This PDF file contains the front matter associated with SPIE Proceedings Volume 12990, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

This paper demonstrates the feasibility of a fast, Frequency-Agile, Stimuli responsive, Tunable (FAST) optical filter using a metamaterial combined with a Phase Change Material (PCM). The FAST Optical Filter (FOF) is optimized to operate in visible to near-infrared (NIR) wavelengths (380-1000 nm, or equivalent frequencies) with the capability to tune to multiple wavelengths, autonomously respond to and function with different polarizations of light and attain frequency agility using external stimuli. A first estimate of their optical performance (reflection and transmission characteristics) is investigated using the transfer matrix method. Incorporation of PCM within a metamaterial (metallo-dielectric stacks) sandwich can enable electronic tuning of the structure through application of an external voltage.

A multimode framework that can be utilized in the analysis and design of general, non-Hermitian periodic systems that comprise contemporary 2D materials is developed. The theoretical framework is based on the concept of Quasinormal Modes (QNMs) that can be used to efficiently retrieve the spectral response of periodic systems. Both sub- and super-wavelength lattice constants are examined with extra care to include propagating higher diffraction orders. The framework is employed in two indicative configurations, a graphene-based periodic metasurface that supports tightly confined graphene surface plasmons, and a periodic dielectric metasurface enhanced with a transition metal dichalcogenide layer that supports quasi bound states in the continuum. Both configurations are of fundamental and practical interest since they exhibit controllable resonant response and, in addition, the electromagnetic properties of the involved 2D materials can be electrically tuned. The Finite Element Method (FEM) is used to retrieve the QNMs, which are then fed to the framework to specify the spectral response for the zeroth and higher diffraction orders. Full-wave FEM simulations are used to verify the validity of obtained results.

In this presentation, we shed light on electromagnetic time interfaces using the principles of quantum optics. We begin by explaining the transformation of bosonic mode operators in response to a temporal discontinuity in the macroscopic parameters. Accordingly, our discussion extends to the general analysis of photon statistics and the degree of second-order coherence of the generated forward and backward modes. Focusing on input number states, we meticulously detail the quantum states produced after the time interface, elucidating the associated probability distributions. By virtue of this study, we specifically highlight the controllable nature of photon statistics of the output forward mode for particular input states. We show that this capability, in general, is not present for the backward mode generated through the time interface. Also, we reveal that photon-pair generation is a fundamental inherent feature of the time interface. By unveiling all these insights, we aim to pave the way for exploring photonic time crystals within the domain of quantum optics.

Flexible holographic metasurfaces offer significant versatility in optical imaging applications due to their ability to conform to non-flat objects and effectively manipulate optical properties. In this paper, we present the design, fabrication, and characterization of two functional imaging applications that utilize flexible holographic metasurfaces when the substrate radius of curvature is comparable to or much larger than the metasurfaces. For the first application, shape-dependent conformable holographic metasurfaces operate in the millimeter wavelength range. The images can be encoded with the target curvature and, more interestingly, the metasurface can encode two independent images within a single hologram. By bending the substrate to either a convex or concave shape, two switchable images can be revealed. The other application is a self-calibrated curvature sensor that operates in the visible range. The sensor consists of two distinct areas formed by metasurfaces that generate images of a two-dimensional reference scale and a dot image. The dot image acts as a displacement indicator and shifts within the scale as the metasurface deforms, providing an instant readout of the radius of curvature of an object that hosts the sensor on its surface, without the need for pre-calibration process. These two demonstrations expanded the use of metasurfaces for out-of-lab applications, particularly in tunable optomechanical sensors for radar and antennas, and in real-time curvature monitoring.

Full-Stokes polarimetric imaging enhances the information available from satellite remote sensing. But the numerous bulky and heavy optical components required to achieve polarimetric imaging limit its use for small-form satellites. We present the modelling of an ultra-thin nanostructured metasurface as a novel solution to the weight and volume constraints faced by small satellites. Positioned in a telescope’s pupil plane, the metasurface diffracts light into five polarization measurements that are imaged onto a detector, restoring the full field of view as the satellite moves over the Earth’s surface. Designed to have effective redundancy, any four out of the five orders enable the reconstruction of the full polarization state, allowing error monitoring. The metasurface material is 1 μm thick silicon repeating patterns on a 460 μm thick sapphire substrate, utilising free-form topology to optimise for throughput efficiency and equal light distribution between the polarization diffraction orders.

