photo-induced force microscopy to study the hyperbolic image phonon-polaritons (HIP) in orthorhombic molybdenum trioxide (α-MoO₃) – a recently discovered polaritonic van der Waals crystal that supports strongly anisotropic mid-infrared phonon-polaritons. As a low-loss substrate for the image modes, we employ the atomically-flat monocrystalline gold flakes. By leveraging the unique physical properties of the gold crystals, we systematically probe different momentum states on the isofrequency surface and measure the full complex-valued propagation constant of the HIP propagating at different angles to the crystallographic axes of α-MoO₃.
We report an exceptionally long lifetime of phonon-polaritons in α-MoO₃: 4.2 ps and 9.7 ps in the second and the third Reststrahlen bands, respectively, owing to the suppression of all substrate-mediated loss channels in our samples. In contrast, the less confined phonon-polaritons in the α-MoO₃ on SiO₂ substrate have been shown to have a significantly shorter lifetime of ~2 ps in the second Reststrahlen band.
Furthermore, our study reveals the drastically different dispersion properties of the HIP with opposite signs of the group velocity: only the modes with positive group velocity (in the second Reststrahlen band) have both significantly larger momentum and lifetime compared to their counterparts on a dielectric substrate, while the image modes with anomalous dispersion (in the third Reststrahlen band) possess smaller momentum.
Summarizing, our near-field study demonstrates a unique combination of the in-plane anisotropic phonon-polaritons in α-MoO₃, the image mode, and the monocrystalline gold substrate, leading to a manifestation of the hyperbolic image phonon-polaritons with simultaneously greater field compression and larger lifetime compared to the phonon-polaritons in α-MoO₃ on a dielectric substrate. Our results spotlight the HIP in α-MoO₃ as an appealing platform for versatile nanophotonic applications.

The field of nanophotonics is based on the ability to confine light to sub-diffractional dimensions. In the infrared, this requires compression of the wavelength to length scales well below that of the free-space values. While traditional dielectric materials do not exhibit indices of refraction high enough in non-dispersive media to realize such compression, the implementation of polaritons, quasi-particles comprised of oscillating charges and photons, enable such opportunities. Two predominant forms of polaritons, the plasmon and phonon polariton, which are derived from light coupled with free carriers or polar optic phonons, respectively, are broadly applied in the mid- to long-wave infrared. However, the short scattering lifetimes of free-carriers results in high losses and broad linewidths for the former, while the fast dispersion and narrow band of operation for the latter result in significant limitations for both forms. Here we will discuss the opportunity to implement polaritonic strong coupling between different media in an effort to dictate the polaritonic dispersion relation, and thus, the propagation and resonant properties of these materials. Further, by employing the extreme anisotropy of crystals ranging from two-dimensional materials such as hexagonal boron nitride and transition metal dichalcogenides to low-symmetry monoclinic to triclinic materials, novel optical phenomena such as hyperbolicity and shear polaritons are observed. The talk will highlight ultra-strong coupling between both forms of polaritons in the context of infrared emitters, as a means to control planar propagation using hyperbolic polaritons, an modifying thermal dissipation at ultrafast time scales.

We present near- and far-field optical properties of silicon nanostructures (Si-NS) under linear polarization (Gaussian beam), and azimuthally or radially focused cylindrical vector beams investigated by finite-difference time-domain method (FDTD) in Meep open-source software.
In addition to the preferential excitation of specific electric or magnetic resonance modes as function of the excitation beam polarization, we show in the case of spheroids that shape anisotropy affects the resonance wavelength and the dipole orientation of the magnetic or electric dipole mode. Depending on the spheroid symmetry axis with respect to the electric field orientation, the electric dipole resonance can be split in two peaks, giving quasi-unidirectional scattering, separated by an anapole mode. The optical properties in both the far-field (scattering pattern) and near-field (electric and magnetic field hot spots) can be tuned by changing the excitation polarization at a fixed wavelength and selecting properly the spheroid shape and dimensions. Similar behavior is obtained on nanostructures such as nanocylinders with circular or elliptic section that could be produced by top-down fabrication techniques.
As magnetic and electric near field hotspots can be engineered, we present photoluminescence (PL) mappings of Eu³⁺-doped homogenous thin films deposited on Si nanostructures produced by electron beam lithography and reactive ion etching of silicon on silica substrate.
On the one hand, we show that the PL enhancement depends on the Si-NS resonance mode at the excitation wavelength. We also evidence that PL maps exhibit a very specific behavior as function of the shape and polarization of the excitation beam. In other words, the PL intensity distribution can be spatially shaped by the near-field, itself shaped by both the excitation beam polarization and the Si-NS properties.
On the other hand, we investigate how the PL maps at the emission wavelength for the different kind of emitting dipolar transitions (magnetic dipole at about 590 nm and electric dipole at about 610 nm) are influenced by either the electric or magnetic Local density of States (LDOS).
Such work constitutes a first step in the design of Si-NS coupled to quantum emitters that can be optimized for many parameters: both excitation and emission wavelengths, emitting dipole nature and orientation, and emission directivity.

There is a lot of interest in utilizing metamaterial perfect absorbers and transforming electrodes into hybrid photodetectors, which can enhance their performance. Due to their exceptional properties, lossy materials, such as titanium, nickel, and others, have been a focus of large research efforts. However, because of the high losses, achieving high light confinement and harvesting in the nanostructures of such materials is challenging.
We design resonant nanostructures to achieve efficient light control and dynamic tuning of nanophotonic elements. We have achieved the excitation of strong lattice resonances and larger absorption in the arrays of lossy nanoparticles, which can result in a higher generation of hot electrons. We explore the optical properties of novel nanostructures, intending to use them in laser devices and for enhanced emission.
A universal approach is developed to achieve the design goals of layered nanostructures with low optical losses and high performance. We have shown that further narrowband and broadband photodetector functionality can be improved using a multiple-period array of nanoparticles by manifesting enhancement in the resonances and absorption profiles. These nanostructures can provide high-efficiency hot-electron photoconversion and be applied in hybrid devices and photodetectors.
This work was performed, 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 by Los Alamos National Laboratory (Contract 89233218CNA000001) and Sandia National Laboratories (Contract DE-NA-0003525). The work is also supported by Contract DE-2375849.

Hyperbolic metamaterials (HMM) are artificially engineered anisotropic materials. These materials are characterized by their hyperbolic dispersion relation where the in-plane dielectric constants have an opposite sign to the out-of-plane dielectric constants. Recently, HMMs received much attention due to their potential applications in super resolution imaging, absorber, biosensors, extreme polarization anisotropy of photoluminescence just to name a few. Metal-halide perovskites have emerged in recent years as a powerful material system in the field of photonics. Due to their promising electronic and optical properties, tremendous progress has been made in perovskites solar cell performance. In this presentation, I will demonstrate the first realization of HMM using solution processable methyl ammonium lead iodide (MAPbI₃) and plasmonic metal, Au. I will discuss the properties of the HMM within the available parameter space (geometry, wavelength, etc.) in the context of effective medium theory as well as the potential applications

I will discuss the opportunities and applications enabled by strain-induced deformations in semiconductor and polymer materials. One such application is persistent electric energy generation in flexoelectric devices that can be achieved by harnessing absorption and radiative heat emission in narrow-gap semiconductors with inversion symmetry broken by inhomogeneous strain [1]. Another is static and dynamic control of directional surface plasmon polariton modes propagation in Weyl semimetals or semiconductors under strain [2,3]. I will outline strategies to engineer and characterize the effects of the externally-induced or in-situ strain on the material structure, polarization, and energy transport properties. Finally, I will discuss the opportunities for solid-state cooling and heating applications based on elastocaloric and twistocaloric effects in strain-activated polymer fibers engineered to achieve temperature control, energy storage, and thermal conductivity modulation.

This research has been supported in part by the MIT Lincoln Laboratory under award No.ACC-777 (for the flexoelectric energy harvesting), the Army Research Office under award No. W911NF-19-1-0279 (for the near-field radiative energy transfer), the DOE-BES program under award No. DE-FG02-02ER45977 (for the strain-controlled thermal conductivity) and the MIT-SUSTech Program (for the elastocaloric effects).

[1] B. Lorenzi, Y. Tsurimaki, A. Kobayashi, M. Takashiri, and S. V. Boriskina, "Self-powered broadband photo-detection and persistent energy generation with junction-free strained Bi2Te3 thin films," Opt. Express 28(19), 27644–27656 (2020).
[2] S. Pajovic, Y. Tsurimaki, X. Qian, and S. V. Boriskina, "Radiative heat and momentum transfer from materials with broken symmetries," Opt. Mater. Express 11(9), 3125–3131 (2021).
[3] S.V. Boriskina, M. Blevins, S. Pajovic, There and Back Again: the nonreciprocal adventures of light, Opt. Photon. News, Sept. 2022.

The use of plasmonic or high refractive index dielectric nanoantennas or metasurfaces is a common strategy for increasing optical absorption and scattering and for localizing light at subwavelength dimensions. Because of their low losses and direct compatibility with large-scale production techniques from microelectronics, semiconductor nanostructures and full noble metal nanostructures are attracting increasing interest in the visible spectrum. Those techniques have limitations in the mid-infrared (MIR), namely at wavelengths ranging from 2 to 15 µm, which is the region of relevance for chemical or biological sensing and thermal imaging.
Doped semiconductors^1-2, in addition to other properties like shape and size, are a promising choice for plasmonics encompassing a broad infrared spectrum, since their LSPR can be controlled by dopant concentration as well as other parameters like shape and size. Massively doped silicon (Si) nanostructure arrays are logical candidates for mass-producing highly integrated low-cost MIR plasmonic devices.
In this framework, we developed all-silicon-based plasmonic metasurfaces generated by submicrometer tiny doped-Si nanodisks (Si-NDs) by processing hyper-doped overlayers of SOI substrates³. We showed that such nanostructures can support localized surface plasmon resonances that are controlled by the Si free carrier density. By modulating the free carrier concentration between 10²⁰ and 10²¹ cm^-3, the plasmon can be tuned in a spectral window between 2 and 5 µm⁴. Active dopant concentrations exceeding 10²¹ cm^-3 are required to reach the NIR regime. We showed that, despite a large mismatch between the wavelength (25.5 µm) and the Si-ND diameter (100 nm), the LSPR allows for a substantial interaction with IR light, resulting in a loss in transmittance of up to 10% at the resonance⁴. We evaluated the optical characteristics of a single nanodisk as well as the entire metasurface using numerical simulations⁵. They demonstrate excellent agreement with the experimental findings and provide physical insights into the influence of nanostructure form and near-field effects on the metasurface's optical characteristics. Our results open highly promising perspectives for integrated all-silicon-based plasmonic devices for instance for chemical or biological sensing or for thermal imaging.

