This PDF file contains the front matter associated with SPIE Proceedings Volume 12568, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

Magnetic topological insulators are a new class of materials that combine magnetism with topology, which leads to exotic quantum phenomena such as the quantum anomalous Hall effect and the axion insulator phase. Of the magnetic topological insulators, those with MnBi₂Te₄ magnetic septuple layers self-assembled in a non-magnetic topological Bi₂Te₃ host material are of particular interest and have recently been extensively studied. Here, we present an overview of our recent advances in understanding the influence of several factors such as the ordering of Mn impurities, omnipresent magnetic disorder, and the position of the Fermi level on ferromagnetism and magnetotransport in such systems. In particular, the consequences of these effects for observation or lack of the quantized anomalous Hall effect are discussed. Both theoretical and experimental research on these issues is crucial for gaining controllable access to the quantum anomalous Hall effect and other spintronic phenomena, which have potential applications in low-power consumption electronic devices, data storage, and quantum computing.

Non-Hermitian Physics has emerged as a fertile ground for a smart control of waves. Here, we present direct and inverse-design strategies to achieve ‘on demand’ dynamical manipulation of light by non-Hermitian potentials. The direct approach is based on our recently proposed generalized Hilbert Transform relating the real and imaginary distributions of the complex permittivity to induce spatial symmetry breaking to control scattering, widening the concept Kramers Kronig relations in space. A recipe to design complex potentials to tailor the propagation of light following any vector field, or to generate invisible potentials where light propagates as in free space. The procedure may be applied on any given arbitrary background permittivity distribution being regular or random, extended or localized. Moreover, it is possible to keep the design parameters within realistic limits, even avoiding gain. Beyond this fundamental approach, we also we also present supervised and unsupervised learning techniques for knowledge acquisition in non-Hermitian systems which accelerate the inverse the “on demand” design process. The different proposals may have direct applications to control the wave dynamics in semiconductor lasers or other linear and nonlinear physical systems including cloaking sensors and arbitrary shaped objects.

We demonstrate spectral and scattering properties of a photonic implementation of a modified Fano-Anderson model. The model includes formation of a symmetry-protected bound state in the continuum (BIC). We interpret the scattering spectra of the structures with the broken symmetry in terms of system eigenmodes. The Fano resonance associated with the excitation of quasi-BIC is explained as arising from the interference between this mode and another leaky mode.

Vertical-cavity semiconductor lasers as well as single units or arrays of Edge Emitting Lasers suffer from dynamical spatiotemporal instabilities leading to temporally unstable and low spatial beam quality. We propose a feasible stabilization mechanism for microlasers based on periodic non-Hermitian potentials, i.e. simultaneous modulations of refractive index and gain-loss. The proposed spatiotemporal modulations can be introduced by a potential directly acting on the field or by carrier modulations. The stabilization effect is based either on the suppression of the modulation instability or on asymmetric couplings in the transverse direction to localize and stabilize the field.

We investigate the effect of dynamic non-Hermitian potentials on the control of turbulence in nonlinear systems. The proposed mechanism consists on the introduction of a complex modulation both in space and time. The non-Hermitian potential is intended to asymmetrically affect the excitation cascade responsible of turbulence. We show how the system effectively promotes or opposes to the excitation cascade through wavenumbers depending on the interplay between the real and the imaginary parts of the temporal modulation. The proposal is proved on the universal Complex Ginzburg Landau Equation and its fractional counterpart.

We design and characterize chiro-optical effects in nanostructured materials, inspired by low-cost self-assembling fabrication, which can be used to obtain both single resonant nanostructures and metamaterials. We numerically show how near- and far-field chirality can be obtained in simple geometries based on plasmonic nanohole arrays or nanowires asymmetrically covered by metal. Then, we perform conventional experiments based on extinction or reflection response with a widely tunable near-infrared laser; these allow us to characterize diverse asymmetrically nanostructured substrates under tunable oblique incidence and sample rotation, thus providing direct insight into the metamaterial-governed resonances. Moreover, we employ photothermal techniques to measure the influence of the chiral response on the thermal signal. Specifically, we use photo-acoustic spectroscopy to directly measure the total absorption in the metasurface, and monitor its dependence on the laser excitation handedness. Finally, the photo-thermal deflection technique provides unconvetional insighit into chirality-governed diffraction effects in metamaterials with asymmetric plasmonic layers.