Miniaturized optical spectrometers have promising prospects, owing to their potential applications in several areas such as healthcare, industrial in-line inspection, remote sensing and so on. Recently, significant progress has been made in the development of computational micro-spectrometers based on compressive sensing techniques that use on-chip photonic filters coupled to the image sensor. We present a hybrid design for a photonic filter chip based on photonic crystal structures that can improve the spectral reconstruction efficiency of computational micro-spectrometers. Our design consists of multiple layers of optical materials, where the thickness of each layer is optimized to facilitate the generation of random and minimally correlated optical transmission through multiple on-chip photonic filters, which are required for the effective execution of compressive sensing algorithms. We verified our design by performing multiple simulated reconstructions of different spectral signals and using reconstruction algorithms.

Metasurfaces have opened the way for gaining control over the frequency and the polarization responses of electromagnetic waves. However, it is still challenging to control their electromagnetic angular response. Here, the potential of diffractive surfaces for breaking the symmetry of the angular transmittance is revealed. By engineering an asymmetric angular response for the scattering particle in each unit cell of the surface, it is possible to break the angular transmittance symmetry in a reciprocal, passive and lossless fashion. In this work, we introduce a metagrating which can filter the positive (or negative) side of the momentum space for obliquely impinging TM waves. Such a system could find several applications in optical analog signal processing. For example, the Schlieren imaging technique, which consists in filtering out half of the momentum space, would be feasible without the need for a bulky free-space system composed of multiple lenses or mirrors.

Between the design and final realization of a working metasurface lies the potential for a myriad of complications: fabrication tolerances, material permittivity uncertainties, alignment issues and localized defects to name just a few. Global methods of characterizing an entire surface are often incapable of separating these candidates and typically one must resort to the simulation of a wide parameter space to begin to understand experimental discrepancies. In this work we introduce a new imaging technique that is able to locate and discern the resonant frequencies of individual antennas in a complex microwave metasurface. This is achieved with a microwave single-pixel camera using patterned optical excitation of a silicon layer adjacent to the metamaterial to achieve super-resolution. This approach allows us to locate and diagnose fabrication defects, spectrally characterize individual meta-atoms, and visualize inhomogeneous broadening across our samples with below λ/20 resolution, over large areas and in near real-time.

We present our progress in the development of metasurface technology for biophotonics applications.

Bound states in the continuum can be defined as non-radiating resonant modes within open environments. These modes share a defining characteristic of being dark, displaying an exceptional degree of field localization. However, their practical accessibility lies in their quasi-bound form, which needs the introduction of perturbations in the system's geometry or material properties. Despite a finite, albeit high, quality factor, the quasi-bound modes manage to retain their characteristic strong field localization. In this presentation, our focus will be directed towards the exploration of symmetry-protected bound states in the continuum, delving into a comprehensive analysis of the impact that the introduction of various types of asymmetries can have on the formation and behavior of their quasi-bound counterparts. In particular, we will focus our attention on metasurfaces made of BaTiO₃, whose constituent elements are periodically arranged nanowires. By investigating the topological features that contribute to certain mode selection rules, our analysis aims to provide a deeper understanding of the underlying mechanisms governing the formation and behavior of these modes. Our findings provide a strategic roadmap for optimizing the implementation of quasi-bound modes and provide a clear path to exploit them in specific applications such as sensing and nonlinear optical processes.