^{1 D.} Li, Ning, C. Z. All-semiconductor active plasmonic system in mid-infrared wavelengths. Opt. Express 2011, 19, 14594.
² T. Taliercio, P. Biagioni, P. Semiconductor infrared plasmonics, Nanophotonics 2019, 8, 949−990.
³ N. Chery, et al. Study of recrystallization and activation processes in thin and highly doped
silicon-on-insulator layers by nanosecond laser thermal annealing. Journal of Applied
Physics 2022, 131, 065301.
⁴J. M. Poumirol et al. Hyper-Doped Silicon Nanoantennas and Metasurfaces for Tunable Infrared Plasmonics. ACS Photonics 2021, 8, 1
393-1399.
⁵ C. Majorel, et al. Theory of plasmonic properties of hyper-doped silicon nanostructures. Opt. Commun. 2019, 453, 124336.

The magnetocaloric effect is the phenomenon in which some magnetic materials experience a change in temperature upon an adiabatic change in magnetization. It is well-known that this process can be exploited to achieve magnetic refrigeration and has other potential applications including drug delivery (particularly controlled release) and hyperthermia to treat cancer [1]. In recent years, there has also been an interest in using magnetic fields to manipulate radiative heat transfer, especially in the near-field, where length scales are approximately less than the peak wavelength of the blackbody spectrum and evanescent modes dominate. For example, it is possible to tune the near-field radiative heat transfer between planar doped semiconductors by applying a magnetic field and modulating its magnitude and direction [2]. This is because the applied magnetic field induces off-diagonal elements in the dielectric tensor, which are linked to nonreciprocal modes and magneto-optic effects, as well as uniaxial or biaxial anisotropy, which can give rise to hyperbolic modes. In this work, we have combined the magnetocaloric effect and magnetic control of radiative heat transfer to model a near-field tunable magnetocaloric thermal switch. We have shown that the heat flux between a magnetocaloric material and a magneto-optic material can be controlled via two mechanisms: the temperature change originating from the magnetocaloric effect; and modes such as surface plasmon polaritons and hyperbolic modes that depend on both the magnitude and the direction of the applied magnetic field. Thus, we predict the applied magnetic field enables control over the directionality of the heat flux—from the magnetocaloric material to the magneto-optic material and vice-versa—and its magnitude, both spectrally and totally. Our work points toward a new class of thermal switches with a potentially wide range of operating temperatures dependent on the Curie temperature of the magnetocaloric material. This work is supported by an ARO MURI (Grant No. W911NF-19-1-0279) via the University of Michigan.

References
[1] A. M. Tishin, Y. I. Spichkin, Int. J. Refrig. 37, 223 (2014).
[2] E. Moncada-Villa, V. Fernández-Hurtado, F. J. García-Vidal, A. García-Martín, J. C. Cuevas, Phys. Rev. B 92, 125418 (2015).

Recent advances in biomedical engineering have led to a growing worldwide effort to develop a retinal prosthesis designed to restore vision to people suffering from retinal dystrophies such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Some of this work has involved interfacing degenerative retinas with organic-semiconductor based devices to elicit an electrical response upon illumination in order to initiate a transduction of stimuli through the remaining retinal network, and thus provide a capability to restore vision. Previous work done by our group has demonstrated several conjugated polymers and small molecules with different bandgaps used towards a device prototype for potential vision restoration in full colour by emulating the each photoresponse from the rods and cones of the human eye.[1]
Commercially available prosthetics to date have been designed primarily as solid-state devices requiring transcranial wiring and associated ancillary optical and power components. Recent research into less invasive and more comfortable devices has turned to organic photovoltaic materials that are lightweight, flexible, and relatively low-cost. Moreover, using organics capitalises on their potential biocompatibility and their electrical properties to create arrays of miniature photovoltaic pixels to stimulate neurons in the retinal pathway and may provide a straightforward and reliable basis for durable visual prosthetics.
However, challenges in using carbon-based semiconductors to elicit enough photocurrent to stimulate a cell in the retinal network remain. Firstly, the power conversion efficiency of thin film organic photovoltaics has only recently exceeded 12%,[2-4] a value that is woefully small. Increasing excitation power until a photocurrent is detectable is a common work-around, however, while it might prove functional in a laboratory setting, the unnaturally powerful source may result in catastrophic damage to a prosthetic device in a human body. It would be much more favourable to utilise the electronic properties of additional interfacial materials to improve device performance.
In this study, we report on the characteristics of devices fabricated with a range of organic semiconductor polymers for active layers as part of our development of an artificial retina prototype. Previous work has identified a range of promising small molecules that emulate trichromatic vision (rods and three cones); here we describe improvements made with “off the shelf” polymers with narrow absorption bands that suitably match those of human photoreceptors. Importantly, we employ electron-transporting / hole-blocking metal oxide interlayers at the current collecting electrode to elicit capacitive charge transfer of sufficient magnitude for neuronal stimulation. By reducing the difference in work functions of the electrode and LUMO level of the active materials, a significantly higher photoinduced current can be produced for all of the molecules in this study, and the resulting capacitive nature of charge transfer can help to dramatically reduce peroxide-induced Faradiac reactions in device-neuron interfaces.[5]
1. Shkunov, M., et al., Pixelated full-colour small molecule semiconductor devices towards artificial retinas. The Journal of Materials Chemistry C, 2021.
2. Meng, L., et al., Organic and solution-processed tandem solar cells with 17.3% efficiency. Science, 2018. 361(6407): p. 1094-1098.
3. Cui, Y., et al., Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages. Nature Communications, 2019. 10(1).
4. Armin, A., et al., A History and Perspective of Non-Fullerene Electron Acceptors for Organic Solar Cells. Advanced Energy Materials, 2021. 11(15): p. 2003570.
5. Ehlich, J., et al. (2022). "Direct measurement of oxygen reduction reactions at neurostimulation electrodes." Journal of Neural Engineering.

Excitons in transition metal dichalcogenides (TMDs) monolayer possess strong binding energy, valley selectivity, and high quantum efficiency, which are useful for various applications in excitonics, quantum many-body physics, and optoelectronics. TMDs support not only excitons but also a variety of exciton complexes such as biexcitons, charged excitons, and dark excitons that have distinct features and capabilities from excitons: Biexcitons in TMDs have high valley selectivity. Charged excitons can be effectively controlled by the application of electric fields. Dark excitons have a longer intrinsic lifetime and thus can diffuse a longer distance. It has therefore been essential to examine and understand the radiative decay dynamics of exciton complexes in various environmental conditions. The Purcell effect in terms of vacuum field interference has been widely used to control the radiative decay rate of the exciton complexes. However, the radiative decays of the exciton complexes occur at different emission wavelengths and their detailed properties vary from specimen to specimen. The conventional planar mirrors or resonators are not practical to provide the target TMD material with various optical environments depending on the position and emission wavelength at the same time, which is required for investigating the decay dynamics of the exciton complexes systematically.
In this research, we present the spatial modulation of radiative decay dynamics of exciton complexes in the WSe₂ monolayer employing a gradient-thickness mirror (GTM). Our GTM structure consists of the Si/Al (80 nm)/SU8 multilayer. The Al layer acts as a bottom mirror, and the thickness of the SU8 layer gradually changes from 80 to 500 nm over a horizontal distance of 20 micrometers with a slope of ~1°. The flake of hBN (150 nm)/WSe₂ monolayer/hBN (70 nm) was transferred onto the surface of the GTM structure. At a given wavelength, the vacuum field interference and the Purcell effect depend on the thickness of the dielectric SU8 layer. The GTM structure supports a white light interference pattern, in which the constructive/destructive interference conditions for the different emission wavelengths of the exciton (720 nm), biexciton (730 nm), and dark exciton (740 nm) in the WSe₂ monolayer appear at different spatial positions and repeat with different spatial frequencies. As a result, the radiative decay dynamics of each exciton complexes can be modulated in different manners along the direction of the thickness gradient, which enables us to examine the decay dynamics of exciton complexes in the identical WSe₂ monolayer under a variety of conditions.
We measured the photoluminescence (PL) spectrum from the exciton complexes in the WSe₂ monolayer pumped with He-Ne laser via a confocal scanning microscopy. The PL intensity ratio between the exciton complexes, related to their relative radiative decay rates, change depending on the position along the thickness gradient of the GTM structure. We demonstrated that the relative PL intensity ratio of biexcitons to bright excitons spatially changes from ~0.28 to 2.1 with one order of magnitude. We also demonstrated an optical situation where the emission from dark excitons dominates in those of other exciton complexes, which have been difficult to realize due to the weak radiative recombination of the dark exciton. In addition, we were able to systematically investigate the Lamb shift of the bright exciton emission spectrum depending on the vacuum field interference strength.

Luminescent solar concentrators (LSCs) are theoretically able to concentrate both direct and diffuse solar radiation with extremely high efficiencies. Photon-multiplier luminescent solar concentrators (PM-LSCs) contain chromophores which exceed 100\% photoluminescence quantum efficiency. PM-LSCs have recently been experimentally demonstrated and hold promise to outcompete traditional LSCs. However, we find that the thermodynamic limits of PM-LSCs are different and are sometimes more extreme relative to traditional LSCs. As might be expected, to achieve very high concentration factors a PM-LSC design must also include a free energy change, analogous to the Stoke shift in traditional LSCs. Notably, unlike LSCs, the maximum concentration ratio of a PM-LSC is dependent on brightness of the incident photon field. For some brightnesses, but equivalent energy loss, the PM-LSC has a greater maximum concentration factor than that of the traditional LSC. We find that the thermodynamic requirements to achieve highly concentrating PM-LSCs differ from traditional LSCs. The new model gives insight into the limits of concentration of PM-LSCs and may be used to extract design rules for further PM-LSC design.