In this work, we investigate intrinsically chiral optical effects – specifically absorption circular dichroism (CD) at normal incidence – in elliptical nanohole arrays (ENHAs) with square lattice realized in thin films of silver, gold, and aluminum on glass. Our purpose is twofold: first, we aim at clarifying the origin of CD and its relation to surface plasmon polariton (SPP) properties upon symmetry reduction in the plasmonic metasurface. Second, we optimize the parameters for CD enhancement, with specific attention on aluminum in the visible-near UV spectral range. The choice of the square lattice (as compared to our previous study on ENHA in Au with a triangular lattice [Petronjievic et al. Opt. Quantum Electronics (2020) 52:176]) yields a more complete picture of chiral properties in ENHAs, and it allows comparing the results of two different simulation methods under ideal conditions, as discussed below. The results shed light on the subtle interplay between two concurring mechanisms of symmetry reduction, namely elliptical nanohole shape and tilting of nanohole axis with respect to the symmetry axes of the array. Moreover, they give guidelines for optimizing the CD from the near-IR (using Au or Ag) to the UV spectral region (using Al metal). Thus, the present work sets the bases for applications of the ENHA to chiroptical spectroscopies, notably in the near-UV region which is especially interesting for various kinds of biomolecules.

The emission pattern of Light-Emitting Diodes (LEDs) is Lambertian, which requires secondary optics to improve directionality. In addition, Gallium Nitride (GaN) based LEDs and micro-LEDs (μLEDs) have low outcoupling efficiency due to the high refractive index difference between air and GaN. Here, we experimentally investigate the impact of introducing a simple design of aluminum (Al) nanoparticles arrays (metasurfaces) to control the far-field emission of InGaN green emitting quantum wells (MQWs). This tailoring of emission originates from the near-field coupling between the InGaN MQWs and the resonant nanoparticles. Fourier microscopy measurements reveal that the period of the Al array controls the angular photoluminescence (PL) emission pattern. Furthermore, we obtain a five-fold enhancement of the collected outcoupled light intensity by implementing Al metasurfaces to the InGaN MQWs.

In this work, we propose a plasmonic absorber structure based on planar thin films metal-dielectric layers, and PCMs (Phase Change Materials) in the infrared spectrum, between 1000-2200 nm, which has high optical contrast in these regions, thus favoring its use. The transition effects between the intermediate to amorphous and crystalline phases of the PCMs layers are analyzed, based on the Lorentz-Lorenz relation. Absorption effects can be controlled using functions in which geometric parameters and crystallization levels can be related. The results presented show high absorption above 95% in both phases of the material, in normal incidence. We also analyzed the structure in oblique incidence in the TE and TM polarization modes. These structures are eligible for next-generation reconfigurable control devices.

This study was aimed to investigate the formation of thin films on a nanostructured surface. Optical characterization revealed the presence of Fano-like resonance phenomenon in such single-layer structure, surrounded by a lower refractive index media. Also, we demonstrate a 5 µm thick photonic multilayer structure composed of alternating high- and low-index materials, providing angular selectivity of light. The proposed 2D structure can be considered as a promising component for intracavity spatial filtering even in high power microlasers. Moreover, the possibility to control polarization with such photonic structures will be presented.

The process of radiative cooling is enabled by a transparency window of the atmosphere in the wavelength range of 8-13 microns. When a material emits blackbody radiation at these wavelengths, its radiation goes through the atmosphere to outer space and the material cools. To have radiative cooling during the daytime, we need materials that are both very emissive in the atmospheric window and have a large reflectivity in the solar range of 0.3-2.5. In this work, we study how metasurfaces can be designed with exactly these properties. In order to make an optimal metasurface, we use inverse design techniques.

In this paper we present a new model to describe the interaction of electromagnetic waves with spheres covered by metamaterials. The model is based on a field expansion and the definition of reflection coefficients as ratios between travelling waves. The introduced formulation allows a clear physical interpretation of several electromagnetic scattering phenomena. In this work, it is used to investigate the scattering suppression that is the phenomenon that allows the cloaking of spherical objects. The obtained results are consistent with other published results and introduce a new interpretation framework that can be used to design future materials in a plethora of applications.