A very interesting class of metamaterials can be defined making use of supersymmetry (SUSY). What makes SUSY very attractive for the design of new optical devices is the possibility to define different spatial refractive index distributions (superpartners) having the same scattering spectra (angularly and spectrally). In this study we explore the possibilities offered by the generation of superpartners of a constant (in space) refractive index distribution (practically an infinite slab), being transmissionless and possessing unit reflection by definition. Interestingly, starting from a critical wavelength, the Darboux transform allows to define 1D materials that are transmissionless (or perfectly reflective) for a continuum of frequencies and incident directions. This behavior can be understood in analogy to total reflection instead realized spectrally. In addition, broken-SUSY can be used to define a transmissionless active cavity capable of amplifying (or absorbing) electromagnetic radiation in the visible. This approach is analytical through the use of the Darboux transform (a type of supersymmetric transformation) for the generation of the optical potential and the calculation of the field, while the transmission/reflection spectra evaluation is done with the transfer matrix method. Finally, the calculated refractive index distributions can be realized in practice as 1D multi-layered structures corresponding to optical filters.

We propose a new application of metasurfaces, namely the harnessing of the visual appearance. We demonstrate that disordered metasurfaces composed of high-index resonant metaatoms, once deposited on macroscopic objects, offer visual appearances with novel visual properties.

Nonlinear dielectric metasurfaces provide a promising platform to control and manipulate the optical frequency conversion at the nanoscale, suggesting new applications in imaging, lasing, and sensing. Here, we engage symmetry-broken silicon metasurfaces to enhance the second- and third-harmonic generation. We design and fabricate amorphous silicon metasurfaces supporting optical bound states in the continuum and guided-mode resonances, which greatly boost light-matter interaction resulting in several-order enhancements in both second-and third-harmonic generation.

This research introduces a highly compact chiral metamaterial design (100 nm) capable of generating Circular Polarization (CP) lasing, a topic of high scientific and technological interest. The proposed metamaterial utilizes pairs of twisted-crosses as meta-atoms, embedded within a gain medium. By controlling the crosses twist-angle we demonstrate control of the emission polarization, ranging from almost purely left- to purely right-circular polarization. Our highly engineerable metamaterial design provides a versatile and easily implementable method for producing robust and tailored CP lasers, opening up significant possibilities for future applications.

The control of wave scattering in unknown complex media (e.g., chaotic cavities) that can be reconfigured by a programmable metasurface is a seemingly daunting task: the mapping from metasurface configuration to scattering matrix is non-linear (due to non-locality), and there is no a priori knowledge of geometry or material composition of the unknown complex medium. Yet, this mapping must be known to enable open-loop wave control. Training a deep neural network to approximate the mapping is a potential solution. However, we show here that formulating a physical model is surprisingly simple and offers 10X advantages over the use of deep learning in terms of the achieved accuracy, the model compactness (number of model parameters) and the number of required calibration measurements. Surprisingly, the physical model can be trained based on phase-insensitive measurements which drastically alleviates the measurement complexity. This unlocks previously unimagined regimes of phaseless coherent wave control combining wavefront shaping and metasurface tuning, e.g., to tune a complex system to coherent perfect absorption (CPA) at a desired frequency and to identify the CPA wavefront without ever measuring phase. This remarkable phase-retrieval ability can be traced back to the mathematical structure of the physical model that imposes strong constraints on the possible model parameter values. The constraints are so strong that one can even estimate an entire scattering matrix (up to some sign ambiguities) by only inputting/outputting waves through a subset of the ports while tuning the loads terminating the remaining ports.

In recent years, the booming development in the field of metamaterials has provided new opportunities to introduce the concept of metamaterials into carbon fiber composites with the aim of enhancing material properties. In order to optimize the interfacial properties of carbon fiber composites, researchers have designed a series of novel interfacial materials, surface modification techniques, and interfacial engineering methods by modulating the interfacial structure and properties. Among them, the interfacial behavior of carbon fiber reinforced resin matrix composites was investigated using Molecular Dynamics (MD) simulation, and a carbon fiber/epoxy shear model with unoxidized and 10% hydroxyl group on the surface was constructed using Material Studio software to simulate the detachment of the carbon fibers from the epoxy resin matrix. The microscopic behavior of the carbon fiber/epoxy interface was investigated, including the change of interfacial mechanical properties under functionalization treatment and loading conditions, and the in-plane shear damage mechanism. The results show that the main mechanism of composite interface damage is shear slip behavior, and under the simulated in-plane shear loading conditions, the elastic phase is firstly entered in the separation damage process, when the interaction energy between carbon fiber and epoxy resin atoms changes, followed by a rapid increase of hydrogen bonding in the non-bonding energy. The treatment of carbon fibers with oxidized functional groups can enhance the interaction between carbon fibers and epoxy resin, which is beneficial for stress transfer and improves the interfacial strength. This study provides an in-depth understanding of the damage mechanism of carbon fiber/epoxy resin materials on a microscopic scale, which is of great theoretical significance for exploring carbon fiber reinforced resin matrix composites.