Metallic thin films and nanostructures are vital building blocks for next generation nanophotonic devices. Nevertheless, the permittivity of pure metals frequently imposes restrictions on the materials utilized and/or on device performance. Metallic mixtures and alloys embody a promising alternative to engineer the optical response of devices, enabling extra control of the spectrum. I will present our recent advances in photonics achieved using metal alloys, comprising super-absorbers using earth-abundant Al-Cu back reflectors, Pd-Au thin films for catalysis, and photodetectors with enhanced response based on Ag-Au films. An in-depth analysis of experimental and theoretical studies of the metallic mixtures optical properties will be also provided, including their correlation with electronic band structure. In the realm of biodegradable photonics, I will share our results regarding color pixels formed by Mg and MgO. Specifically, the green chemistry reaction between these materials and water enables structural colors to disappear within a few seconds. These devices find applications in environmental science, encryption, and health care. Overall, materials beyond coin-age metals can enable a pathway for further controlling the electromagnetic spectrum.

Nonlinear optics has proven a powerful probe for understanding the intrinsic crystal structure and phase of material. Recently, it has been shown that the 2D class of vdW materials exhibit remarkable nonlinear optical properties. Among the exciting family of 2D materials that are being investigated for their nonlinear optical properties, the Rashba family of semiconductors, including BiTeI, has emerged as an excellent candidate for studying the origins of optical nonlinearities in polar semiconductors. BiTeI and related materials have a large Rashba spin-orbit coupled state, and their large polarity is expected to persist down to the monolayer limit. Here, we used second harmonic generation (SHG) microscopy to show that BiTeI is an extremely nonlinear material using layer-dependent studies and extracting its nonlinear coefficients. This then enabled us to exploit SHG microscopy to image polar domains in BiTeI and correlate it this with piezoresponse force microscopy (PFM), providing key insights into the origin of the domains.

The ability to accurately identify and monitor bacterial pathogens in environmental samples such as rivers, streams, and wastewater is critical to public health. Our group has recently shown that Raman spectroscopy combined with machine learning (ML) can be a powerful tool for identifying bacterial species, strain, and antibiotic susceptibility [1, 2, 3]. Gold nanorods can enhance the Raman scattering signature, owing to their ability to concentrate the incident light and selectively bind to bacteria via electrostatic interactions. In wastewater, however, anions mask the surface charges of the nanorods, disrupting interactions with bacteria and weakening the Raman enhancement.

Here, we achieve bacterial surface enhanced Raman spectroscopy (SERS) in wastewater by conjugating 12-residue gold-binding peptides (GBPs) to the bacterial surface. These peptides interact with gold through van der Waals interactions, overcoming the need for positive surface charges on the nanorods, and form bonds with gold 10 times more stable than electrostatic interactions [4]. As a proof of concept, we design a plasmid encoding a GBP conjugated to a circularly permuted outer membrane protein X (CPX) scaffold and use this plasmid to express GBPs on the E. coli cell surface. We describe the effects of GBP surface expression on adsorption affinity of bacteria to nanorods. Next, we use filtered wastewater samples and spike in GBP-expressing E. coli at a concentration of 10⁷ cells/ml, a realistic total bacterial concentration for wastewater. With our surface expression of GBPs, SERS can detect E. coli at this concentration using only 10 second integration times in a liquid wastewater sample. Finally, to demonstrate broad application to any bacterial species, we conjugate synthetic GBPs to dibenzocyclooctyne (DBCO). These conjugates can be selectively cross-linked to azides via copper-free click chemistry, enabling GBP cross-linking to any bacteria that have been metabolically labeled with azide-conjugated cell wall and outer membrane components. Hence, by combining advances in nanophotonics, ML, and biological engineering, we demonstrate SERS can be used to detect a wide variety of bacterial species and strains in complex environmental samples, allowing rapid and label-free monitoring of a community’s bacterial infections through wastewater.

[1] Ho et al., Nat. Comm. 10, 4927 (2019).
[2] Tadesse et al., Nano Lett. 20, 7655-7661 (2020).
[3] Safir et al., ArXiv (2022).
[4] Hnilova et al., Langmuir 24, 12440-12445 (2008).

Phonon polaritons are supported within a material-specific spectral region of polar materials called the reststrahlen band, which is bounded by the transverse and longitudinal optical phonons. Surface phonon resonances (SPhPs) are widely recognized in polar semiconductor materials such as SiC and GaN, including our demonstrating active tuning of nanopillar SPhPs and Berreman modes. We have recently shown that phonon resonances are also supported by other materials, including inorganic salts and molecular crystals. Some materials have anisotropic and even hyperbolic optical properties similar to hexagonal boron nitride, in which they behave optically like a dielectric and a metal along different crystal axes. We describe IR reflection results for calcite nanopillar and nanohole arrays which support hyperbolic phonon polaritons (HPs). The HP modes observed in the nanohole arrays appear at higher frequencies than those in the nanopillar arrays. We have developed a qualitative understanding of the nature of the HP modes in the nanohole arrays, in which the modes are most likely confined to the constrictions between holes. In such a model, changing the pitch and/or the diameter of the holes will change the dimensions of the constrictions and result in changing the resonance frequency of the HP modes in a predictable way. Being able to theoretically describe HPs in calcite is a first step towards understanding the nature of these modes in a variety of materials in order to expand the range of phonon polariton materials available for mid-IR nanophotonic applications.

Progress in understanding resonant subwavelength optical structures has fueled a worldwide explosion of interest in both fundamental processes and nanophotonic/plasmonic devices for imaging, sensing, solar energy conversion, and information processing. However, plasmonic platforms in the visible region with robust, high performance, thermo-stable, and low-cost remains remain unexplored. In this presentation, I will particularly discuss emerging plasmonic platforms based on transition metal nitrides. I will present an overview of my research works over the past five years on the plasmon-enhanced light-matter interactions in the visible regions and its applications [1-6], including the plasmonic nanolasers [1-2], tunable plasmonic modulators [3], plasmonic phototransistors [4], plasmon-enhanced solar energy harvesting [5], and the refractory plasmonic colors for back-light free displays [6]. My group discovered several unique working mechanisms that utilize plasmonic nanostructures to improve optoelectronic device performance. By engineering the local electromagnetic field confinement, the light-matter interaction strength can be enhanced, which results in efficient energy conversion in the designed nanosystem. The detailed mechanisms and possible applications will be discussed. These results have broad implications for the use of plasmonic crystals/metasurfaces in high-performance optoelectronic devices with efficient energy conversion.

References
[1]. Y-H Hsieh, B-W Hsu, K-N Peng, K-W Lee, C W Chu, S-W Chang, H-W Lin*, T-J Yen*, and Y-J Lu*, Perovskite Quantum Dot Lasing in a Gap-Plasmon Nanocavity with Ultralow Threshold, ACS Nano 14, 11670 (2020). (Issue cover)
[2]. Y-J Lu*, T L Shen, K-N Peng, P-J Cheng, S-W Chang, M-Y Lu, C W Chu, T-F Guo, H. Atwater*, Upconversion Plasmonic Lasing from an Organolead Trihalide Perovskite Nanocrystal with Low Threshold. ACS Photonics 8,335–342 (2021).
[3]. Y-J Lu, R. Sokhoyan, W-H Cheng, G. Kafaie Shirmanesh, A. Davoyan, R. A. Pala, K. Thyagarajan, and H. A. Atwater*, Dynamically Controlled Purcell Enhancement of Visible Spontaneous Emission in a Gated Plasmonic Heterostructure, Nature Communications 8, 1631 (2017).
[4]. H-Y Lan, Y-H Hsieh, Z-Y Chiao, D. Jariwala, M-H Shih, T-J Yen, O. Hess, and Y-J Lu*, Gate-Tunable Plasmon-Enhanced Photodetection in a Monolayer MoS₂ Phototransistor with Ultrahigh Photoresponsivity. Nano Lett. 21, 3083 (2021).
[5]. M-J Yu, C-L Chang, H-Y Lan, Z-Y Chiao, Y-C Chen, H W H. Lee, Y-C Chang, S-W Chang, T. Tanaka, V. Tung, H-H Chou*, and Y-J Lu*, Plasmon-Enhanced Solar-Driven Hydrogen Evolution Using Titanium Nitride Metasurface Broadband Absorbers. ACS Photonics 8, 3125–3132 (2021).
[6] Z-Y Chiao, Y-C Chen, J-W Chen, Y-C Chu, J-W Yang, T-Y Peng, W-R Syong, H W H. Lee, S-W Chu, and Y-J Lu*, Full-Color Generation Enabled by Refractory Plasmonic Crystals. Nanophotonics 11, 2891-2899 (2022)

Spiky hedgehogs can be self-assembled from twisted ribbons formed by gold-copper-cysteine complexes.¹ Furthermore, chirality of these twisted ribbons can be controlled by the cysteine amino acid while their arrangement into supraparticles is guided by the ratio of L- vs D-form during the initial stages of self-assembly. Here we report that the spherically symmetric arrangement of twisted ribbons displays a maltese cross pattern typical of spherulites and liquid crystals under cross-polarizers.² This optical effect is observed at the single particle level³ with tunable diameters of 1-10 um. We investigate the optical effects in chiral hedgehogs using single particle methodology and find that the emergence of Maltese cross pattern is dependent on the sphericity of the supraparticle and the spacing between individual spikes. We understand the interaction of light with hierarchically self-assembled structures through finite-element based electromagnetic simulations and reconstruct the distribution of electromagnetic fields around the hedgehogs in three dimensions. These hedgehogs are omnidispersible in a variety of solvents and offset the limitations of commonly observed organic molecule based spherulitic arrangement for biomolecular sensing under harsh environments.⁴

1. Jiang, W. et al. Emergence of complexity in hierarchically organized chiral particles. Science (1979) 368, 642–648 (2020).
2. Shtukenberg, A. G., Punin, Y. O., Gunn, E. & Kahr, B. Spherulites. Chemical Reviews 112, 1805–1838 (2011).
3. Bahng, J. H. et al. Mie Resonance Engineering in Meta-Shell Supraparticles for Nanoscale Nonlinear Optics. ACS Nano 14, 17203–17212 (2020).
4. Sivakumar, S., Wark, K. L., Gupta, J. K., Abbott, N. L. & Caruso, F. Liquid crystal emulsions as the basis of biological sensors for the optical detection of bacteria and viruses. Advanced Functional Materials 19, 2260–2265 (2009).

