Reversible optical tuning of GeSbTe phase-change metasurface spectral filters for mid-wave infrared imaging

Infrared phase-change chiral metasurfaces with tunable circular dichroism

Integrating phase-change materials in metasurfaces has emerged as a powerful strategy to realize optical devices with tunable electromagnetic responses. Here, phase-change chiral metasurfaces based on GST-225 material with the designed trapezoid-shaped resonators are demonstrated to achieve tunable circular dichroism (CD) responses in the infrared regime. The asymmetric trapezoid-shaped resonators are designed to support two chiral plasmonic resonances with opposite CD responses for realizing switchable CD between negative and positive values using the GST phase change from amorphous to crystalline. The electromagnetic field distributions of the chiral plasmonic resonant modes are analyzed to understand the chiroptical responses of the metasurface. Furthermore, the variations in the absorption spectrum and CD value for the metasurface as a function of the baking time during the GST phase transition are analyzed to reveal the underlying thermal tuning process of the metasurface. The demonstrated phase-change metasurfaces with tunable CD responses hold significant promise in enabling many applications in the infrared regime such as chiral sensing, encrypted communication, and thermal imaging.

Nonvolatile Phase-Only Transmissive Spatial Light Modulator with Electrical Addressability of Individual Pixels

Active metasurfaces with tunable subwavelength-scale nanoscatterers are promising platforms for high-performance spatial light modulators (SLMs). Among the tuning methods, phase-change materials (PCMs) are attractive because of their nonvolatile, threshold-driven, and drastic optical modulation, rendering zero-static power, crosstalk immunity, and compact pixels. However, current electrically controlled PCM-based metasurfaces are limited to global amplitude modulation, which is insufficient for SLMs. Here, an individual-pixel addressable, transmissive metasurface is experimentally demonstrated using the low-loss PCM Sb2Se3 and doped silicon nanowire heaters. The nanowires simultaneously form a diatomic metasurface, supporting a high-quality-factor (∼406) quasi-bound-state-in-the-continuum mode. A global phase-only modulation of ∼0.25π (∼0.2π) in simulation (experiment) is achieved, showing ten times enhancement. A 2π phase shift is further obtained using a guided-mode resonance with enhanced light-Sb2Se3 interaction. Finally, individual-pixel addressability and SLM functionality are demonstrated through deterministic multilevel switching (ten levels) and tunable far-field beam shaping. Our work presents zero-static power transmissive phase-only SLMs, enabled by electrically controlled low-loss PCMs and individual meta-molecule addressable metasurfaces.

Multi-band reprogrammable phase-change metasurface spectral filters for on-chip spectrometers

Active optical metasurfaces provide a platform for dynamic and real-time manipulation of light at subwavelength scales. However, most active metasurfaces are unable to simultaneously possess a wide wavelength tuning range and narrow resonance peaks, thereby limiting further advancements in the field of high-precision sensing or detection. In the paper, we proposed a reprogrammable active metasurface that employs the non-volatile phase change material Ge2Sb2Te5 and demonstrated its excellent performance in on-chip spectrometer. The active metasurfaces support magnetic modes and feature Friedrich-Wintgen quasi bound states in the continuum, capable of achieving multi-resonant near-perfect absorption, a multilevel tuning range, and narrowband performance in the infrared band. Meanwhile, we numerically investigated the coupling phenomenon and the intrinsic relationship between different resonance modes under various structural parameters. Furthermore, using the active metasurfaces as tunable filters and combined with compressive sensing algorithms, we successfully reconstructed various types of spectral signals with an average fidelity rate exceeding 0.99, utilizing only 51 measurements with a single nanostructure. A spectral resolution of 0.5 nm at a center wavelength 2.538 µm is predicted when the crystallization fractions of GST change from 0 to 20%. This work has promising potential in on-site matter inspection and point-of-care (POC) testing.

Versatile spaceborne photonics with chalcogenide phase-change materials

Recent growth in space systems has seen increasing capabilities packed into smaller and lighter Earth observation and deep space mission spacecraft. Phase-change materials (PCMs) are nonvolatile, reconfigurable, fast-switching, and have recently shown a high degree of space radiation tolerance, thereby making them an attractive materials platform for spaceborne photonics applications. They promise robust, lightweight, and energy-efficient reconfigurable optical systems whose functions can be dynamically defined on-demand and on-orbit to deliver enhanced science or mission support in harsh environments on lean power budgets. This comment aims to discuss the recent advances in rapidly growing PCM research and its potential to transition from conventional terrestrial optoelectronics materials platforms to versatile spaceborne photonic materials platforms for current and next-generation space and science missions. Materials International Space Station Experiment-14 (MISSE-14) mission-flown PCMs outside of the International Space Station (ISS) and key results and NASA examples are highlighted to provide strong evidence of the applicability of spaceborne photonics.