Nanowire metamaterials play a pivotal role in a wide variety of applications including antennas research field and cancer detection in clinical practice. In the following we make a step forward by investigating properties of surface waves at the boundary of ellipsoidal nanowire metamaterial, specifically, we study effective properties of the ellipsoidal medium. The former could result in a possible application in clinical practice dealing with the cancer detection especially by having a deep insight into derived medium treatment approach due to the chosen model having similarities with the real biological systems containing cancer cells as the ellipsoidal inclusions. The future prospects could include translation of the obtained effective medium properties applying effective medium approximation models into cancer detection in clinical practice. The ultimate goal is to treat the investigated biological medium from the perspectives of the metamaterial theory.

Reduced Graphene Oxide (rGO) and other graphene-based two-dimensional (2D) materials exhibit promising potential across various fields, such as energy storage, solar cells, and sensors. However, concerns regarding the toxic nature of rGO in biomedical applications have been raised. In this study, we investigate the feasibility of utilizing hole arrays as a sensing platform for detecting the presence of rGO through the modulation of Surface Plasmon Resonance (SPR) via Extraordinary Transmission phenomena (EOT) in the Terahertz (THz) regime. The rGO is prepared using the Hummers method and subsequent reduction of Graphene Oxide (GO). Characterization of the reduced material is performed using Fourier-Transform Infrared spectroscopy (FTIR) and X-Ray Diffraction (XRD), confirming successful reduction. Deposition of rGO on top of hole arrays resulted in changes in SPR frequency, indicating a responsive sensing platform sensitive to changes in rGO thickness. Experimental findings are further verified through numerical simulations. Our study highlights the potential of surface plasmon-based sensing platforms for the detection of rGO using hole arrays in the THz regime.

The state-of-the-art spectroscopy instruments are designed to collect as much light as possible, especially for astronomy applications. In this context, a blazed sub-wavelength disperser (called a metasurface) is a promising alternative to the widely used sawtooth blazed gratings with a metal coating. In order to find the optimal opto-geometric characteristics of such a device, topology optimization based on Finite Element modelling of Maxwell’s equations is used. This paper deals with the application of this powerful optimization process, using different materials and introducing the manufacturability constraints. The reflection averaged on the [400,1500]nm wavelength range can reach 80% with a broadband optimization on silica. It is 28% higher than that of the sawtooth blazed grating (which reaches 52%) in absolute terms, and 54% in relative terms. First samples of metasurface gratings have been manufactured.

Gaia mission is providing the most accurate astrometric and photometric star catalogue (June 2022 DR3), in addition it is providing, as various projects on earth or in space, numerous alerts concerning different celestial objects and transient events. To make comparison between the observations on earth, it is recommended that they be well standardized, using optical filters adapted to Gaia three spectral bands. In this work, we propose to reproduce these three spectral bands using multilayer filter containing non periodic structures using optimization algorithms, such as Levenberg-Marquardt algorithm, “needle” algorithm and genetic algorithm. Optimizing multilayers for optical applications is still a major issue today, with the aim of achieving the best possible specifications with the fewest possible layers. Three multi-layer filters are proposed made up respectively of 20, 40 and 60 layers deposited on BK7 substrate, to match the specifications provided by the RAPAS network. We also investigated the use of other substrates and thin metal layers, which helped us to reduce the number of layers in the optimization process.

In this paper, we will report on our progress in the development and application of flexible holographic metasurfaces designed for AR NEDs. Our discussion will detail the innovative methods employed, address the challenges encountered, and present our prototype design concept and testing results under different conditions such as (RGB illumination etc.) on non-flat substrates, maintaining high image quality.