By using metamaterial, scattering and absorption properties of systems can be engineered to meet certain criteria. Although metamaterials are usually based on the structuralization of matter, we demonstrate that electromagnetic radiation can be manipulated via randomly distributed nanoparticles. On the one hand, scattering in nanoparticle suspensions occurs traditionally whenever the size of the nanoparticles is non-negligible compared to the wavelength, which induces high incoherent radiation. However, high scattering is also obtained with subwavelength nanoparticle clouds in spectral bands where resonant behaviors appear. In such configuration, we demonstrate that scattering and incoherence are not correlated anymore. On the other hand, high absorption is achieved by means of the plasmonic response of the nanoparticles. Because the system is resonant, the required representative volume element to include the numerous interactions becomes very large, rendering three-dimensional computation impractical. Therefore, we have implemented most of our simulations in two-dimensional systems where the computational load is manageable. Nevertheless, 3D computations are still performed on reduced systems. Interestingly, we found that the relation between 2D and 3D representative volume is non-linear. Finally, the control of the mechanisms at stake in polydisperse-disordered media allows us to engineer systems in which the absorption is greatly enhanced by critical coupling effect.

Plasmonic enhancement has a great potential for performance improvement of high operating temperature (HOT) photodetectors, especially those optimized for long-wavelength infrared (LWIR). Conventional HOT photodetectors exhibit poor quantum efficiency (QE) due to short carrier diffusion lengths of narrow bandgap semiconductors and relatively low absorption coefficients within the LWIR range. Plasmon-driven subwavelength light confinement enables high absorption even in a very thin absorber that provides efficient carrier collection, boosting the detector QE. We propose a photovoltaic detector equipped with a two-dimensional subwavelength hole array (2DSHA) in gold metallization on InAs/InAsSb type-II superlattice (T2SL) heterostructure. Our numerical study utilizing the finite-difference time-domain (FDTD) method predicts five times increased absorption in comparison with a conventional, back-side illuminated device. The simulated behavior of the plasmonic structure was confirmed experimentally by transmittance measurements, which revealed resonant features corresponding to various plasmonic modes.

We experimentally demonstrate high coupling of light to surface polaritons by means of an optimized scatterer placed at a suitable distance from a polariton-supporting surface. Specifically, we consider poorly-absorbing gold disks acting as nearly-perfect resonant scatterers, which we separate from a gold film by means of a dielectric silica spacer. This configuration leads to resonant coupling between externally incident light and plasmon polaritons in the film with associated cross sections that approach and surpass the fundamental limit ~λ² imposed by the light wavelength λ. Our method introduces a disruptive, efficient way to solve the in/out-coupling problem in nanophotonics.

In accordance with growing scientific interests in nanoplasmonic structures, along with the increasing ability to fabricate them using proper nanotechnologies, and current interest in nonlocal optical responses, first, we have developed a methodology to incorporate nonlocal optical responses, described with a simple hydrodynamic model, into the numerical Fourier modal method (FMM) technique, to enable broadening of the simulation portfolio of such physical phenomena in plasmonic nanostructures. It is generally accepted that nonlocal interactions are most pronounced on structures with nanometer unit sizes and affect the shape of spectral functions of characterizing quantities in the resonance region. We have relied on our previous profound experience, mainly with the periodic (for one-dimensional - 1D and two-dimensional - 2D cases) extensively to various rather complex problems. Here, based on this experience, we have newly incorporated the nonlocal-response approximation into the periodic FMM technique, described with a proper hydrodynamic model, showing in several examples that this implementation is capable of numerically analyzing periodic plasmonic systems, such as nonlocal periodic multilayers and resonant gratings. Secondly, for some simple structures it is possible to find analytical solutions which can then be used to build semi-analytical approaches for the analysis of some more complex structures. Thus the second part is focused on the analytical description of nonlocal manifestations of both a planar metal layer and a bilayer, using the transfer matrix approach.

In this paper, we report the design and analysis of a metamaterial for multiband microwave applications with epsilon negative (ENG), mu-negative (MNG), and negative effective refractive index characteristics. The suggested metamaterial comprises a 0.16 cm thick FR-4 substrate with a unit cell dimension of 1 cm × 1 cm. Design and simulation of the proposed metamaterial structure are performed with the Finite element method using COMSOL Multiphysics software in the frequency range of 1-15 GHz. The effective medium parameters i.e., effective permittivity, effective permeability and effective refractive index were calculated using Nicolson Ross weir method. Further, its Effective medium ratio (EMR), which is obtained 9.9 at 3 GHz operating resonance frequency, indicating the compactness of the structure.