Dual-Channel Switchable Metasurface Filters for Compact Spectral Imaging with Deep Compressive Reconstruction

Spectral imaging technology, which aims to capture images across multiple spectral channels and create a spectral data cube, has been widely utilized in various fields. However, conventional spectral imaging systems face challenges, such as slow acquisition speed and large size. The rapid development of optical metasurfaces, capable of manipulating light fields versatilely and miniaturizing optical components into ultrathin planar devices, offers a promising solution for compact hyperspectral imaging (HSI). This study proposes a compact snapshot compressive spectral imaging (SCSI) system by leveraging the spectral modulations of metasurfaces with dual-channel switchable metasurface filters and employing a deep-learning-based reconstruction algorithm. To achieve compactness, the proposed system integrates dual-channel switchable metasurface filters using twisted nematic liquid crystals (TNLCs) and anisotropic titanium dioxide (TiO2) nanostructures. These thin metasurface filters are closely attached to the image sensor, resulting in a compact system. The TNLCs possess a broadband linear polarization conversion ability, enabling the rapid switching of the incidence polarization state between x-polarization and y-polarization by applying different voltages. This polarization conversion facilitates the generation of two groups of transmittance spectra for wavelength-encoding, providing richer information for spectral data cube reconstruction compared to that of other snapshot compressive spectral imaging techniques. In addition, instead of employing classic iterative compressive sensing (CS) algorithms, an end-to-end residual neural network (ResNet) is utilized to reconstruct the spectral data cube. This neural network leverages the 2-frame snapshot measurements of orthogonal polarization channels. The proposed hyperspectral imaging technology demonstrates superior reconstruction quality and speed compared to those of the traditional compressive hyperspectral image recovery methods. As a result, it is expected that this technology will have substantial implications in various domains, including but not limited to object detection, face recognition, food safety, biomedical imaging, agriculture surveillance, and so on.

Roadmap for phase change materials in photonics and beyond

Phase Change Materials (PCMs) have demonstrated tremendous potential as a platform for achieving diverse functionalities in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum, ranging from terahertz to visible frequencies. This comprehensive roadmap reviews the material and device aspects of PCMs, and their diverse applications in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum. It discusses various device configurations and optimization techniques, including deep learning-based metasurface design. The integration of PCMs with Photonic Integrated Circuits and advanced electric-driven PCMs are explored. PCMs hold great promise for multifunctional device development, including applications in non-volatile memory, optical data storage, photonics, energy harvesting, biomedical technology, neuromorphic computing, thermal management, and flexible electronics.

Review: tunable nanophotonic metastructures

Tunable nanophotonic metastructures offer new capabilities in computing, networking, and imaging by providing reconfigurability in computer interconnect topologies, new optical information processing capabilities, optical network switching, and image processing. Depending on the materials and the nanostructures employed in the nanophotonic metastructure devices, various tuning mechanisms can be employed. They include thermo-optical, electro-optical (e.g. Pockels and Kerr effects), magneto-optical, ionic-optical, piezo-optical, mechano-optical (deformation in MEMS or NEMS), and phase-change mechanisms. Such mechanisms can alter the real and/or imaginary parts of the optical susceptibility tensors, leading to tuning of the optical characteristics. In particular, tunable nanophotonic metastructures with relatively large tuning strengths (e.g. large changes in the refractive index) can lead to particularly useful device applications. This paper reviews various tunable nanophotonic metastructures' tuning mechanisms, tuning characteristics, tuning speeds, and non-volatility. Among the reviewed tunable nanophotonic metastructures, some of the phase-change-mechanisms offer relatively large index change magnitude while offering non-volatility. In particular, Ge-Sb-Se-Te (GSST) and vanadium dioxide (VO2) materials are popular for this reason. Mechanically tunable nanophotonic metastructures offer relatively small changes in the optical losses while offering large index changes. Electro-optically tunable nanophotonic metastructures offer relatively fast tuning speeds while achieving relatively small index changes. Thermo-optically tunable nanophotonic metastructures offer nearly zero changes in optical losses while realizing modest changes in optical index at the expense of relatively large power consumption. Magneto-optically tunable nanophotonic metastructures offer non-reciprocal optical index changes that can be induced by changing the magnetic field strengths or directions. Tunable nanophotonic metastructures can find a very wide range of applications including imaging, computing, communications, and sensing. Practical commercial deployments of these technologies will require scalable, repeatable, and high-yield manufacturing. Most of these technology demonstrations required specialized nanofabrication tools such as e-beam lithography on relatively small fractional areas of semiconductor wafers, however, with advanced CMOS fabrication and heterogeneous integration techniques deployed for photonics, scalable and practical wafer-scale fabrication of tunable nanophotonic metastructures should be on the horizon, driven by strong interests from multiple application areas.