On-chip optical trapping as an alternative to bulky microscope objectives has been primarily explored in the near-infrared spectrum with the aid of metalenses. This limitation arises from the tendency of dielectric metasurfaces to be constrained to the infrared transparency window, resulting in significant absorption losses, especially in the visible spectrum and particularly at shorter wavelengths. However, optical trapping in shorter wavelengths can provide higher trap stiffness that can stabilize the trapping platform. Silicon and titanium dioxide as common materials that can be utilized for optical trapping in the visible range pose practical limitations due to either the nature of the material or the fabrication process involved. Here, we introduce zirconium dioxide as a reliable alternative to other material platforms, having a high refractive index with weakly-dispersive behavior in the ultraviolet and visible range with negligible absorption loss, along with excellent mechanical strength. We experimentally report the first demonstration of on-chip devices for optical trapping below 500 nm and holographic image projection applications across the entire visible range. Specifically, we created metasurfaces for optical trapping applications with a high numerical aperture (NA=1.2) at wavelengths as low as 532 nm and 488 nm, with trap stiffness greater than 130 pN/(μmW) and 200 pN/(μmW), respectively. Importantly, the efficient developed fabrication scheme consists of two steps, including E-beam lithography and atomic layer deposition with operational temperature as low as 80°C. This fabrication scheme encapsulates the meta-atoms and offers a flat biocompatible layer atop the structure.

Epsilon-Near-Zero (ENZ) materials have gained interest due to their unusual linear and large nonlinear optical response. Here, we demonstrate that plasmonic dipole nanoantennas on a graded ENZ substrate have optical properties significantly different from antennas on a conventional ENZ substrate. Our hybrid nanoantenna-graded ENZ metasurface offers an efficient nonlinear optical platform.

Solar energy is a clean and renewable energy source that solves energy and climate emergencies. The near-perfect broadband solar absorbers can offer necessary technical assistance to follow this route and develop an effective solar energy-harvesting system. This work designed a metamaterial perfect absorber that operates in the ultraviolet to near-infrared spectral range. It is made up of titanium (Ti) parabolic nanoarrays with a hexagonal lattice structure. By using the commercially available 3D-electromagnetic simulator (Lumerical FDTD) operating in the finite-difference time-domain approach, we have found the strong absorption (⪆ 90%) of the absorber across the broadband range of the wavelength 200–3000 nm. Furthermore, we observed that increasing the height of nano-pillars can lead to an expansion in the bandwidth of the absorber. Our study highlights the significant roles played by the localized surface plasmon resonances of Ti parabolic nanoarrays, the inherent loss of titanium material, and the coupling of resonance modes between adjacent parabolic nanoarrays in facilitating this broadband perfect absorption phenomenon. Additionally, we demonstrate that the absorber exhibits some excellent features desirable for the practical absorption and harvesting of solar energy, such as precision tolerance, polarization independence, and large angular acceptance.

Designing acoustic resonators at ultra-high frequencies is a promising pathway for important technological advancements, including quantum technologies. Resonators operating in the tens-of-gigahertz range are usually based on semiconductor multi-layered Distributed Bragg Reflectors (DBRs). However, their inherent lack of tuning capability imposes constraints on specific applications, such as sensing. Mesoporous materials, with pores at the nanoscale, could potentially bridge the gap for responsive acoustic resonators. These cost-effective materials support gigahertz acoustic resonances, and their high surface-to-volume ratio allow for engineering resonators with novel functionalities, such as optoacoustic sensors or switches. For instance, the infiltration of liquids and vapors into the pores modifies the material’s optical and elastic parameters, leading to direct modification of the acoustic resonances. Here, we theoretically propose open-cavity multi-layered acoustic resonators based on SiO₂ mesoporous materials that are responsive to the environment, e.g., relative humidity changes. The resonators are formed by a DBR of dense SiO₂/TiO₂ multilayers, followed by a Ni acousto-optical transducer, to both generate and detect acoustic phonons, and finalized by the mesoporous as the last layer, so it has access to the environment. In this design, gigahertz acoustic phonons can be effectively confined at the mesoporous layer. On one hand, the acoustic DBR prevents their leakage toward the substrate, and on the other hand, they cannot propagate through the air, creating an ideal acoustic mirror at the mesoporous-air interface. We present two case studies based on this approach and show, by numerical simulations using transfer matrix method, the responsiveness and tunability of such resonators upon ambient humidity changes. The simulation results also indicate that these devices can be characterized through transient reflectivity experiments. Our study could guide the way for designing effective nanoacoustic sensing and adaptable devices, utilizing cost-effective fabrication techniques.