Metal nanohole arrays are a famous example of plasmonic nanostructured materials, which are crucial plasmonic devices that display resonances and high electromagnetic confinement in the visible and near-infrared range. Therefore, they have been suggested for use in many applications, including communications and biosensing. In this work, we present the asymmetry in nanoholes and examine its impact on the electromagnetic response using numerical models and broadband experimental measurements. We fabricated a 2D hexagonal array of asymmetric nanoholes in Ag using a low-cost production method called nanosphere lithography combined with tilted silver evaporation. Our experimental setup is based on a laser with fine input and output polarization control that is broadly controllable in the near-infrared spectrum. When the nanohole array is activated with linear polarization, we next determine the circular polarization degree of the transmitted light. We explain the asymmetry of the nanohole, which is supported by numerical simulations, as the cause of the imbalance between left and right transmitted light. We propose that such straightforward plasmonic shape could be optimized to create multipurpose flat-optic devices.

Analytical techniques such as Kubelka Munk theory, four flux theory, diffusion theory, Monte Carlo simulation technique are used to calculate radiative properties of the disordered coatings for various applications. These techniques deviate significantly from exact numerical solver when particles are in dependent scattering regime and/or when matrix absorption is high. To overcome this, we have proposed a semi-analytical technique to calculate the optical properties of disordered coatings over the entire wavelength range from visible to IR. In this work, we have shown the applicability of semi-analytical technique to model the radiative properties of monodisperse and polydisperse coating for passive daytime radiative cooling. We have used TiO₂/PDMS for monodisperse and TiO₂/ZrO₂/PDMS for polydisperse coating.

Metamaterials (MM) are artificially designed structures in which resonant subwavelength elements act as unit cells. However, metallic resonators in terahertz frequency range often have high losses. To address the issue of losses, enhancing the quality factor of resonances in MMs is an important field of study. Toroidal excitations are a class of electromagnetic excitations that take place when magnetic moments develop an end-to-end formation. The toroidal excitation has the advantage of lower radiative losses. One unique way of decreasing linewidth of resonances is to couple the MM resonance to the first-order lattice mode of the resonance. In our study, we enhance the quality factor (Q) of a toroidal resonance in a THz MM by coupling it to the lattice mode of the MM. Our proposed MM displays a broad toroidal resonance at 0.5 THz. On coupling the toroidal mode to the lattice mode by setting a P=170 μm, a 64% increase in the Q of the resonance is observed The Q is calculated to be 47. The lattice coupling is confirmed by varying the periodicity at 0.5 THz and observing modulation of resonances. Such an increase in the Q could be useful in the design of highly sensitive THz sensors for chemical and biological sensing.

Metalenses have shown their great ability in efficient manipulation of light fields and have been proposed for variety of devices with specific functionalities making the world more compact and flatter. However, a high meta-atom aspect ratio is still a drawback as it causes difficulty in fabrication of metalens. In this paper, we propose a design principle to lower the meta-atom aspect ratio in near infrared region. The designed metalens is made by arranging hollow cylindrical nanopillars made up of crystalline Silicon and the substrate is of material SiO₂. The simulation results show that the required aspect ratio of our design is much smaller than that of solid single material meta-atom thus without compromising with the transmission efficiency of the meta-atom. Also, the designed metalens have a high focusing efficiency of nearly 90% and with polarization independent characteristics. Our proposed design may pave the way towards easier fabrication and thus application of near infrared devices.

Passive radiative cooling has garnered significant attention in recent years due to its potential in addressing the energy consumption of conventional cooling systems. Plasmonic and metamaterial structures have been found to be effective broadband absorbers due to their selective emissive spectra, thin thickness, design flexibility, and the ability to excite plasmonic or photonic resonances. This study explores the use of bowtie shape plasmonic metamaterials for the development of novel, structurally simple radiative cooling devices. We show that by designing and optimizing a periodic high index-low index alternating layers (SiO₂-TiO₂), broadband reflection in visible and near-infrared spectrums is achievable. While to achieve broadband absorption in the transparency window (8-13 um), metamaterial is utilized.

Radiative losses in nanophotonic devices are a fundamental challenge in their miniaturization. Plasmonic metals overcome the radiation losses, but high ohmic losses hinder the optical performance. Supercavity modes, also known as quasi-bound states in the continuum (BIC), help circumvent this problem. In this work, we propose a low refractive index 2D-periodic array of slotted disk that supports symmetry protected BIC and accidental BIC at off-gamma point. This BIC point is very fascinating to study the exciton-cavity interaction. To study the exciton-cavity, TMDCs have the great potential to generate the exciton. This exciton is coupled with BIC mode to generate the polariton state in a strong coupling region.