Recent developments in Chalcogenide phase change material-based nanophotonics

There is now a deep interest in actively reconfigurable nanophotonics as they will enable the next generation of optical devices. Of the various alternatives being explored for reconfigurable nanophotonics, Chalcogenide phase change materials (PCMs) are considered highly promising owing to the nonvolatile nature of their phase change. Chalcogenide PCM nanophotonics can be broadly classified into integrated photonics (with guided wave light propagation) and Meta-optics (with free space light propagation). Despite some early comprehensive reviews, the pace of development in the last few years has shown the need for a topical review. Our comprehensive review covers recent progress on nanophotonic architectures, tuning mechanisms, and functionalities in tunable PCM Chalcogenides. In terms of integrated photonics, we identify novel PCM nanoantenna geometries, novel material utilization, the use of nanostructured waveguides, and sophisticated excitation pulsing schemes. On the meta-optics front, the breadth of functionalities has expanded, enabled by exploring design aspects for better performance. The review identifies immediate, and intermediate-term challenges and opportunities in (1) the development of novel chalcogenide PCM, (2) advance in tuning mechanism, and (3) formal inverse design methods, including machine learning augmented inverse design, and provides perspectives on these aspects. The topical review will interest researchers in further advancing this rapidly growing subfield of nanophotonics.

Optical transparent metamaterial with multi-band compatible camouflage based on inverse design

Infrared (IR) thermal camouflage and management are deeply desirable in the field of military and astronomy. While IR compatible with laser camouflage technology is extensively studied to counter modern detection systems, most existing strategies for visible light camouflage focus on color matching, which is not suitable for scenarios requiring transparency. In this work, we propose an optically transparent metamaterial with multi-band compatible camouflage capability based on the inverse design. The metamaterial consists of Ag grating, Si3N4 dielectric spacer layer, Ag reflection layer, and Si3N4 anti-reflective layer. An ideal multi-band compatible spectrum is involved in the inverse design algorithm. Calculated results demonstrate high transmittance (T0.38-0.78µm = 0.70) in the visible region, low reflectance (R1.55µm = 0.01) in laser working wavelength, high reflectance (R3-5µm = 0.86 and R8-14µm = 0.92) in the dual-band atmospheric window, and high emissivity (ɛ5-8µm = 0.61) for the non-atmospheric window. The radiative heat flux in the detected band is 31W/m2 and 201W/m2 respectively. Furthermore, the incident and polarized insensitivity of the proposed metamaterial supports applicability for practical situations. This work, emphasizes an effective strategy for conducting optically transparent design with compatible IR-laser camouflage as well as radiative cooling properties by an automated design approach.

Toward Accurate Thermal Modeling of Phase Change Material-Based Photonic Devices

Reconfigurable or programmable photonic devices are rapidly growing and have become an integral part of many optical systems. The ability to selectively modulate electromagnetic waves through electrical stimuli is crucial in the advancement of a variety of applications from data communication and computing devices to environmental science and space explorations. Chalcogenide-based phase-change materials (PCMs) are one of the most promising material candidates for reconfigurable photonics due to their large optical contrast between their different solid-state structural phases. Although significant efforts have been devoted to accurate simulation of PCM-based devices, in this paper, three important aspects which have often evaded prior models yet having significant impacts on the thermal and phase transition behavior of these devices are highlighted: the enthalpy of fusion, the heat capacity change upon glass transition, as well as the thermal conductivity of liquid-phase PCMs. The important topic of switching energy scaling in PCM devices, which also helps explain why the three above-mentioned effects have long been overlooked in electronic PCM memories but only become important in photonics, is further investigated. These findings offer insight to facilitate accurate modeling of PCM-based photonic devices and can inform the development of more efficient reconfigurable optics.

As-Grown Miniaturized True Zero-Order Waveplates Based on Low-Dimensional Ferrocene Crystals

As basic optical elements, waveplates with anisotropic electromagnetic responses are imperative for manipulating light polarization. Conventional waveplates are manufactured from bulk crystals (e.g., quartz and calcite) through a series of precision cutting and grinding steps, which typically result in large size, low yield, and high cost. In this study, a bottom-up method is used to grow ferrocene crystals with large anisotropy to demonstrate self-assembled ultrathin true zero-order waveplates without additional machining processing, which is particularly suited for nanophotonic integration. The van der Waals ferrocene crystals exhibit high birefringence (Δn (experiment) = 0.149  ±  0.002 at 636 nm), low dichroism Δκ (experiment) = -0.0007 at 636 nm), and a potentially broad operating range (550 nm to 20 µm) as suggested by Density Functional Theory (DFT) calculations. In addition, the grown waveplate's highest and the lowest principal axes (n1 and n3 , respectively) are in the a-c plane, where the fast axis is along one natural edge of the ferrocene crystal, rendering them readily usable. The as-grown, wavelength-scale-thick waveplate allows the development of further miniaturized systems via tandem integration.

Mid-infrared plasmonic filter arrays using nanoholes in gold and silver films

Plasmonic filters based on subwavelength nanohole arrays are an attractive solution for creating arrays of filters with varying passbands in a single lithography step. In this work, we have developed a fabrication method which allows fabrication of nanohole arrays in silver by use of a thin layer of aluminum oxide, which serves the dual purpose of both capping layer and hardmask for metal patterning. We demonstrate arrays of gold and silver mid-infrared plasmonic filters, fabricated on silicon, intended for use in optical filter blocks or for future integration with infrared imagers. The filter arrays are designed for the wavelength range 2-7 µm, and exhibit peak filter transmission efficiencies around 70%.