Metasurfaces require the integration of materials with accurate control over position, size and shape for high optical efficiency. This is commonly achieved using well-established lithographic or chemical processes. Nevertheless, such processes suffer from intrinsic limitations over throughput or flexibility, and hence remain difficult to scale up and adapt to large area, flexible or stretchable substrates. Here, we present a novel fabrication approach based on controlled templated fluid instabilities of thin optical glasses, to self-organize a variety of large index contrast all-dielectric metasurfaces. Given the right annealing time-temperature settings, initial film thickness and underlying pattern, the breakup of the film can occur at prescribed locations resulting in nano-objects of tunable position, shapes and sizes. Such control paves the way towards simple fabrication route of advanced 2D and quasi-3D photonic architectures (Das Gupta, Sorin et. al. Nature Nanotechnology, 14 (4), 320 (2019); Martin-Monier, Sorin et. al. Physical Review Applied 3, 034025 (2021)). Low processing temperatures enable large-scale use of rigid but also unconventional flexible and stretchable substrates. Such structures enable strong electromagnetic field confinement, and are shown to have varieties of applications in sensing, light management and second harmonic generation. In particular, by dewetting successively increasingly thick layers, inter-particle gap down to 10 nm could be achieved. By critically coupling the in-plane diffractive mode with the radiative dipolar mode in such structures, sharp Fano-type resonances are demonstrated in the near visible region. These resonances are exploited to monitor protein monolayer concentrations down to 0.5mg/ml (Das Gupta, Sorin et. al. Nature Nanotechnology, 14 (4), 320 (2019)). Tailoring the underlying texture, we will also discuss recent results of chalcogenide based architectures exhibiting high second harmonic conversion efficiency of 10^-6 in the UV region (Das Gupta, Sorin et. al., Nanophotonics 10, 3465 (2021)). Finally, we will further highlight the flexibility of our approach by presenting recent results on highly reflecting all-dielectric nanostructures, as well as tailored architectures for improved index sensing.

The growing demand on reconfigurability in neuromorphic computing, integrated photonics and microwave photonics is attracting increasing attention towards design and optimization of active photonic components. By changing a phase of the phase change material or the state of hydrogel near a functional photonics building block, it is possible to realize (re)configurable, time dependent and (re)programmable components and materials. Phase change materials are materials in which phase transitions can be induced quickly and reversibly, resulting in pronounced changes in their physical properties. Hydrogels are macromolecular cross-linked networks swollen by the solvent in which they are dissolved. The unique property of hydrogels is their ability to dramatically change volume in response to external stimuli. Here we report on our recent developments of a multiphysics description of complex composite active photonic components incorporating phase change materials and hydrogels as their building blocks. Such a multiphysics description requires self-consistent treatment of the electromagnetic, heat transfer, mechanical deformation and phase transition models. Possible applications in the fields of integrated photonics, mid-infrared and microwave metasurfaces are discussed.

Solid-state quantum emitters represent a valuable path for quantum technologies. Engineering and control of single photon emitters (SPEs) in diamond and silicon carbide serve as a benchmark for quantum emission from crystals. However, their intrinsic 3D nature limits some of their applications. 2D materials can circumvent this issue, plus they can also be integrated with standard CMOS technology [1].
In this framework, SPEs in wide bandgap 2D hexagonal boron nitride (hBN) have many advantageous properties including high brightness, photostability, and most importantly, room-temperature operation [2]. Different forms of hBN have been studied ranging from bulk crystals epitaxial films to nanocrystals, and several techniques have been exploited to induce quantum emitting crystal imperfections.
In the present contribution, we characterize the efficacy of ion irradiation to generate emitters in hBN. We compare the throughput of different ions (Ne, Ga, C) in producing the largest number of defects within target hBN flakes. We also tune the ion energy to address a specific penetration depth inside the mechanically exfoliated hBN. Additionally, the ion fluence is also spanned across a wide range.
Irrespectively of the ionic species, a strong and broad peak in the photoluminescence spectra from irradiated hBN always appears at about 820 nm. Additionally, upon fixing the same implantation depth, gallium irradiated flakes are an order of magnitude brighter.
We conclude that the role of the impinging ions is to trigger the formation of intrinsic defects. We surmise that boron atoms are displaced from lattice sites, creating boron vacancies. These become negatively charged [3] and leads to emission in the NIR.
Such a defect, which can act as a SPE with a spin-dependent optical transition, can be employed for example in quantum communication protocols, where a spin-photon interface is highly desirable.

References
[1] Aharonovich, Igor, Dirk Englund, and Milos Toth. "Solid-state single-photon emitters." Nature photonics 10.10 (2016): 631-641.
[2] Tran, Toan Trong, et al. "Quantum emission from hexagonal boron nitride monolayers." Nature nanotechnology 11.1 (2016): 37-41.
[3] Gottscholl, Andreas, et al. "Initialization and read-out of intrinsic spin defects in a van der Waals crystal at room temperature." Nature materials 19.5 (2020): 540-545.

The utilization of exciton-polariton resonances in dielectric materials represents a novel pathway for the realization of next generation sensors and optical devices. Recently, the J-aggregation of cyanine dyes has been demonstrated to produce a Frenkel excitonic absorption features strong enough to enable the formation of surface exciton polariton states at room temperatures. The implementation of these optical dyes has an inherent advantage for the development of low cost, optically resonant surfaces, as they can be easily deposited from aqueous solutions to a variety of differing substrates. Here, we demonstrate the massively parallel printing of J-aggregate dyes using polymer pen lithography to fabricate large scale arrays of optical antenna nanostructures. Here, through the control of the contact parameters of the polymer pen array and annealing conditions, we demonstrate the multiplexed fabrication of dye-based optically resonant nanostructures. Reducing structure sizes into the nanoscale to reach sub-wavelength dimensions, we explore the formation of localized exciton polariton resonances in these materials. We further relate the connection between the local morphology of the J-aggregate dye nanostructures to localized optical resonances through a combination of characterization techniques including atomic force microscopy, scattering spectroscopy, and finite element method optical modeling of the near-and far- field optical response.

Phase change materials or PCMs are truly remarkable compounds whose unique switchable properties have fueled an explosion of emerging applications in electronics and photonics. Traditional PCMs suffer from large optical losses even in their dielectric states, which fundamentally limits the performance of optical devices based on the materials. To resolve the issue, we have demonstrated a new PCM Ge-Sb-Se-Te (GSST) with broadband low loss characteristics. In this talk, we will review an array of reconfigurable photonic devices enabled by the low-loss PCM, including electrically tunable metasurfaces, beam steering optics, and parfocal zoom lenses.

Roughness-induced light scattering is an important limitation of the performance of various on-chip optical technologies such as microresonators and quantum photonic integrated circuits. Here, we report a plasma-thermal atomic layer etching (ALE) method which enables etching and smoothing of aluminum nitride (AlN) films with Angstrom-scale precision. The ALE process employs sequential exposures of SF₆ plasma and trimethylaluminum (Al(CH₃)₃, TMA). In each cycle, the surface is fluorinated by flourine radicals, followed by a ligand-exchange reaction to yield etching. We observed a maximum etch rate of 1.9 Å/cycle at 300 ^○C as measured using ex-situ ellipsometry. After ALE, the surface roughness of AlN was measured to decrease ~35% from 4.3 Å to 2.7 Å. In addition, the etched surface was found to contain a lower concentration of oxygen compared to the original surface. We anticipate that the ALE process may be applied as a post-treatment to improve the performance of on-chip photonic devices which exploit the simultaneous second-order and cubic-order optical nonlinearities of AlN. ALE processes may be generally applicable to photonic materials to decrease losses arising from surface roughness scattering.
Reference: Wang, Haozhe, et al. "Isotropic plasma-thermal atomic layer etching of aluminum nitride using SF₆ plasma and Al (CH₃) ₃" arXiv preprint arXiv:2209.00150 (2022).

Vortices, swirling textures with singularities, have been observed in various physical entities. In condensed matter, the emergence of the vortices originates from exotic physical phenomena, such as Bose-Einstein condensation and magnetic flux quantization, that provides quasi-particle nature to the textures. The optical vortex (OV) is an optical analogy to the vortices in other physical entities, exhibiting zero-intensity point with spiral phase distribution in the electromagnetic field. OVs support the orbital angular momentum of light, which provides an extra degree of freedom in fundamental studies and applications, such as optical manipulation, forbidden electronic transition, and classical/quantum information processing. However, the generation of OVs has relied on structural singularities, such as spiral phase plates and metasurfaces, rather than critical phenomena, which prevents the OVs from possessing quasiparticle-like behavior, such as dynamic and interactive characteristics.
In this study, we propose and demonstrate the spontaneous generation of real-space optical vortex and anti-vortex and their active control using an external magnetic field based on a gradient-thickness optical cavity (GTOC) [1]. The GTOC consists of a magneto-optic Ni layer, between two SiO₂ layers, on top of an Al mirror. We found singular solutions in the calculated optical reflection of GTOC depending on the Ni layer thickness (h_Ni) in the generalized parameter space of top and bottom SiO₂ layer thicknesses (h₁, h₂), which correspond to the OVs with winding number of w = ±1. Regarding to the emergence of the OVs, topological phase of given system can be classified as trivial (h_Ni < h_c1 and h_Ni > h_c2) and non-trivial phase (h_c1 < h_Ni < h_c2), where h_c1 = 6.34 nm and h_c2 = 13.15 nm correspond to the critical points.
We experimentally realized the topological phases of GTOC in an actual media by the bijective projection of the generalized parameter space (h₁, h₂) into real space (x, y). The thickness changes of the top and bottom SiO₂ layers are achieved by non-uniform sputtering in the order of tens of nanometer variation in centimeter-scale distances. We fabricated GTOC samples at trivial (h_Ni = 5, 15 nm) and non-trivial (h_Ni = 10 nm) phases and characterized them using confocal reflectance scanning and off-axis holography. For the non-trivial phase, the OVs are observed in the reflection distribution as singular minima and phase singularities. For the trivial phases, weak single minimum and continuous phase distributions are observed.
The most striking point of our topological media is the magnetic field activity. The magneto-optic effect applies an effective change to the optical thickness of the Ni layer, which can perturb the topological phase of GTOC. Under the external magnetic field (B), we observed magneto-optically-driven vortex dynamics, which induce topology- and polarization-dependent movement of the OVs. From B = -0.5 to +0.5 T, the displacements showed (Δx, Δy) = (0.649 mm, -0.344 mm) and (-0.385 mm, 0.010 mm) for the optical vortex (w = +1) and antivortex (w = -1), respectively, under the right circularly polarized incidence. The displacement directions were reversed when the incidence polarization changed into left circular. Finally, we demonstrated the magnetic-field-induced generation of the optical vortex-antivortex pair at the critical point (h_Ni ~ h_c2) of the topological phase. This is the first observation of field-induced topological phase transition in photonic media. We believe that our findings will pave a way to study topological photonic interactions and inspire the exploration of quasiparticle-like nature in various topological photonic textures, such as toroidal vortices, polarization/vortex knots, and optical skyrmions.