Hybrid systems based on porous silicon microcavities and quantum emitters (QEs) are of much interest in terms of both basic research and development of new hybrid photoluminescent (PL) materials to be used in photonic, optoelectronic, and sensing applications. In these systems, light-matter coupling is established, whose strength could be increased to achieve the strong coupling regime by enhancing the quality factor of the microcavity. Incorporation of plasmonic nanoparticles (PNPs) also promotes an increase in the coupling strength and establishment of the coupling regime via the formation of hierarchical plasmon-optical cavities. Here we present the results of a numerical study of hybrid systems comprising porous silicon microcavities and plasmonic arrays placed inside them. These hybrid systems enable hierarchical plasmon-optical coupling with exciton transitions in QEs embedded into a porous silicon microcavity. We used numerical simulations to estimate the critical parameters for achieving light-matter coupling, including the Purcell factor and expected field enhancement, as well as the spatial distribution of the electromagnetic field within the structure. We speculate that light-matter coupling between the PL of QEs and the hierarchical cavity mode is stronger than in a microcavity not containing PNPs.

In this paper we demonstrate a high numerical aperture (NA) mid Infra-red (MIR) meta-lens with high focusing efficiency in band (2μm - 3μm) based on inverse design with topology optimization. We reformulate the optimization problem to benefit multi-wavelengths using maximal mini formulation. The meta-surfaces based on all-doped titanium dioxide (TIO2). The designed meta-lens based on this inverse design methodology produces a maximum focusing efficiency 93.71%, an average focusing efficiency 75.66%, a maximum full width half maximum (FWHM) 2.2μm and an average FWHM 1.8µm, and all these results laying under very high numerical aperture condition 0.8.

Fano resonance is a universal phenomenon that has been widely exploited to achieve spectrally sharp resonances for a variety of applications. Recently, all dielectric metasurface design composed of two silicon resonators, nanodisk and nanobar, exhibits an induced transparency window with ultrahigh quality factor in polarization direction along the nanobar long axis relying on the intriguing physics of Fano resonance. However, the design demonstrates a dispersive behavior within a short spectral range, a phenomenon that hinders its practical usage as a Biosensor due to the lack of the spectral selectivity. In this work, we report the tunable design along the range of wavelengths from 1.2 μm to 1.75 μm through changing the radius of the nanodisk from 200nm to 350nm. Although the design demonstrates a spectral red shift of the reflection with increasing the nanodisk radius, the reflectance behavior is inconsistent indicating the formation of transparency windows in most cases. Worthwhile, the structure design of radius (r=300nm) enables attaining a single sharp Fano resonance addressing the aforementioned short range dispersion obstacle. Meanwhile, the optimized design of radius of (r=300nm) provides a significant quality factor of (~808) and ultrahigh sensitivity of (275nm/RIU) with a figure-of-merit of (148) at the operating wavelength of (λ=1.583μm). The proposed design demonstrates a tremendous impact in refractometric biosensing applications due to its high performance and the ease of fabrication.

Fano resonance is an intriguing physical phenomenon that could be achieved by engineering a destructive interference between a superradiant bright mode and a subradiant dark mode. A variety of hybrid systems of metasurfaces have been widely explored demonstrating sharp Fano resonances with high sensitivity. However, plasmonic metasurfaces have limitations of Ohmic losses that constrain the achievable quality factor. Meanwhile, the dielectric metasurfaces provide sharp Fano resonance but with limited sensitivity and figure-of-merit (FOM) compared to plasmonic metasurfaces. In this paper, we report an ultra-sharp, ultra-sensitive refractormetric gas sensor based on Fano resonance using all dielectric metasurface. Our proposed design is composed of hybrid system of nano-bar/nano-elliptic all dielectric silicon. The proposed design has reported a sensitivity of 1,852 nm/RIU with a significant quality factor of 1225, leading to a figure-of-merit (FOM) of 411 at the operating wavelength of 5.5128 μm, within the spectral window of the nitric oxide. Our design brings a considerable impact as a cheap and easily fabricated sensor for gas chemical recognition.

In this paper, we present a new treatment of cloaking strategies proposed by Belın et. al., in their work on Ideallens Cloaks. By calculating material properties we design a metamaterial structure that acts as an invisibility cloak, based on the principle of an Abyss Cloak, which is a device that shifts the image of the cloaked object to the exterior position. The use of this approach comes with a better affinity for experimental realization. To support this claim, we execute a simple experiment, using a setup consisting of four optical wedges. Our work significantly stretches the arsenal of transformation optics devices, as the presented devices can be employed not only as invisibility cloaks, but also as building blocks for other devices, e.g. optical analogues of celestial mechanics.