Wavelength-tunable infrared chiral metasurfaces with phase-change materials

Optical phase-change materials exhibit tunable permittivity and switching properties during phase transition, which offers the possibility of dynamic control of optical devices. Here, a wavelength-tunable infrared chiral metasurface integrated with phase-change material GST-225 is demonstrated with the designed unit cell of parallelogram-shaped resonator. By varying the baking time at a temperature above the phase transition temperature of GST-225, the resonance wavelength of the chiral metasurface is tuned in the wavelength range of 2.33 µm to 2.58 µm, while the circular dichroism in absorption is maintained around 0.44. The chiroptical response of the designed metasurface is revealed by analyzing the electromagnetic field and displacement current distributions under left- and right-handed circularly polarized (LCP and RCP) light illumination. Moreover, the photothermal effect is simulated to investigate the large temperature difference in the chiral metasurface under LCP and RCP illumination, which allows for the possibility of circular polarization-controlled phase transition. The presented chiral metasurfaces with phase-change materials offer the potential to facilitate promising applications in the infrared regime, such as chiral thermal switching, infrared imaging, and tunable chiral photonics.

Recent Advances in Tunable Metasurfaces and Their Application in Optics

Metasurfaces can be opportunely and specifically designed to manipulate electromagnetic wavefronts. In recent years, a large variety of metasurface-based optical devices such as planar lenses, beam deflectors, polarization converters, and so on have been designed and fabricated. Of particular interest are tunable metasurfaces, which allow the modulation of the optical response of a metasurface; for instance, the variation in the focal length of a converging metalens. Response tunability can be achieved through external sources that modify the permittivity of the materials constituting the nanoatoms, the substrate, or both. The modulation sources can be classified into electromagnetic fields, thermal sources, mechanical stressors, and electrical bias. Beside this, we will consider optical modulation and multiple approach tuning strategies. A great variety of tunable materials have been used in metasurface engineering, such as transparent conductive oxides, ferroelectrics, phase change materials, liquid crystals, and semiconductors. The possibility of tuning the optical properties of these metamaterials is very important for several applications spanning from basic optics to applied optics for communications, depth sensing, holographic displays, and biochemical sensors. In this review, we summarize the recent progress on electro-optical magnetic, mechanical, and thermal tuning of metasurfaces actually fabricated and experimentally tested in recent years. At the end of the review, a short section on possible future perspectives and applications is included.

Toward Water-Immersion Programmable Meta-Display

Heading toward next-generation intelligent display, dynamic control capability for meta-devices is critical for real world applications. Beyond the conventional electrical/optical/mechanical/thermal tuning methods, liquid immersion recently has emerged as a facile tuning mechanism which is easily accessible (especially water) and practically implementable for large tuning area. However, due to the longstanding and critical drawback of lacking independent-encoding capability, the state-of-art immersion approach remains incapable of pixel-level programmable switching. Here a water-immersion tuning scheme with pixel-scale programmability for dynamic meta-displays is proposed. Tunable meta-pixels can be engineered to construct spectral selective patterns at prior-/post- immersion states, such that a metasurface enables pixel-level transforming animations for dynamic multifield meta-displays, including near-field dual-nanoprints and far-field dual-holographic displays. The proposed water-immersion programmable approach for meta-display, benefitting from its large tuning area, facile operation and strong repeatability, may find a revolutionary path toward next-generation intelligent display with practical applications in dynamic display/encryption, information anticounterfeit/storage, and optical sensors.

Reconfigurable Radiation Angle Continuous Deflection of All-Dielectric Phase-Change V-Shaped Antenna

All-dielectric optical antenna with multiple Mie modes and lower inherent ohmic loss can achieve high efficiency of light manipulation. However, the silicon-based optical antenna is not reconfigurable for specific scenarios. The refractive index of optical phase-change materials can be reconfigured under stimulus, and this singular behavior makes it a good candidate for making reconfigurable passive optical devices. Here, the optical radiation characteristics of the V-shaped phase-change antenna are investigated theoretically. The results show that with increasing crystallinity, the maximum radiation direction of the V-shaped phase-change antenna can be continuously deflected by 90°. The exact multipole decomposition analysis reveals that the modulus and interference phase difference of the main multipole moments change with the crystallinity, resulting in a continuous deflection of the maximum radiation direction. Thus, the power ratio in the two vertical radiation directions can be monotonically reversed from -12 to 7 dB between 20% and 80% crystallinity. The V-shaped phase-change antenna exhibits the potential to act as the basic structural unit to construct a reconfigurable passive spatial angular power splitter or wavelength multiplexer. The mechanism analysis of radiation directivity involving the modulus and interference phase difference of the multipole moments will provide a reference for the design and optimization of the phase-change antenna.