[1] Kim, D., et al., Spontaneous generation and active manipulation of real-space optical vortex. arXiv:2202.02335 (2022). (Accepted in Nature)

Every object has a temperature greater than zero Kelvin can emit thermal emission according to the fundamental principles of statistical mechanics. Thermal emission, especially in the long-wave infrared region (LWIR), is of great importance in a variety of fields, including spectroscopy, imaging, communication, and energy management. Recently, the possibility of controlling thermal emission based on nanophotonic structures has been intensively investigated with a view towards a new generation of far-field thermal emitters. Such control is demonstrated in the aspects of engineering narrowband, polarized, and spatial coherent (directional) thermal emission. While narrow-band thermal emitters that provide IR LED-like performance have been demonstrated in photonic crystals, perfect absorber geometries, metallic bullseye structures, and periodic arrays of nanoantennas, spectral tunability and control of the spatial coherence within a given platform is challenging.
We present phonon-polaritonic thermal emitter designs to control the spectral and spatial dispersion of thermal emission. Surface phonon polaritons (SPhPs) are quasiparticles comprising a photon and a coherently oscillating charge on a polar lattice. Localized SPhPs (LSPhPs) can be supported by periodic SiC nanopillars, offering exceptionally high Purcell enhancements that are in excess of plasmonic counterparts, which result from the narrow resonance line widths. We demonstrate that such large-scale narrowband thermal emitters, featuring near-unity absorption, can serve as LWIR sources with effectively no net power consumption, enabling their operation entirely by waste heat from conventional electronics [1]. Furthermore, increasing the complexity of the LSPhPs unit cell can serve to modify the resonant near-field and intra- and inter-unit-cell coupling as well as to dictate spectral tuning in the far-field. We also exploit more complicated unit-cell structures to realize new LSPhPs modes with additional degrees of design freedom [2]. Unlike LSPhPs, which offer no spatial coherence, spatially coherent thermal emission can be induced by a one-dimensional grating with propagating SPhPs. However, only a single spatially coherent mode is supported by purely periodic grating structures. We explore superstructure gratings to enable polaritons with different frequencies/wavevectors in a single grating, manifesting as additional spatial modes in the thermal emission profile [3]. By combining the corresponding virtues of both localized and propagating SPhPs through the introduction of strong coupling, we demonstrate a new hybrid mode with narrowband, spatially coherent thermal emission signature. Additionally, through coupling to a third zone-folded longitudinal optic phonon (ZFLO) mode, the realization of electrically driven emitters is possible [4]. Thus, our results show that phonon polaritons and strong coupling phenomenon enable thermal emitters, which meet the requirements for a host of LWIR applications in a simple, lightweight, and intense emitter.
1. Lu, G., et al., Narrowband Polaritonic Thermal Emitters Driven by Waste Heat. ACS Omega, 2020. 5(19): p. 10900-10908.
2. Lu, G., et al., Collective Phonon-Polaritonic Modes in Silicon Carbide Subarrays. ACS Nano, 2021.
3. Lu, G., et al., Multi-frequency coherent emission from superstructure thermal emitters. Applied Physics Letters, 2021. 118(14).
4. Lu, G., et al., Engineering the Spectral and Spatial Dispersion of Thermal Emission via Polariton-Phonon Strong Coupling. Nano Lett, 2021. 21(4): p. 1831-1838.

The application of epsilon-near-zero (ENZ) materials to photonic devices has resulted in a wealth of fascinating phenomena such as electromagnetic supercoupling, resonance pinning, perfect optical absorption, and ultrafast optical switching. However, the effect of ENZ materials on quantum fluctuations is much less developed. Here, we will present out recent work on use of epsilon-near-zero (ENZ) materials to engineer quantum effects and discuss how they can be applied to various technologies. Specifically, we show how ENZ materials can be used to control the spontaneous emission rate of quantum emitters and present the concept of electromagnetic bandgaps for nanoparticles comprising an ENZ material. Further, we will show how tunable ENZ materials can be used to convert electrical bias into mechanical motion through the modification of the allowed modes within a cavity composed of an ENZ material. Finally, we will discuss the outlook for using the concepts in future device architectures.

In this talk, I will discuss our recent progress in the area of polaritonics and its role in advancing the field of metamaterials and metasurfaces. Tailored material resonances, based on excitons, phonons, or electronic transitions, can be strongly coupled to light in natural or engineered metasurfaces, unveiling new degrees of freedom for the control of light-matter interactions with light-based quasi-particles. I will discuss how, as the degree of lattice symmetries in the involved materials and in optically engineered structures is reduced, new opportunities emerge in manipulating light and matter at the nanoscale, unveiling giant nonlinearities and topological phenomena in real and reciprocal space.

We report on nonlinear optical dynamics and structural reconstruction of Co-doped BaFe₂As₂ single-crystal superconducting films within a broad range of temperatures. Films were grown by pulsed laser deposition on (La,Sr)(Al,Ta)O3 [LSAT] and LaAlO₃ [LAO] crystalline substrates with epitaxial SrTiO₃ [STO] template buffer layer used for higher epitaxial bonding of BaFe₂As₂. Samples show superconducting transition with T_c~15.5-20 K. Temperature-dependent optical transmittance measurements in the range from 290 K down to 10 K were conducted using a supercontinuum laser source, revealing a distinct increase of the transmittance within the wavelengths range from λ=400 nm to λ=1.6 µm, as the sample temperature drops below T_c. Reflectivity measurements at λ=800 nm also show an abrupt decrease of the reflectance below T_c. Drude-Lorentz model was applied to fit the optical transmittance spectra in order to demonstrate the contribution of a free-carriers component in the optical signal. The noticeable change of both, carrier scattering rate and plasma frequency near ~150 K is associated with the spin density wave (SDW) transition in BaFe₂As₂. The angle-resolved hemispherical elastic light scattering surface metrology measurements performed with a light scatterometer at cryogenic temperatures indicate that this transition is accompanied by surface reconstruction. High-resolution angle-resolved hemispherical scattering imaging of multi-scale nonequilibrium processes in BaFe₂As₂ reveals that the small change in the elastic energy upon SDW formation results in the splitting of the film into domains due to the same or different structural phases since this process is accompanied by tetragonal-to-orthorhombic phase transition. Several competitive transition processes at ~150 K alter the correlation length of the surface and optical properties. In the region of superconducting transition, BaFe₂As₂ films demonstrate abrupt changes in surface morphology (surface autocorrelation length, optical isotropy, and root-mean-square roughness). A strong correlation between the observed temperature dependence of the surface statistics and quasiparticle dynamics near T_c was found. Time-resolved measurements of transient reflectivity performed in a relatively high-fluence regime, of several mJ/cm², reveal a strong dependence of the excited-states-dynamics on sample temperature, morphology, and laser excitation energy. Femtosecond photoexcitation demonstrates complex reflectance traces attributed to two distinct ultrafast processes: (i) instantaneous formation of the nonequilibrium state of quasiparticles with its subsequent thermalization within ~3 ps with characteristic time τ₁, and (ii) slower few-picoseconds phonon-phonon scattering. Temperature-dependence of τ₁ shows its noticeable drop as the temperature rises above Tc. Observed nonlinear optical dynamics correlate with the temperature dependence of the surface morphology near T_c and provide a new understanding of light-induced processes in the superconducting state of BaFe₂As₂.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have attracted great interests because of their novel excitonic properties, providing new opportunities in nanophotonic devices, excitonic physics, and polaritonic systems. Efficient modulation of the 2D excitons in TMDs is attractive yet challenging. Coupling TMDs with single Mie resonators at the subwavelength scale can be a promising strategy.
Here, playing with the electric (ED) and magnetic dipole (MD) modes, we apply single hydrogenated amorphous Silicon nanospheres (a-SiNS:Hs) to facilitate favorable far-field or near-field light manipulations for effective control of TMD excitonic properties. The a-SiNS:H Mie resonators are designed via bandgap engineering to support near-zero losses at full visible wavelengths and tunable refractive index based on hydrogen dopants (Nature Communications 2020, 11, 5055). Different from the high index in typical Mie resonators, a unique moderate refractive index of ~2.1 is proposed for broadband and highly interacted ED and MD. The size dependence of Mie resonances is also utilized and sphere diameters from 400 down to 200 nm are chosen for various modulation functions.
(1) Targeting at the directional modulation of far-field excitonic radiation patterns, we control the a-SiNS:H sizes between 300 and 400 nm with partially overlapped ED and MD. The a-SiNS:H Mie resonator is proposed as a highly miniaturized platform for controlling exciton emission through both the excitation and emission processes. Based on the mutual interference of Mie resonances, efficient modulation of directional excitation and exciton emission have been achieved in different TMD materials. A modified Mie theory for dipole-sphere hybrid systems is derived to instruct the optimal design for desirable modulation, leading to the potential of multifunctional integrated photonics. (Advanced Materials 2021, 33, 2007236)
(2) By coupling single a-SiNS:Hs of 250 to 290 nm in diameters with monolayer WS₂ in water, we successfully tune the WS₂ homogeneous exciton linewidth between 35 and 7 meV at room temperature (RT), providing a practical method to modulate exciton quantum dynamics at the subwavelength scale. Impressively, with an optimal sphere size of 268 nm, we achieve near-intrinsic exciton linewidth at RT, approaching the theoretical limit at 0 Kelvin. The linewidth-narrowing mechanism manifests the dynamic competition between exciton and trion decay channels. Specifically, the a-SiNS:H Mie resonator is designed to selectively boost the trion radiative decay, which breaks the charge balancing in inherently n-doped WS₂ and rebuilds the excitonic relaxation processes with suppressed exciton nonradiative decays. (Advanced Materials 2022, 34, 2108721)
(3) For a sphere diameter of 240 nm, ED and MD are found to be perfectly overlapped among the whole visible wavelengths. We employ this unique a-SiNS:H Mie resonator for broadband ultra-strong light-matter interactions with 2D excitonic materials. By coupling it with monolayer TMDs, we have achieved multi-Rabi splittings and robust exciton-polariton emission at RT.
In conclusion, by using a moderate-refractive-index a-SiNS:H Mie resonator, we have shown the efficient modulation of 2D TMD excitons in multiple dimensions. The subwavelength nature of the proposed Mie resonator ensures the good coupling efficiency with 2D excitons and practically enables the miniaturization of functional photonic devices.