The challenge of designing crossings in one-dimensional grating waveguides (1DGWs) arises from the noticeable asymmetry in Bloch mode profiles, which causes the guided modes to be compressed toward the outer sidewall. The proposed solution involves using digitized metamaterials in an extensively corrugated 1DGW on a silicon-on-insulator platform. The resulting crossing structure has an ultra-miniaturized and ultra-low loss design, with a broad bandwidth spanning from 1500 to 1580 nm, while maintaining a minimal footprint of merely 2.1×2.1 μm². The fabricated device is found to have an insertion loss of -2.52 dB within the wavelength range of 1500 to 1580 nm. This design represents a significant advancement in the pursuit of compact and low-energy silicon photonic waveguide crossings.

The ICR (ionic current rectification) phenomenon in nanopores is influenced by both the shape and material of the nanopore, as well as the ion concentration within the electrolyte that permeates the nanopore. Specifically, nanopores composed of multiple materials can induce ICR due to variations in ion distribution along the nanopore-electrolyte interface across different sections of the material. In this study, we investigate the impact of configurations involving up to two layers of different materials (Si₃N₄ and Au) and six different electrolytes on solid-state nanoporous ICR. We aim to assess how different configurations of surface charges can influence ICR. The main advantage of nanopores made from gold and other metals may be their use as an ICR amplifier when illuminating nanopores with a wavelength of light lying in the plasmon resonance region of the metal used.

All naturally occurring materials show a fixed combination of refractive index and bandgap energy, which correlates with their absorption. Both properties are material constants. Metamaterials are an alternative approach to vary these constants independently and increase the transparency range of existing optics in the UV. These metamaterials can be produced using Quantum Nanolaminates (QNLs). The special type of material sequence in the QNLs coatings opens up a wide range of possible material parameters without the process problems which occur with mixed materials that are commonly used in this context. This presentation is going to investigate the concept of nanolaminates made from the high-refractive index material hafnia (HfO₂) and the low-refractive index material silica (SiO₂). In order to reach a blue shift of the absorption edge, the QNLs were applied in an anti-reflex coating as a substitute for the high-refractive coating material. All layers were produced by ion beam sputtering.

Structured light beams have attracted significant interest due to their versatile spatial or spatiotemporal configurations, which can potentially transform various real-world applications not achievable using conventional Gaussian beams. Bessel beams, with their non-diffracting properties, have proved themselves to be a phenomenal alternative in numerous scenarios where Gaussian beams were traditionally expected to excel. Therefore, numerous devices have been proposed to generate such non-diffracting beams via ultrathin and compact platforms of artificially engineered subwavelength thick structures, acknowledging the significance of chip-scale integration. However, selecting element/elements with their geometry influences and imposes limitations on many applications. Here, we demonstrate a geometric phase-based all-dielectric, highly efficient (≈ 80%) polarization-insensitive compact transmissive structure. Our metastructure utilizes zinc sulfide as an elementary material, designed to operate in the visible spectrum (475− 650 nm) due to its favorable optical constants. The proposed design philosophy is affirmed by a straightforward approach that effectively handles the persistent challenge of polarization-insensitive light structuring across a broader spectrum using a distinct single-cell-driven, single-layered metastructure. For proof of concept, numerical simulations utilizing the Finite Difference Time Domain (FDTD) method validate our idea, demonstrating second-order Bessel beams under various polarization states (e.g., linear and circular) for the visible spectrum. We are confident that generating polarization-independent Bessel beams of zeroth or higher order within a visible domain of the electromagnetic spectrum can unlock promising possibilities for numerous advanced applications, such as imaging, laser fabrication, optical manipulation, and beyond.