Shortwave infrared single-pixel spectral imaging based on a GSST phase-change metasurface

Shortwave infrared (SWIR) spectral imaging obtains spectral fingerprints corresponding to overtones of molecular vibrations invisible to conventional silicon-based imagers. However, SWIR imaging is challenged by the excessive cost of detectors. Single-pixel imaging based on compressive sensing can alleviate the problem but meanwhile presents new difficulties in spectral modulations, which are prerequisite in compressive sampling. In this work, we theoretically propose a SWIR single-pixel spectral imaging system with spectral modulations based on a Ge₂Sb₂Se₄Te₁ (GSST) phase-change metasurface. The transmittance spectra of the phase-change metasurface are tuned through wavelength shifts of multipole resonances by varying crystallinities of GSST, validated by the multipole decompositions and electromagnetic field distributions. The spectral modulations constituted by the transmittance spectra corresponding to the 11 phases of GSST are sufficient for the compressive sampling on the spectral domain of SWIR hyperspectral images, indicated by the reconstruction in false color and point spectra. Moreover, the feasibility of optimization on phase-change metasurface via coherence minimization is demonstrated through the designing of the GSST pillar height. The concept of spectral modulation with phase-change metasurface overcomes the static limitation in conventional modulators, whose integratable and reconfigurable features may pave the way for high-efficient, low-cost, and miniaturized computational imaging based on nanophotonics.

Mid-infrared reconfigurable all-dielectric metasurface based on Ge2Sb2Se4Te1 phase-change material

In this paper, a reconfigurable all-dielectric metasurface based on Ge₂Sb₂Se₄Te₁ (GSST) phase-change material is proposed. By changing GSST from amorphous state to crystalline state, the metasurface can achieve high circular dichroism (CD) and wideband polarization conversion for circularly polarized waves in the mid-infrared (MIR) band. The maximum CD value reaches 0.95 at 74 THz, and circular polarization conversion efficiency is more than 90% in the wideband range of 41 THz - 48 THz. In addition, based on Pancharatnam-Berry phase, three kinds of wavefront manipulation of light have been realized: abnormal refraction, orbital angular momentum vortex beam and orbital angular momentum vortex beam splitting. This work has potential applications in the future MIR optical integrated system.

A review of tunable photonics: Optically active materials and applications from visible to terahertz

The next frontier of photonics is evolving into reconfigurable platforms with tunable functions to realize the ubiquitous application. The dynamic control of optical properties of photonics is highly desirable for a plethora of applications, including optical communication, dynamic display, self-adaptive photonics, and multi-spectral camouflage. Recently, to meet the dynamic response over broad optical bands, optically active materials have been integrated with the diverse photonic platforms, typically in the dimension of micro/nanometer scales. Here, we review recent advances in tunable photonics with controlling optical properties from visible to terahertz (THz) spectral range. We propose guidelines for designing tunable photonics in conjunction with optically active materials, inherent in wavelength characteristics. In particular, we devote our review to their potential uses for five different applications: structural coloration, metasurface for flat optics, photonic memory, thermal radiation, and terahertz plasmonics. Finally, we conclude with an outlook on the challenges and prospects of tunable photonics.

Multifunctional metasurfaces for switchable polarization selectivity and absorption

A multifunctional metasurface capable of dynamic control for polarization selectivity and absorption is proposed by controlling the phase of Ge₂Sb₂Te₅ (GST) in the near-infrared region. At amorphous state of GST (a-GST), the proposed GST strip array realized polarization selectivity in transmission-reflection integrated modes. The high-efficiency asymmetric transmission (AT = 0.92) and asymmetric reflection (AR = -0.82) are both obtained by selectively exciting Mie multipole resonances. With the transition from a-GST to crystalline (c-GST) state, the giant polarization selectivity almost disappeared, and the absorptions climb from < 0.1 to > 0.9. The maximum modulation depth reaches 94%. The mechanism of the dynamic switching between polarization selectivity and absorption is quantitively analyzed via multipole expansion. The GST based metasurfaces simultaneously possess excellent switchable capability for AT, AR, and absorption without refabricating structures, which is promising to the applications for next generation optical devices.

Tunable mid-infrared selective emitter based on inverse design metasurface for infrared stealth with thermal management

Infrared (IR) stealth with thermal management is highly desirable in military applications and astronomy. However, developing selective IR emitters with properties suitable for IR stealth and thermal management is challenging. In this study, we present the theoretical framework for a selective emitter based on an inverse-designed metasurface for IR stealth with thermal management. The emitter comprises an inverse-designed gold grating, a Ge₂Sb₂Te₅ (GST) dielectric layer, and a gold reflective layer. The hat-like function, which describes an ideal thermal selective emitter, is involved in the inverse design algorithm. The emitter exhibits high performance in IR stealth with thermal management, with the low emissivity (ɛ_{3-5 µm} =0.17; ɛ_{8-14 µm} =0.16) for dual-band atmospheric transmission windows and high emissivity (ɛ_{5-8 µm} =0.85) for non-atmospheric windows. Moreover, the proposed selective emitter can realize tunable control of thermal radiation in the wavelength range of 3-14 µm by changing the crystallization fraction of GST. In addition, the polarization-insensitive structure supports strong selective emission at large angles (60°). Thus, the selective emitter has potential for IR stealth, thermal imaging, and mid-infrared multifunctional equipment.