Epsilon-near-zero materials have been shown to be as one of the most promising optical materials in the recent years as the electromagnetic field inside media with near-zero permittivity has been shown to exhibit unique optical properties. I will review our recent studies on the active linear, nonlinear, and emission properties of conducting oxide and metallic nitride epsilon-near-zero materials. Discovery of novel active materials is fundamental for photonic applications. The epsilon-near-zero (ENZ) materials have been shown to be as one of the most promising optical materials in the recent years as the electromagnetic field inside media with near-zero permittivity has been shown to exhibit unique optical properties, including strong electromagnetic wave confinement, enhanced quantum emission near ENZ media, non-reciprocal magneto-optical effects, unique topological properties, and abnormally large optical nonlinearity. While ENZ optics have been investigated extensively in the last few years, the current commonly used ENZ materials suffer from limited optical enhancement because of the lack of precise control of ENZ frequency, loss, and thickness for efficient ENZ mode excitation. In addition, the active tunability and enhanced nonlinearity and emission properties in ENZ materials have yet been fully experimentally explored. This talk will review our recent studies on the active linear, nonlinear, and emission properties of conducting oxide and metallic nitride epsilon-near-zero materials in planar and optical fiber platforms [1-7]. I will present a method to engineer the field intensity enhancement of the Al-doped zinc oxide (AZO) ENZ thin films synthesized by atomic layer deposition (ALD) technique. I will then discuss the observation of abnormal nonlinear temporal dynamic of hot electrons and enhanced optical nonlinearity in AZO and ITO ENZ thin films under different pump fluences and excitation angles using a degenerate pump-probe spectroscopy technique. I will present the first comprehensive study of photoluminescence from a 2D material placed near ENZ films and its dependence on the losses of materials, as well as the spectral response of such emitters with excitation wavelengths across the ENZ regime. Finally, I will also discuss the first experimental demonstration of optically confined ENZ resonance excitation in an optical fiber waveguide uniformly coated with AZO nanolayer. These studies enrich the fundamental understanding of emission and nonlinear properties on ENZ thin films and the integration of ENZ materials and optical fiber will open the path to revolutionary ultracompact in-fiber optical devices for optical communication, imaging, sensing/laser applications. This work was supported in part by the National Science Foundation (grant number: 2113010), and Air Force Office of Scientific Research (FA9550-21-1-02204) (AFOSR-AOARD, FA2386-21-1-4057)

Weyl semimetals (WSMs) are a material of emerging interest due to their distinct electronic band structure, which supports pairs of chiral Weyl nodes formed from intersecting, linearly dispersing bands. The Weyl nodes in a pair each act as a source and a sink of Berry curvature respectively, resulting in a net flux of Berry curvature between them and zero Berry flux outside of them in momentum space. The flux of Berry curvature between the Weyl nodes gives rise to the unique, conductive Fermi arc surface states of WSMs.
Either time-reversal symmetry or inversion symmetry breaking is required for the existence of a WSM. Time-reversal symmetry breaking gives rise to the anomalous Hall effect in WSMs. On the other hand, inversion symmetry breaking WSMs have exhibited an exceptionally large bulk photogalvanic effect (PGE) under illumination from linearly polarized infrared (IR) light. This linear bulk PGE results in part from the Berry curvature induced shirt current, in which an electron excited by linearly polarized light experiences a spatial shift upon transition from the valence to the conduction band, resulting in a photocurrent without the need for a p-n junction or external field. The detection of mid-IR light with conventional noncentrosymmetric semconductors is limited by a lack low energy bandgap materials but is enabled in a WSM by the gapless nature of its bandstructure. The use of the bulk PGE in WSMs has great potential for IR imaging and energy harvesting applications and for surpassing the efficiency limits inherent to p-n junction type detectors [1,2].
In addition to exploiting the bulk PGE, WSM surfaces can be engineered to control the surface photogalvanic effects, which opens the door to manipulating both bulk and surface photocurrents in WSMs for enhanced IR detection and devices. Surfaces of WSMs with broken time-reversal symmetry also support non-reciprocal surface plasmon-polariton modes (SPP) propagation, that enable directional energy transfer, Kirchhoff’s law violation, and tunable near-field radiative heat transfer [3].
A key step in the development of WSM SPP engineering is to establish means to control them by external stimuli, especially optical stimuli. In this work, we model the impact of the bulk and surface photogalvanic effects as well as material strain on the WSMs surface states. Recent works have shown the Lorentz reciprocity of materials is broken by applying dc currents which produce a Doppler frequency shift of the electron plasma (Fizeau drag) [4]. Here, we exploit the fact that spontaneous photocurrent in WSMs acts as a dc current in the illuminated sample and imparts Fizeau drag on the electron plasma, providing an additional mechanism to control the nonreciprocal behavior of WSM SPPs by optical means. We also compare the impact of the PGE-induced Fizeau drag to the impacts of strain [5] on the SPP modes. A combination of these two mechanisms enables effective means for both static and dynamic control of nonreciprocal SPP propagation.
[1] S. Pajovic, et al, Radiative heat and momentum transfer from materials with broken symmetries: opinion, Optical Materials Express, 11(9), 3125-3131, 2021.
[2] S.V. Boriskina, M. Blevins, S. Pajovic, There and Back Again: the nonreciprocal adventures of light, Opt. Photon. News, Sept. 2022.
[3] S. Pajovic, et al, Intrinsic nonreciprocal reflection and violation of Kirchhoff’s law of radiation in planar type-I magnetic Weyl semimetal surfaces, Phys. Rev. B 102(16), 165417, 2020.
[4] K. Y. Bliokh, et al, Electric-current-induced unidirectional propagation of surface plasmon-polaritons, Opt. Lett. 43, 963-966, 2018.
[5] Bugaiko, O. & Gorbar, E. & Sukhachov, Pavlo. (2020). Surface plasmon polaritons in strained Weyl semimetals. Physical Review B. 102. 10.1103/PhysRevB.102.085426.
This research has been supported by the Army Research Office (W911NF-13-D-0001), Lincoln Laboratory, Massachusetts Institute of Technology (ACC-777), and a Draper Fellowship to M.B.

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs) show extraordinarily strong second- and third-order nonlinear optical responses which, thanks to their unique electronic properties and despite their atomic thickness, allow a wide range of fundamental studies and technological applications. In this presentation we will discuss some of our recent results on the study of the nonlinear optical properties of two-dimensional materials.
We show that the third-harmonic generation efficiency in graphene can be increased by almost two orders of magnitude by controlling the Fermi energy and the incident photon energy [1]. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from resonant multiphoton transitions. Thanks to the linear dispersion of the massless Dirac fermions, gate controllable third-harmonic enhancement can be achieved over an ultrabroad bandwidth, paving the way for electrically tunable broadband frequency converters.
Using semiconducting TMDs, we demonstrate single-pass optical parametric amplification at the ultimate thickness limit, down to a single atomic layer [2]. Second-order nonlinear interaction at the two-dimensional limit bypasses phase-matching requirements and achieves ultrabroad amplification bandwidths. In agreement with first-principle calculations, we observe that the amplification process is independent of the in-plane polarization of signal and pump fields.
We investigate 3R-stacked TMD crystals which combine broken inversion symmetry and aligned layering. By measuring second harmonic generation (SHG) of 3R-MoS₂ with various thickness, from monolayer (0.65 nm) to bulk (»1 μm), we present the first measurement of the SHG coherence length (» 530 nm) at 1520nm and achieve record nonlinear optical enhancement from a van der Waals material, > 10⁴ stronger than a monolayer. We find that 3R-MoS₂ exhibits similar conversion efficiency as lithium niobate, but with more than 100-fold shorter propagation lengths [3 ].
Finally, we demonstrate a novel approach for the all-optical control of SHG polarization in MoS₂ and show that this can be used for all-optical modulation of the SHG efficiency with modulation depth close to 100% and speed limited only by the fundamental frequency pulse duration [4].

References
[1] G. Soavi et al., Broadband, electrically tunable third-harmonic generation in graphene. Nat. Nanotechnol. 13, 583–588 (2018).
[2] C. Trovatello et al., Optical parametric amplification by monolayer transition metal dichalcogenides. Nat. Photon. 15, 6–10 (2021).
[3 X. Xu et al., Towards compact phase-matched and waveguided nonlinear optics in atomically layered semiconductors, Nat. Photon., in press (2022).
[4] S. Klimmer et al., All-optical polarization and amplitude modulation of second-harmonic generation in atomically thin semiconductors, Nat. Photon. 15, 837 (2021).

Noble-transition metal alloys are a promising class of materials for interband-driven hot hole generation with NIR light.¹ NIR hot-carrier generation in pure, noble metals suffers from insufficient energy to exceed the interband energy threshold (IET) (e.g.,> 2 eV), and in pure transition metals, rapid hot-carrier thermalization. Band hybridization in select noble-transition metal alloys can overcome these issues, yielding emergent properties that facilitate tuning the hot-carrier distribution and lifetime.

We deposit disordered Cu_xPd_1-x alloy thin films under non-equilibrium conditions and investigate their stoichiometry-dependent optical, structural, electronic, and transient near-infrared absorption properties.² We show experimentally through ultraviolet photoelectron spectroscopy and computationally through the projected density of states calculations that strong hybridization between the Cu 3d and Pd 4d bands exists in the alloy films. Unlike the Au 5d band, the Cu 3d band does not exhibit strong spin-orbit coupling. Therefore, we observe the formation of a common band between the full Cu 3d and Pd 4d bands. This strong d-band hybridization in the alloy films drives emergent behavior across various measurement techniques.

Time-resolved terahertz spectroscopy with near-infrared (e.g., 1550 nm) excitation displays composition tunable electron dynamics. We posit that the negative peak in ΔT/T below 2 ps from dilute Pd alloys is due to non-thermalized hot-carrier generation and field-driven electron emission. On the other hand, Pd-rich alloys exhibit an increase in ΔT/T due to thermalization effects upon ultrafast NIR photoexcitation. Cu_xPd_1-x alloys in the dilute Pd regime may be a promising material for future ultrafast NIR optoelectronic devices.

