This PDF file contains the front matter associated with SPIE Proceedings Volume 12195, including the Title Page, Copyright information, Table of Contents, and Conference Committee Page.

We present a systematic and computationally efficient methodology to design scattering responses for lossless reciprocal 2-port systems, emphasizing compact high-order standard filters. Our approach is based on universal analytical criteria for the resonances (quasi-normal modes) of the structure, which can be used as tractable optimization (root-finding) objectives. We demonstrate our method by designing multiple microwave metasurfaces configured for polarization-preserving transmission, reflective polarization conversion, or diffractive anomalous reflection, and exhibiting responses that precisely match standard bandpass or bandstop filters of various types, orders and bandwidths, with focus on the best-performing elliptic filters.

We study the role of non-Markovian effects, originating from optical dispersion of metal dielectric function, in the emission spectrum of a quantum emitter resonantly coupled to a surface plasmon in a metal-dielectric structure as the system transitions to strong coupling regime. By using a quantum approach to interacting plasmons that incorporates the effects of dispersion and losses in the coupling parameters, we obtain analytically the emission spectrum for an exciton-plasmon system with characteristic size below the diffraction limit. In the strong coupling regime, the dispersion-induced non-Markovian effects lead to dramatic changes in the emission spectra by causing inversion of spectral asymmetry, as compared with classical and quantum models based on the Markov approximation, which results in a strong enhancement of the lower frequency polaritonic band.

Recent advances in understanding of ultrashort pulse propagation in multilayered AZO/ZnO ENZ metamaterial are presented. The influence of the material parameters for AZO/ZnO metamaterial on chromatic dispersion are discussed. Numerical approach based on a full wave analysis of the ultrashort pulse propagation in the presence of enormous second-order dispersion was used to investigate ultrashort pulse propagation through ENZ AZO/ZnO metamaterial. An approach using an adaptive pre-shaping algorithm for ultrashort pulse distortion compensation during the propagation at the ENZ spectral point is introduced. The results based on auxiliary differential equation finite-difference time domain method that show a dramatic change in shape for the probe pulse modulated using pump pulses of various duration (100-500 fs) and amplitude (106−1010 V/m) are presented.

Thermal regulation is essential for numerous applications across multiple industries such as the efficient temperature control of indoor facilities and the reliable operation of many electronic systems. Vanadium dioxide (VO₂) is a phase change material that is well-suited for thermal regulation as a result of its ultrafast, reversible, solid-state transition at 68°C that produces a significant contrast in its infrared (IR) emissive properties. To meet application demands, VO₂’s transition temperature can be tuned via doping with a reduction in temperature of ~22 °C per atomic percent tungsten (at. % W⁶⁺). However, historically this decrease in the transition temperature has coincided with a reduction in IR optical contrast between the two phases. In this investigation, we demonstrate that by patterning VO₂ thin film composites with preoptimized thicknesses, a thermal regulation system with a tunable transition temperature and no significant degradation of contrast between the states is produced. Through carefully selected user-defined patterning of the undoped VO₂ layer within the multilayer film, a 64% operating optical contrast was achieved across the 8 – 13 μm spectral region as compared to 42% in the as-deposited film. Additionally, at a doping level of 1.7%, the transition temperature in a VO₂ thin film composite with micron-scaled patterning was reduced to 25°C while maintaining 58% emissive contrast in the 8 – 13 μm spectral region.

Light emitted by fluorophores can be computed from the knowledge of the absorption spectrum. However, at long wavelengths, the calculated emission may diverge if the decay of the imaginary part of the permittivity is not modelled accurately. We report a technique to obtain the permittivity of fluorophores such as dye molecules from fluorescence measurements. We find that the Brendel-Bormann model enables to fit the emission spectra accurately.

We present the theory of parametrically resonant surface plasmon polaritons. We show that a temporal modulation of the dielectric properties of the medium adjacent to a metallic surface can lead to efficient energy injection into the surface plasmon polariton modes supported at the interface. When the permittivity modulation is induced by a pump field exceeding a certain threshold intensity, such field undergoes a reverse saturable absorption process. We introduce a time-domain formalism to account for pump saturation and depletion effects. Finally, we discuss the viability of these effects for optical limiting applications.

We develop a quasinormal mode theory (QNMT) of the scattering matrix S, satisfying fundamental symmetries (such as reciprocity and time-reversal or PT symmetries) even for a small truncated set of resonances. It is a useful and accurate reduced-order model for S based on the resonant frequencies and mode-to-port coupling coefficients, obtained from an eigensolver without the need for QNM normalization. We further show that a slowly varying background, useful to describe Fano-shaped spectra, can be extracted using high-loss modes. We demonstrate the improved accuracy of our formulation using various electromagnetic metasurfaces.