Switchable distributed Bragg reflector using GST phase change material

We demonstrate the design, fabrication, and measurement of a switchable distributed Bragg reflector (DBR) that can be thermally switched from a close-to-zero reflective OFF state to a more than 70% reflection in its ON state. This is accomplished using a multilayer thin film stack using germanium (Ge) and the phase change material (PCM) Ge₂Sb₂Te₅ (GST). The refractive indexes of Ge and GST in the amorphous state are closely matched, resulting in a nearly zero interface reflection. With appropriate antireflection coatings at the cavity ends, the overall reflection can be designed to be close to zero. When the GST is switched to the crystalline state, the refractive index contrast between the Ge and GST layers will increase dramatically contributing to the DBR reflection. Using this unique feature, we were able to design and experimentally demonstrate more than 70% reflection in the ON state and close to zero reflection in the OFF state at a wavelength of 2 µm.

Free-Space Optical Merging via Meta-Grating Inverse-Design

Despite various advances in achieving arbitrary optics steering, one of the longstanding challenges is to achieve optical merging for combining multidirectional beams through single-time reflection/transmission in free space. Typically, dual-directional beam merging is conducted by combining half-transmission and half-reflection using beam splitters; however, it leads to a bulky system with stray light and low merging efficiency. The difficulty of free-space beam merging lies in imparting respective distinct wavevectors to different directional beams. Herein, we originally proposed and successfully demonstrated the free-space optical merging (FOM) functionality based on the inverse-designed meta-grating architecture in the visible regime. By utilizing the inverse problem solver, two proposed meta-grating schemes experimentally enable merging of dual-directional beams into the same outgoing angle for the first time merely through single-time reflection. We envision that the creation of free-space merging performance can be widely applicable to the future optical system and facilitate the miniature optical devices and integration.

Extremely Narrow and Actively Tunable Mie Surface Lattice Resonances in GeSbTe Metasurfaces: Study

Mie surface lattice resonances (SLRs) supported by periodic all-dielectric nanoparticles emerge from the radiative coupling of localized Mie resonances in individual nanoparticles through Rayleigh anomaly diffraction. To date, it remains challenging to achieve narrow bandwidth and active tuning simultaneously. In this work, we report extremely narrow and actively tunable electric dipole SLRs (ED-SLRs) in Ge₂Se₂Te₅ (GST) metasurfaces. Simulation results show that, under oblique incidence with TE polarization, ED-SLRs with extremely narrow linewidth down to 12 nm and high quality factor up to 409 can be excited in the mid-infrared regime. By varying the incidence angle, the ED-SLR can be tuned over an extremely large spectral region covering almost the entire mid-infrared regime. We further numerically show that, by changing the GST crystalline fraction, the ED-SLR can be actively tuned, leading to nonvolatile, reconfigurable, and narrowband filtering, all-optical multilevel modulation, or all-optical switching with high performance. We expect that this work will advance the engineering of Mie SLRs and will find intriguing applications in optical telecommunication, networks, and microsystems.

Switchable Induced-Transmission Filters Enabled by Vanadium Dioxide

An induced-transmission filter (ITF) uses an ultrathin metallic layer positioned at an electric-field node within a dielectric thin-film bandpass filter to select one transmission band while suppressing other bands that would have been present without the metal layer. We introduce a switchable mid-infrared ITF where the metal can be "switched on and off", enabling the modulation of the filter response from a single band to multiband. The switching is enabled by the reversible insulator-to-metal phase transition of a subwavelength film of vanadium dioxide (VO₂). Our work generalizes the ITF─a niche type of bandpass filter─into a new class of tunable devices. Furthermore, our fabrication process─which begins with thin-film VO₂ on a suspended membrane─enables the integration of VO₂ into any thin-film assembly that is compatible with physical vapor deposition processes and is thus a new platform for realizing tunable thin-film filters.

Nanophotonic Color Routing

Recent advances in low-dimensional materials and nanofabrication technologies have stimulated many breakthroughs in the field of nanophotonics such as metamaterials and plasmonics that provide efficient ways of light manipulation at a subwavelength scale. The representative structure-induced spectral engineering techniques have demonstrated superior design of freedom compared with natural materials such as pigment/dye. In particular, the emerging spectral routing scheme enables extraordinary light manipulation in both frequency-domain and spatial-domain with high-efficiency utilization of the full spectrum, which is critically important for various applications and may open up entirely new operating paradigms. In this review, a comparative introduction on the operating mechanisms of spectral routing and spectral filtering schemes is given and recent progress on various color nanorouters based on metasurfaces, plasmonics, dielectric antennas is reviewed with a focus on the potential application in high-resolution imaging. With a thorough analysis and discussion on the advanced properties and drawbacks of various techniques, this report is expected to provide an overview and vision for the future development and application of nanophotonic color (spectral) routing techniques.