1. Sara KF Stofela, Orhan Kizilkaya, Benjamin T Diroll, Tiago R Leite, Mohammad M Taheri, Daniel E Willis, Jason B Baxter, William A Shelton, Phillip T Sprunger, Kevin M McPeak, “A Noble-Transition Alloy Excels at Hot-Carrier Generation in the Near Infrared,” Advanced Materials 32 (23), 1906478 (2020)

2. Orhan Kizilkaya, Gregory Manoukian, Sergi Lendinez, Luis D. B. Manuel, Tiago R. Leite, Karunya S. Shirali, William A. Shelton, Phillip T. Sprunger, Jason B. Baxter, Kevin M. McPeak “Emergent Properties from CuPd Alloy Films under Near-Infrared Excitation,” J Chem Phys (Under Review)

We report on the nonequilibrium dynamics of photoinduced collective excitations in FeSe_0.8Te_0.2 films in a superconducting (SC) state. Epitaxial films were grown on CaF₂ single-crystal substrates by pulsed laser deposition. The SC properties were tuned by varying the strain level by altering the film thickness of the epitaxial film. It was found that increased strain in film can depress SC when the film thickness riches a threshold minimum, while the increased thickness can result in static domain structure via generation of the dislocation network, but with the introduction of new grain boundaries or microcracks. This fact was observed in angle-resolved light scattering measurements of the surface morphology within a broad temperature range, down to 7 K. It was also found that the surface domains undergo temperature-dependent reorganization when FeSe_0.8Te_0.2 film undergoes a solid-to-solid structural phase transition. Ultrafast time-resolved measurements of reflectivity were performed with 35-fs laser pulse excitation, 1 kHz repetition rate, the central wavelength of 800 nm, and orthogonal polarization for pump and probe pulses. The FeSe_0.8Te_0.2 film was maintained in its SC state at 7.5 K, while excitation fluence was tuned from up to ~10 mJ/cm², sufficiently below the damage threshold. The obtained reflectance traces show three distinct regions attributed to different dynamical processes in the material within the 50 ps time scale: (i) drop of the reflectivity signal within 200 fs, (ii) its subsequent rise within 1 ps, and (iii) much longer process of tens picoseconds. First two processes are associated with collective excitations and photoexcitation of quasi-particles (QP) in FeSe_0.8Te_0.2 . A gradual increase of the excitation energy results in a gradual growth of QP density. Moreover, the QPs decay rate slows down as excitation fluence increases. The transient measurements manifest a gradual time-shift of the specific signal minimum on the reflectivity diagram, since the QPs decay rate slows down as excitation fluence increases. The characteristic time of the process (ii) shows a linear increase with excitation fluence, while the process (i) is more complex. The process (iii) is associated with phonon-phonon scattering demonstrating a decrease of its characteristic time as excitation increasing. This could be a signature of photoinduced structural transformation or a hidden phase in the film. A strong acoustic phonon generation starts at ~ 100 ps, right after the process (iii) completes. This reveals new features of photoinduced dynamics of superconducting FeSe_0.8Te_0.2 in high excitation regime.

Transition metal dichalcogenides (TMDCs) are an emerging class of materials with extraordinary physical properties, which are investigated for a wide range of electronic, photonic, and optoelectronic applications. While TMDCs have been extensively explored as an excitonic platform with novel optical and electronic functionalities, it is less appreciated that TMDCs also have a high refractive index. The presence of excitons in high-index TMDC nanoresonators enables the formation of unique self-hybridized exciton polaritons, which remarkably enrich the capabilities of nanophotonic systems.

Here, we present strong coupling between excitons and Mie resonances in individual colloidal TMDC nanowires. Molybdenum disulfide (MoS₂) nanowires are chemically synthesized with tailorable dimensions to support tunable Mie-exciton polaritons in visible wavelengths. Compared to previously reported TMDC nanoresonators that are fabricated by lithographical methods (e.g., electron beam lithography), colloidal MoS₂ nanowires provide additional possibilities in the design and fabrication of nanophotonic devices. Specifically, the chemical synthesis approach permits the scalable and high-throughput production of TMDC nanoresonators with tunable optical responses, which can be easily integrated with other techniques for on-demand dispersion, transfer, manipulation, and assembly. Moreover, we show that the MoS₂ excitonic properties and spectral splittings can be optothermally modulated under light illumination, which offers a promising platform for the development of active photonic devices, such as all-optical switches and sensors.

Quantum Electrodynamic Density Functional Theory (QEDFT) is a generalization of time-dependent DFT in the presence of quantized modes of the electromagnetic field. QEDFT presents a good compromise between accuracy and computational effort that makes it possible to treat quantum light-matter interactions even for large electronic systems and many modes of the electromagnetic field. At present, however, the light-matter coupling strengths are essentially free parameters and it is unclear how to relate them to a real cavity setup.
In this talk, I will discuss how we can formulate the length-gauge light-matter Hamiltonian in terms of the quantized fields from Macroscopic QED and the extent to which it can be solved using QEDFT. This formulation lets us relate all cavity coupling parameters to the classic Green’s function of the electromagnetic field and eliminates all free parameters related to the light-matter coupling strength. Our approach provides a step towards a parameter-free, quantitative description of light-matter interactions in arbitrary electromagnetic environments.

Actively tunable optical nanoantennas are of great importance for dynamic control over the wavefront of light, where real-time and reprogrammable functions are required. Electro-optical tuning mechanisms based on electro-refraction effects induced by the free carriers in doped semiconductors are of growing interest for the realization of active metasurfaces. In particular, indium tin oxide (ITO) is a degenerately doped semiconductor, whose incorporation into the subwavelength unit cells of the geometrically-fixed metasurfaces enables post-fabrication electrical tuning of the optical response at near-infrared frequencies. The epsilon-near-zero transition in the ITO as well as novel design paradigms leveraging high-Q resonant modes allow for extreme light-matter interactions for enhancing the tunability of optical response. Despite the fruitful progress, the operating principle of ITO-integrated quasi-static metasurfaces relies on the modulation of resonant modes between the over- and under-coupled regimes. This limits the performance of such quasi-static metasurfaces to negligibly narrowband regime due to the strong resonant dispersion. Introducing time-modulation into the nanoantennas as an additional degree of freedom, renders a four-dimensional design space and surmounts the limitations of the quasi-static metasurfaces by converting the incident signal at the fundamental frequency to the higher-order sidebands. The phase of higher-order sidebands generated by a time-modulated metasurface can be tuned with uniform amplitude via a non-resonant dispersionless geometric phase shift induced by the modulation phase delay. This dispersionless phase elevates time-modulated metasurfaces 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. The current architecture of deep space and local area networks calls for high capacity and high-speed platforms that can simultaneously address multiple users with minimal crosstalk. Time-modulated nanoantennas are promising candidates for multi-frequency functioning as they feature all-angle and all-wavelength optical response due to the access to dispersionless phase span at the sidebands. To demonstrate the adaptive multi-frequency multi-beam steering by the time-modulated metasurface, an array of plasmonic nanostrips integrated with ITO in metal-insulator-metal configuration is considered, wherein two sets of time-varying biasing signals are independently applied to the dual-gated metasurface for modulating the permittivity of ITO in space and time. The aperture of the metasurface is then divided into several interleaved orthogonally modulated sub-array nanoantennas with distinct modulation frequencies to render a shared-aperture metasurface in space and time. The spatially interleaved sub-arrays are programmed to exploit multi-beam scanning via a pixelated control over the modulation phase delays assigned to their constituent elements. The number of sub-arrays and distinct channels can be scaled easily without suffering from crosstalk due to the orthogonality of the channels or the efficient metasurface design. The results point toward high-capacity platforms with low size, weight, and power for next-generation free-space optical (FSO) communication systems.

Emerging plasmonic materials such as semimetals (transition metal nitrides and carbides), highly doped semiconductors, 2D and quasi-2D materials such as MXenes are playing an increasingly important role in emerging technologies for sustainable energy and photocatalysis [1]. For example, enhanced hot electrons generation in transition metal nitride nanoparticles compared to conventional materials such as gold enables more efficient light-to-electricity conversion [2,3]. MXenes, a class of 2D nanomaterials formed of transition metal carbides and carbon nitrides, form a promising material platform for tailorable nanophotonics. They offer a number of unusual properties and are being applied to realize novel electromagnetic shields, metal-ion batteries, super capacitors, lasers, and sensors. We utilized the plasmonic response of titanium carbide (Ti₃C₂T_x) MXene thin films in the near infrared spectral window to create a metamaterial for broadband absorption [4] and explore the usage of MXenes for applications in photonics, energy harvesting, and desalination. Expanding the application realm for transition metal nitrides into sustainable, scalable technologies, we also demonstrated ultra-broadband light absorbers with titanium nitride (TiN) nanoparticles obtained through fast, large-scale and environmentally-friendly processes [5]. To advance photonic applications of novel materials further, we apply machine-learning-assisted data analysis techniques coupled with topology optimization and tailorable material platforms to obtain non-intuitive photonic designs for highly efficient optical devices and energy conversion.
Support from the Air Force Office of Scientific Research (grant FA9550-17-1-0243) is acknowledged.
References
[1] Brongersma, M. L., Halas, N. J. & Nordlander, P. Plasmon-induced hot carrier science and technology. Nature Nanotechnology 10, 25–34 (2015)
[2] A. Naldoni, U. Guler, Z. Wang, M. Marelli, F. Malara, X. Meng, L. V. Besteiro, A. O. Govorov, A. V. Kildishev, A. Boltasseva, V. M. Shalaev, “Broadband Hot Electron Collection for Solar Water Splitting with Plasmonic Titanium Nitride,” Advanced Optical Materials 5 (15) 1601031 (August 2017)
[3] A. Naldoni, F. Riboni, U. Guler, A. Boltasseva, V. M. Shalaev, A. V. Kildishev, “Solar-powered plasmon-enhanced heterogeneous catalysis,” Nanophotonics 5 (1) 112–133 (June 2016)
[4] K. Chaudhuri, M. Alhabeb, Z. Wang, V. M. Shalaev, Y. Gogotsi, A. Boltasseva, “Highly Broadband Absorber Using Plasmonic Titanium Carbide (MXene),” ACS Photonics 5 (3) 1115-1122 (2018)
[5] M. Li, U. Guler, Y. Li, A. Rea, E. K. Tanyi, Y. Kim, M. A. Noginov, Y. Song, A. Boltasseva, V. Shalaev, N. A. Kotov, “Plasmonic Biomimetic Nanocomposite with Spontaneous Subwavelength Structuring as Broadband Absorbers,” ACS Energy Letters 3, 1578–1583 (2018)

Silicon photonics technology has been widely applied in communication and data centers. In this work, we experimentally demonstrate a special class of true bound state in the continuum (BIC), lattice resonance, and Kerker effect in group IV metasurface, including Si-based semiconductor and germanium (Ge). The photodetection in silicon photonics has become the very important research area. In silicon photonics, germanium (Ge) is an ideal candidate for on-chip photodetection due to the small bandgap of 0.66 eV and the compatibility with the modern CMOS technology. It has widely been used in near infrared (NIR) photodetection. However, the bulky and thick Ge film for photodetector limits the development of system minimization. By the design of metasurface, the absorption in thin Ge metasurface could have much larger absorption than the pure Ge thin-film. In addition, we also present directional emission normal to Si3N4 BIC metasurface laser with hybrid surface lattice resonances (SLRs).