Exceptional points (EP) in a non-Hermitian system involve the simultaneous coalescence of two or multiple eigenvalues and associated eigenvectors. Known for their novel functionalities, they have been demonstrated in multiple deterministic systems. Here, we experimentally demonstrate lasing over exceptional points in quasi-one-dimensional Anderson localizing structures with optical gain. Simultaneous spectro-spatial analysis of emission modes revealed coupled lasing modes in several configurations. Systematic analysis of spectral splitting and associated eigenfunctions revealed simultaneous coalescence of eigenvalues and eigenvectors. The former was directly measured in the spectra, while the latter was endorsed by observation of asymmetric amplification in one of the two cavities. The square-Lorentzian lineshape certified Anderson-localized lasing over exceptional points. Enhanced functionality manifested as manyfold increase in lasing intensity at the exceptional point.

This research develops data-driven methods for metamaterial design using generative modeling and reinforcement learning (RL). Previously, both generative modeling1 and RL2 showed exciting results for acoustic cloak design. We want to generalize both frameworks for acoustic lens design. The proposed 2D-Global Optimization Networks (2D-GLOnets) maximize the root mean square (RMS) of the absolute pressure at the focal point at discrete wavenumber values to enable acoustic lens design. The 2D-GLOnets1 are adapted with a reparametrization technique that constrains the scatterers’ positions into a feasible region. The pressure amplitude can converge to optimal values faster because of the gradients computed analytically from a multiple scattering solver.3 The loss function with respect to the weights is utilized to update the generator’s weights. In addition, Deep Deterministic Policy Gradient (DDPG) algorithm is applied to the acoustic lens design. DDPG controls the positions of the cylinders and assigns rewards based on the absolute RMS pressure amplitude at the focal point. The reward function assigns a higher value to the state of absolute pressure amplitude. As the agent iteratively completes episodes, the reward is maximized. The agent searches for the configuration of the scatterers that produce the enhanced focusing effect. The numerical results are presented for both models considering uniform configurations of scatterers with a varying number of scatterers and wavenumbers.

Metasurfaces are composed of periodic pattern of subwavelength structure. They have been of great interest to the scientific community due to their interaction with light in ways that cannot be found in nature. Among various types of metasurfaces, metalenses are ones with promising applications because they can bend and focus light in a confined space. In contrast to conventional refractive lenses, the metalenses are ultra-thin slab with only a few nanometers in thickness suitable for limited space system. However, designing a very thin metalens with requirement of high efficiency is a huge challenge. We designed a high NA metalens with a diameter of less than 25 μm to focus an 830 nm laser beam onto the waveguide situated only a few microns from the laser. The metalens were designed with FDTD simulation and fabricated by distributing Si nanofin structures in a particular pattern using standard electron beam lithography. Determining the efficiency of the metalens itself is a challenge due to small focal spot and background light. Thus, grating of similar physical shape of nano-bricks was used instead for finding efficiency of the metalens. By inspecting diffracted light from metasurface grating, the efficiency of metalens was determined. Here we present the process of designing the metalens, fabrication and testing its efficiency to provide the best solution for limited space optical system.

An angle-insensitive metamaterial spectral filter (MS) that demonstrates great potential as a spectral disperser within hyperspectral imaging (HSI) was simulated and analyzed. The innovation of the MS is its operation on the principle of coupled resonances, whereby coupling the classical narrowband Fabry-Perot (FP) resonance and a broadband cavity mode (CM) resonance can tune its dispersive spectral behavior. This results in the MS transmitting a narrow passband within a broad stopband for a focused light cone. Compared to conventional methods, this novel approach has the potential to reduce the size, weight, and power (SWaP) of a HSI system. Currently, hyperspectral sensors require bulky dispersion controlling optics to collimate the incoming beam due to physical limitations set forth by the disperser. Because the disperser is usually a transmission/reflection grating, the angle of the incident beam significantly impacts whether the correct wavelength is incident on the sensor. At even a slightly off-normal (AoI), the beam could either miss the sensor entirely, or create cross talk between adjacent pixels. This fundamental limitation produces difficulties in managing obliquely incident light, hence the need for collimation. To get around this, the AoI insensitive metamaterial will be used in the place of the disperser and collimated optics to properly deliver obliquely incident light to the detector. When applied correctly, the MS can be used within a remote sensing detector to provide high performance spectroscopy that is similar to larger heritage sensors.