Tunable optical filter using phase change materials for smart IR night vision applications

In this paper we present a tunable filter using Ge₂Sb₂Se₄Te₁ (GSST) phase change material. The design principle of the filter is based on a metal-insulator-metal (MIM) cavity operating in the reflection mode. This is intended for night vision applications that utilize 850nm as the illumination source. The filter allows us to selectively reject the 850nm band in one state. This is illustrated through several daytime and nighttime imaging applications.

Ge2Sb2Te5-based reconfigurable metasurface for polarization-insensitive, full-azimuth, and switchable cloaking

Electromagnetic (EM) metasurface mantles afford an alternative avenue, allowing for the possibility of rendering arbitrary-shape objects unobservable. But the available mechanisms either depend on the states of polarization or the azimuth of wave incidence, or cannot dynamically manipulate cloaking responses without altering the structures. Herein, a three-dimensional closed-ring-based metasurface carpet cloak involving Ge₂Sb₂Te₅ that circumvents current drawbacks of metasurface structures is proposed. By judiciously designing meta-atoms on the external surface of a spherical object, the scattered wavefront, including the distributions of EM fields and polarizations, can be reconstructed, resembling what is deflected from a flat plane. Enabled by the perfectly symmetric distribution of meta-atoms, the carpet cloak is demonstrated to work well under arbitrary states of polarization and arbitrary azimuthal angles of incident light. Meanwhile, by converting Ge₂Sb₂Te₅ from the amorphous to crystalline state, the designed scheme is empowered with the ability to switch "ON" and "OFF" of stealth states. Furthermore, the unique design achieves invisibility over ±20^∘ angular span in the mid-infrared range from 8800 to 9450 nm. The validated recipe empowers robust steps forward to achieve full-polarization, full-azimuth operation, and switchable cloaking in the real-world, showing great potential applications in stealth, camouflage, and illusion fields.

Smart Materials for Dynamic Thermal Radiation Regulation

Thermal radiation in the mid-infrared region profoundly affects human lives in various fields, including thermal management, imaging, sensing, camouflage, and thermography. Due to their fixed emissivities, radiance features of conventional materials are usually proportional to the quadruplicate of surface temperature, which set the limit, that one type of material can only present a single thermal function. Therefore, it is necessary and urgent to design materials for dynamic thermal radiation regulations to fulfill the demands of the age of intelligent machines. Recently, the ability of some smart materials to dynamically regulate thermal radiation has been evaluated. These materials are found to be competent enough for various commands, thereby, providing better alternatives and tremendously promoting the commercial potentials. In this review, the dynamic regulatory mechanisms and recent progress in the evaluation of these smart materials are summarized, including thermochromic materials, electrochromic materials, mechanically and humidity responsive materials, with the potential applications, insufficient problems, and possible strategies highlighted.

Non-Layered Gold-Silicon and All-Silicon Frequency-Selective Metasurfaces for Potential Mid-Infrared Sensing Applications

We report simulations on the spectral behavior of non-layered gold-silicon and all-silicon frequency-selective metasurfaces in an asymmetric element configuration in the mid-infrared spectral window of 5-5.8 μm. The non-layered layout is experimentally feasible due to recent technological advances such as nano-imprint and nano-stencil lithography, and the spectral window was chosen due to the multitude of applications in sensing and imaging. The architecture exhibits significant resonance in the window of interest as well as extended tunability by means of variation of cell element sizes and relative coordinates. The results indicate that the proposed metasurface architecture is a viable candidate for mid-infrared absorbers, sensors and imaging systems.

Active meta-device for angular dispersion elimination of dual-polarized transmission windows

Metasurface-based strategy of tailoring electromagnetic waves has aroused huge attention in both academic and engineering communities owing to great potential in a large portfolio of applications. Commonly, however, the artificially designed metasurfaces are sensitive to the oblique incident waves which results in the angular dispersion and inevitably deteriorates the performances. Here, we propose a paradigm of an active meta-device to effectively eliminate the angular dispersion in two orthogonal polarization states of transmission waves. By loading varactor diodes into a transmissive meta-atom, the transmission responses for traverse electric (TE) and traverse magnetic (TM) waves are actively tunable by a voltage-driven manner. Accordingly, the blue shifts of transmission windows can be ingeniously compensated via tailoring the corresponding dispersion characteristics of varactor diodes. A triple-layer meta-atom loaded with varactor diodes is designed as a dual-polarization proof-of-principle, in which the varactor diodes can be applied to independently control two polarization states. The numerical simulations and experimental verification are in good agreement, indicating the proposed paradigm possesses the potential in versatile applications, including radome, wireless communications, and other dispersionless systems.