Coherent control of vibrational modes is a promising route to achieving ultrafast manipulation of material properties with light. Through excitation of specific phonon modes that are strongly coupled to bulk material properties of interest, researchers have shown examples including enhancement of superconductivity and switching of the magnetic, ferroelectric, and structural phases with ultrafast optical pulses. With the development of intense terahertz (THz) sources, coherent control with THz frequency pulses, which provide phase-sensitive access to phonon modes with no parasitic electronic excitation, has gained significant attention. To achieve this future degree of control over matter, it is essential to characterize the multitude of different linear and nonlinear excitations that occur when pumped with an intense, broadband THz pulse. Lithium niobate (LiNbO₃) is an ideal candidate for developing techniques that can provide detailed information on the potential energy landscape due to its importance in nonlinear THz polaritonic applications. Here, we report two-dimension THz-THz-Raman (2D-TTR) measurements on x-cut LiNbO₃ with selective detection of the third-order nonlinear signal and significantly improved THz bandwidth (up to 9 THz). We demonstrate that 2D-TTR spectroscopy can provide insight into the excitation mechanism, nonlinear phonon-phonon couplings, and sources of anharmonicity in the system. We show that the E(TO₁) and E(TO₃) phonon-polaritons are excited through resonant THz one-photon absorption (1PA) as opposed to THz sum-frequency (SF) excitation. We directly observe THz and Raman nonlinear transitions between the E(TO₁) and E(TO₃) modes. Due to selection rules of the E-symmetry phonon modes, distinct symmetry-allowed Feynman pathways are observed for different THz polarizations. Further, our models show that the set of Feynman pathways observed are induced via mechanical anharmonicity of the phonon-polariton modes as opposed to electronic anharmonicity. Such information is traditionally only available via density functional theory (DFT) calculations. These findings provide a concrete foundation for future coherent control applications. For example, the nonlinear transitions between E(TO₁) and E(TO₃) may be optimized using tailored THz pulse sequences to achieve efficient population transfer between the two modes.

Using the benefit of flat optics metasurface, polarization-sensitive imaging technology has been developing in recent years with a reasonable promise to utilize it as a sensing method. Instead of imaging with light intensity, polarization-sensitive detection can take advantage of the orientation of the electric field to reveal the edge, hidden features, and many details of a target. Metasurface can serve as a stokes intensity splitter based on the incidence polarization state of light, and this utility has become a benchmark of optical sensing. This work explores high contrast dielectric metasurface design with Finite Difference Time Domain (FDTD) simulation to optimize dielectric material choice, layer combination, geometric shape, and assembly to make a total phase, transmission, and polarization control metasurface for 532 nm working wavelength. However, suitable material and design for the total phase control for this wavelength already exist, but the crucial limit of perfect nanofabrication, the design margin, needed to be relaxed. So the main focus of this research is on reducing the height of the nanoantenna and exploring the useful geometric features that can influence the mutual interaction of the metasurface element. For shape optimization, our metasurface element will go beyond the axially rotational symmetric component to affect the orthogonal phase differently. This will continue implementing anisotropic metasurface elements, which may improve the efficiency of detecting scattered light beams as expected from the diverse scattering medium.

Recently emerged high-index materials have been a subject of active studies in recent years. Ultra-thin optical elements can be engineered based on transdimensional photonic lattices that include 3D-designed nanoparticles supporting multipole resonances and arranged in 2D arrays to enhance collective effects in the nanostructure. Nanostructures with a high refractive index have been suggested as a promising alternative to conventional metals supporting plasmonic resonances. Here we show that the antennas made out of high-index material support several multipole resonances enabled by the supporting propagating modes with a high effective index and their reflection from the antenna boundaries. Different multipole resonances appear for the large nanoantenna, as well as a decrease of reflection from the nanoantenna array and highly directional resonant scattering. Transdimensional lattices consisting of resonant nanoantennas in the engineered periodic arrays have great potential to serve as functional elements in flat optical components and photonic devices.
This work was performed, 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 by Los Alamos National Laboratory (Contract 89233218CNA000001) and Sandia National Laboratories (Contract DE-NA-0003525). The work is also supported by Contract DE-2375849.

Due to their capability to overcome ohmic losses, and the unique ability to manipulate, control and confine electromagnetic waves at the nanoscale dimensions, high index dielectric metastructures have become promising candidates for numerous photonic device applications such as beam steering, imaging, sensing and solar energy harvesting. We numerically study the scattering behavior of an array of silicon nanocylinders, and we show that varying the silicon nanocylinder heights results in the excitation of Fano resonance creating asymmetric scattering spectral profiles. We also demonstrate strong mode coupling behavior called quasi-bound states in the continuum where the modes are localized (trapped) in the nanostructure as a result of suppression of radiative losses leading to a high quality factor due to the disappearance of the width of the Fano lineshape. Our studies also reveal the overlap of the modes between the electric dipole and magnetic dipole moments leading to suppression of light reflectance and transmittance in the metastructure array.

Interaction of light with collective free electron oscillations termed plasmon-polariton and with polar lattice vibrations termed phonon-polariton, are essential to confine the light in subwavelength dimensions and for strong optical resonances. Traditionally doped-semiconductors and conducting metal oxides (CMO) are used to achieve plasmon-polaritons in the near-to-mid infrared (IR), while polar dielectrics are utilized for realizing phonon-polaritons in the long-wavelength IR (LWIR) spectral regions. However, demonstrating plasmon- and phonon-polariton in one host material with low loss is challenging due to the mutually conflicting physical property requirements.

In this work, we present high-quality plasmon- and phonon-polaritons in epitaxial polar ScN thin films deposited on (001) MgO substrates using dc-magnetron sputtering in the ultra-high vacuum (1 × 10^-9 Torr). As-deposited ScN thin films exhibit an n-type carrier concentration of (2-4) × 10²⁰ cm^-3 primarily due to the presence of oxygen impurities. Due to such high carrier concentrations, the Fermi level in ScN resides inside the conduction band, about 0.2-0.3 eV above the band edge. Mg-hole doping is used to reduce the high carrier concentration, and p-type ScN is achieved along with high hole mobility. Scanning transmission electron microscopy (STEM) is used to characterize the microstructure of the film. Both oxygen and magnesium-doped ScN films deposited in this work on (001) MgO substrates at high-temperatures are epitaxial and nominally single-crystalline with [001](001) ScN || [001](001) MgO relationships.

Spectroscopic ellipsometry and Fourier-transform infrared spectroscopy (FTIR) analysis indicate that the highly n-type doped ScN thin films exhibit low-loss short-wavelength IR plasmon resonance in the 1500-2500 nm spectral range. Due to the polar semiconducting nature, Mg-doped low carrier concentration ScN also exhibits phonon-polaritons between the 340 cm^-1 - 677 cm^-1 spectral range. Excitation of the surface phonon-polariton modes is also demonstrated by polarization-dependent reflectivity measurement in the attenuated-total-reflection (ATR) mode using the Kretschmann configuration. A clear dip in the p-/s-polarized reflection spectrum at ~ 1.95 µm and at ~ 16 µm correspond to the surface plasmon-polariton (SPP) and surface phonon-polariton (SPhP) resonances respectively.

The plasmonic properties of ScN film are comparable to other IR plasmonic materials such as transparent conductive oxides and doped semiconductors. Also, ScN exhibit a very high figure-of-merit for SPhP propagation compared to its peer materials. Demonstration of plasmon- and phonon-polariton in one host material, ScN through carrier concentration control make it attractive for applications in epsilon-near-zero metamaterials, optical communication, solar-energy harvesting and other nanophotonic applications.


References:

K. C. Maurya, D. Rao, S. Acharya, P. Rao, A. I. K. Pillai, S. K. Selvaraja, M. Garbrecht and B. Saha, "Polar Semiconducting Scandium Nitride as an Infrared Plasmon and Phonon-Polaritonic Material" Nano letters (In-press, 2022).

K. C. Maurya, A. I. K. Pillai, M. Garbrecht and B. Saha, "Simultaneous Light-Harvesting at Visible and Mid-Infrared Frequencies with Epitaxial TiN/Al0.72Sc0.28N/TiN Metal/Polar-dielectric/Metal Metamaterials" (In-review, 2022).

K. C. Maurya, V. M. Shalaev, A. Boltasseva and B. Saha, "Reduced Optical Losses in Refractory Plasmonic Titanium Nitride (TiN) Thin Films Deposited With Molecular Beam Epitaxy" Opt. Mater. Express. 10, 2679 (2020).

Van der Waals materials have emerged as a promising materials platform for electronics and photonics. In this talk I will overview our recent theoretical and experimental efforts on creating ultra-confined nanophotonics devices. Owing to unique and strong resonances van der Waals materials exhibit very high refractive indices and a strong anisotropy. High reflective index, in turn enables, a conceptually new set of photonic devices that outperform current semiconductor and metallic devices. I will discuss the use of high refractive index transition metal chalcogenides, such as MoS2, and hexagonal boron nitride for visible and mid-IR nanophotonics. I will show that these van der Waals materials allow extreme optical field confinement in deeply subwavelength structures. Our studies suggest that smaller and more efficient devices may be created with the use of van der Waals materials.