It is commonly believed that electromagnetic waves cannot propagate in lossy conductive media and that they quickly decay inside such media over short length scales of the order of the so-called skin depth. Here we prove that this common belief is incorrect if the conductive medium is stratified. We demonstrate that electromagnetic waves in stratified lossy conductive media may have propagating character, and that the propagation length of such waves may be considerably larger than the skin depth. Our findings enable novel electromagnetic metamaterial designs by mediating the effect of losses on electromagnetic signal propagation in metamaterials. Our results demonstrate a new class of inherently non-Hermitian electromagnetic media with high dissipation, no gain, and no PT-symmetry, which nevertheless have almost real eigenvalue spectrum.

We extend the transfer matrix method to study the propagation of beams and arbitrary profiled fields through anisotropic metamaterial slabs, and to demonstrate the negative refractive index property resulting in linear self-focusing of beams in hyperbolic metamaterials. Specifically, the transfer matrix method, commonly used to analyze bi-directional plane wave propagation, is developed to analyze beam propagation. By expressing a Gaussian beam as an angular spectrum of plane waves, an anisotropic transfer matrix, which is also obtained using the eigenvalues mentioned above, can be applied to calculate the beam spectrum at an arbitrary distance of propagation through a hyperbolic metamaterial. With given incident and emergent media, say, air, linear self-focusing within the metamaterial slab and subsequent reimaging in the emergent medium are numerically investigated for one transverse dimensional TM polarized Gaussian beam. Simulation results are compared with results from the unidirectional transfer function approach. The anisotropic transfer matrix method can be used to study beam transmission and reflection at the interfaces, and can be applied to analyze optical propagation through anisotropic metamaterial on uniaxial electro-optic substrates. The technique can be extended to arbitrary initial optical field profiles in one transverse dimension to assess the imaging quality of metamaterial slabs.

Hyperbolic metamaterials are valuable potential single-photon emitters because of their large density of states at phase boundaries. We grow metamaterial stacks using alternating layers of undoped GaSb (dielectric) and Si-doped InAsSb (metallic). This combination can act as a dielectric in the sample plane, but a metal perpendicular to the plane, forming a hyperbolic metamaterial (HMM) state depending on the density of free electrons. We demonstrate this behavior by injecting free electrons using an ultrafast 1300-nm pump laser, while probing the differential reflectivity and transmittivity with a linearly polarized probe in the range of 4-5 µm. The difference in results for s- and p-polarized probes demonstrates the anisotropic nature of the hyperbolic state, suggesting that single photons at mid infrared (MIR) frequencies may be efficiently emitted in a highly directional manner. The HMM state is also dependent on the metal fraction, which we control via the relative thicknesses of the layers. Additionally, spectroscopic ellipsometry reveals that the metal fraction is consistently lower than the nominal value, a phenomenon we attribute to doped carriers being squeezed to the center of the InAsSb layers. Our analysis of the pump-related shift of the metal/dielectric/HMM phase diagram shows that our sample structure is a highly tailorable avenue to MIR spontaneous photon emission.

In this work, we present a study by finite-difference time-domain FDTD simulations of metalenses with hyperbolic phase profile, formed by multilayer cylindrical scatters. The multilayers are constructed by alternating high refractive material (α-Si) and a low refractive (GaN). In this way, it is shown that these multilayer scatters improve some properties such as transmission and the reduction of the aspect ratio of the pillars to achieve the 2π phase control. And we studied the chromatic focal shift of various simulated metalenses with different numbers of layers.

Electrochromic polymers incorporated into plasmonic systems provide a possibility to control plasmonic properties with the applied voltage. Using gold-polyaniline (PANI) bilayers, we study the effect of coloration switching on surface plasmon polaritons propagating at the PANI-gold interface. The width of the resonance, magnitude of the plasmon wave-vector and dielectric permittivity of PANI are estimated as the function of the applied voltage.

The health of Earth’s atmosphere and its ecosystems are of vital importance to humanity. To assess the current state of the atmosphere and its rate of degradation, the monitoring of atmospheric gasses and particulates is necessary. The development of next-generation Low size, weight, and power (SWaP) sensors and instruments which are required for this task is a high priority for NASA’s Earth Science Technology Office (ESTO). The primary tool to monitor atmospheric gasses is hyperspectral imaging (HSI). Current HSI systems are composed of a large and complex assortment of lenses, filters and cameras that are large, heavy, expensive, and intolerant to physical shocks—all things that make them challenging for use in space-based sensing and imaging applications. As an alternative, a Low SWaP sensor is made possible by integrating a compact HSI sensor onto a CubeSat or SmallSat platform, which is much cheaper to deploy vs. a conventional satellite. To facilitate this, metamaterials are employed at the detector level to reduce the optical components required for HSI, while still providing comparable performance. The metamaterial studied here replaces a conventional grating disperser in a HSI system, by being compatible with a focused beam (fast optics) while spectrally filtering a particular spectral channel.