Nonvolatile Optically Reconfigurable Radiative Metasurface with Visible Tunability for Anticounterfeiting

Control of thermal emission underpins fundamental science, as it is related to both heat and infrared electromagnetic wave transport. However, realizing nonvolatile reconfigurable thermal emission is challenging due to the inherent complexity or limitation in conventional radiative materials or structures. Here, we experimentally demonstrate a nonvolatile optically reconfigurable mid-infrared coding radiative metasurface. By applying laser pulses, infrared emissive patterns are directly encoded into an ultrathin (∼25 nm) Ge₂Sb₂Te₅ layer integrated into a planar optical cavity with the optically crystallized Ge₂Sb₂Te₅ spots, and the peak spectral emissivity is repeatedly switched between low (∼0.1) and high (∼0.7) values. In addition, the visible scattering patterns are independently modulated with submicron-sized bumps generated by high-power laser pulses. An anticounterfeiting label is demonstrated with spatially different infrared emission and visible light scattering information encoded. This approach constitutes a new route toward thermal emission control and has broad applications in encryption, camouflage, and so on.

Phase Change Metasurfaces by Continuous or Quasi-Continuous Atoms for Active Optoelectronic Integration

In recent decades, metasurfaces have emerged as an exotic and appealing group of nanophotonic devices for versatile wave regulation with deep subwavelength thickness facilitating compact integration. However, the ability to dynamically control the wave-matter interaction with external stimulus is highly desirable especially in such scenarios as integrated photonics and optoelectronics, since their performance in amplitude and phase control settle down once manufactured. Currently, available routes to construct active photonic devices include micro-electromechanical system (MEMS), semiconductors, liquid crystal, and phase change materials (PCMs)-integrated hybrid devices, etc. For the sake of compact integration and good compatibility with the mainstream complementary metal oxide semiconductor (CMOS) process for nanofabrication and device integration, the PCMs-based scheme stands out as a viable and promising candidate. Therefore, this review focuses on recent progresses on phase change metasurfaces with dynamic wave control (amplitude and phase or wavefront), and especially outlines those with continuous or quasi-continuous atoms in favor of optoelectronic integration.

Dynamically tunable transmissive color filters using ultra-thin phase change materials

Structural color filters (i.e. plasmonics and nano-cavities) provide vivid and robust color filtering in applications such as CMOS image sensors but lack simplicity in fabrication and dynamic tuning. Here we report a dynamically tunable, transmissive color filter by incorporating an ultra-thin phase change layer inside a thin-film optical resonator. The transmitted color spectrum can be designed over the entire visible range and shifted by around 50 nm after phase transition. Angle dependence shows little color variation within a ±30° viewing angle. Crucially, only film deposition is required to fabricate our phase change color filter, showing great potential for large-scale and inexpensive production. The dynamically tunable color filter, described in this paper, could be a promising component in display, CMOS sensor, and solar cell technology.

Nonvolatile, Reconfigurable and Narrowband Mid-Infrared Filter Based on Surface Lattice Resonance in Phase-Change Ge2Sb2Te5

We propose a nonvolatile, reconfigurable, and narrowband mid-infrared bandpass filter based on surface lattice resonance in phase-change material Ge2Sb2Te5. The proposed filter is composed of a two-dimensional gold nanorod array embedded in a thick Ge2Sb2Te5 film. Results show that when Ge2Sb2Te5 transits from the amorphous state to the crystalline state, the narrowband reflection spectrum of the proposed filter is tuned from 3.197 μm to 4.795 μm, covering the majority of the mid-infrared regime, the peak reflectance decreases from 72.6% to 25.8%, and the corresponding quality factor decreases from 19.6 to 10.3. We show that the spectral tuning range can be adjusted by varying the incidence angle or the lattice period. By properly designing the gold nanorod sizes, we also show that the quality factor can be greatly increased to 70 at the cost of relatively smaller peak reflection efficiencies, and that the peak reflection efficiency can be further increased to 80% at the cost of relatively smaller quality factors. We expect that this work will advance the engineering of Ge2Sb2Te5-based nonvalatile tunable surface lattice resonances and will promote their applications especially in reconfigurable narrowband filters.

Tunable metasurfaces using phase change materials and transparent graphene heaters

Tunable metasurfaces enable us to dynamically control light at subwavelength scales. Here, using phase change materials and transparent graphene heaters, a new structure is proposed to develop tunable metasurfaces which support first-order Mie-type resonance in the near-IR regime. In the proposed structure, by adjusting the bias voltages applied to transparent graphene heaters, the crystallization levels of the phase change materials are controlled, which in turn modifies the response of the metasurface. The proposed metasurface is able to modulate the phase of the reflected wave in the range of 0° to -270° at the telecommunication wavelength of λ = 1.55 µm. A comprehensive Joule heating analysis is performed to investigate the thermal characterizations of the proposed structure. The results of this analysis show that there is a suitable thermal isolation between adjacent unit cells, making individual control on unit cells possible. The potential ability of the proposed metasurface as a beam steering device is also demonstrated. By using the proposed unit cells, a beam-steering device is designed and numerically studied. This study shows that the device can reflect a light normally incident on it in the range of ±65° with reasonably low sidelobe levels. The proposed structure can be used in developing low-cost integrated LiDARs.