Dynamic Hybrid 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.

Configurable swellability of hydrogel microstructure for structural-color-based imaging concealment/encryption

Optical information concealment/encryption technologies are of great importance to structural color applications. Although a series of responsive materials have been developed for dynamic structural color, the shortcomings of the high-quality synthesis process, the complex controlling method, and the low-resolution capability limit their practical use. Herein, we proposed a novel strategy of humidity-driven structural-color-based imaging concealment/encryption by utilizing metal-hydrogel-metal (MHM) nanocavities with configurable swellablity response to humidity change. With varied exposure doses, multi-stage MHM nanocavities with swellable hydrogel interlayers are achieved, generating dynamic structural color covering the visible spectrum. We revealed that the swelling ratio of hydrogel microstructures can be gradually adjusted between 1.05 and 2.08 by varying the exposure dose. We demonstrated that a hydrogel-based structural color image can be concealed with humidity changes by configurating swellable and non-swellable hydrogel pixels together. Furthermore, we developed the double exposure method in which the first exposure can generate pixel arrays for the deceptive image and the second exposure can locally suppress the swellablity of certain pixels. This method can highlight hidden images in a moist state, demonstrating a powerful strategy for high-density optical information encryption.

Switchable unidirectional emissions from hydrogel gratings with integrated carbon quantum dots

Directional emission of photoluminescence despite its incoherence is an attractive technique for light-emitting fields and nanophotonics. Optical metasurfaces provide a promising route for wavefront engineering at the subwavelength scale, enabling the feasibility of unidirectional emission. However, current directional emission strategies are mostly based on static metasurfaces, and it remains a challenge to achieve unidirectional emissions tuning with high performance. Here, we demonstrate quantum dots-hydrogel integrated gratings for actively switchable unidirectional emission with simultaneously a narrow divergence angle less than 1.5° and a large diffraction angle greater than 45°. We further demonstrate that the grating efficiency alteration leads to a more than 7-fold tuning of emission intensity at diffraction order due to the variation of hydrogel morphology subject to change in ambient humidity. Our proposed switchable emission strategy can promote technologies of active light-emitting devices for radiation control and optical imaging.

A Quick Method for Predicting Reflectance Spectra of Nanophotonic Devices via Artificial Neural Network

Nanophotonics use the interaction between light and subwavelength structures to design nanophotonic devices and to show unique optical, electromagnetic, and acoustic properties that natural materials do not have. However, this usually requires considerable expertise and a lot of time-consuming electromagnetic simulations. With the continuous development of artificial intelligence, people are turning to deep learning for designing nanophotonic devices. Deep learning models can continuously fit the correlation function between the input parameters and output, using models with weights and biases that can obtain results in milliseconds to seconds. In this paper, we use finite-difference time-domain for simulations, and we obtain the reflectance spectra from 2430 different structures. Based on these reflectance spectra data, we use neural networks for training, which can quickly predict unseen structural reflectance spectra. The effectiveness of this method is verified by comparing the predicted results to the simulation results. Almost all results maintain the main trend, the MSE of 94% predictions are below 10-3, all are below 10-2, and the MAE of 97% predictions are below 2 × 10-2. This approach can speed up device design and optimization, and provides reference for scientific researchers.

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.

Programmable nanophotonic planar resonator filter-absorber based on phase-change InSbTe

Reconfigurable plasmonic-photonic electromagnetic devices have been incessantly investigated for their great ability to optically modulate through external stimuli to meet today's emerging needs, with chalcogenide phase-change materials being promising candidates due to their remarkably unique electrical and optics, enabling new perspectives in recent photonic applications. In this work, we propose a reconfigurable resonator using planar layers of stacked ultrathin films based on Metal-dielectric-PCM, which we designed and analyzed numerically by the Finite Element Method (FEM). The structure is based on thin films of Gold (Au), aluminum oxide (Al2O3), and PCM (In3SbTe2) used as substrate. The modulation between the PCM phases (amorphous and crystalline) allows the alternation from the filter to the absorber structure in the infrared (IR) spectrum (1000-2500 nm), with an efficiency greater than 70% in both cases. The influence of the thickness of the material is also analyzed to verify tolerances for manufacturing errors and dynamically control the efficiency of transmittance and absorptance peaks. The physical mechanisms of field coupling and transmitted/absorbed power density are investigated. We also analyzed the effects on polarization angles for Transversal Electric (TE) and Transversal Magnetic (TM) polarized waves for both cases.

Phase-change Fano resonator for active modulation of thermal emission

Optical modulation of heat emission using spectrally selective infrared (IR) metasurface nanoantenna designs has found potential applications in various fields, including radiative cooling and thermal camouflage. While radiative cooling requires emitters to emit within atmospheric transmissive windows (mainly located at 8-14 μm), thermal camouflage structures have to operate within the non-transmissive window (5-8 μm) to hide an object from thermal imaging systems and cameras. Therefore, a passive nanoantenna structure cannot satisfy both conditions simultaneously. In this paper, we propose an adaptive nanoantenna emitter made of samarium nickelate (SmNiO₃) phase change material to cover both functionalities with a single Fano resonator-based design. As the temperature rises, the thermal signature of the nanoantenna at the transmissive window is suppressed; therefore, a better camouflage performance is achieved. The dynamic tunability of switching from radiative cooling to thermal camouflage of the proposed Fano resonator-based design is quantitatively demonstrated using emissive power calculations under different conditions.

Shielding Performance of Electromagnetic Shielding Fabric Implanted with "Split-Ring Resonator"

The electromagnetic shielding (EMS) fabric is an important electromagnetic protection product, which is widely applied in various fields. The improvement of its shielding effectiveness (SE) has always been the focus of research. This article proposes to implant a metamaterial structure of a "split-ring resonator (SRR)" in the EMS fabrics, so that the fabric not only maintains the porous and lightweight characteristics, but also obtains the SE improvement. With the help of the invisible embroidery technology, stainless-steel filaments were used to implant hexagonal SRRs inside the fabric. The effectiveness and influencing factors of the SRR implantation were described by testing the SE of the fabric and analyzing the experimental results. It was concluded that the SRR implantation inside the fabric can effectively improve the SE of the fabric. For the stainless-steel EMS fabric, the increase amplitude of the SE reached between 6 dB and 15 dB in most frequency bands. The overall SE of the fabric showed a decrease trend with the reduction of the outer diameter of the SRR. The decrease trend was sometimes fast and sometimes slow. The decreasing amplitudes were different in various frequency ranges. The number of embroidery threads had a certain effect on the SE of the fabric. When other parameters remained unchanged, the increase of the diameter of the embroidery thread resulted in the increase of the SE of the fabric. However, the overall improvement was not significant. Finally, this article also points out that other influencing factors of the SRR need to be explored, and the failure phenomenon may occur under certain situations. The proposed method has the advantages of the simple process, convenient design, no pore formation, SE improvement retaining the original porous characteristics of the fabric. This paper provides a new idea for the design, production, and development of new EMS fabrics.

Hydrogel-Based, Dynamically Tunable Plasmonic Metasurfaces with Nanoscale Resolution

Flat metasurfaces with subwavelength meta-atoms can be designed to manipulate the electromagnetic parameters of incident light and enable unusual light-matter interactions. Although hydrogel-based metasurfaces have the potential to control optical properties dynamically in response to environmental conditions, the pattern resolution of these surfaces has been limited to microscale features or larger, limiting capabilities at the nanoscale, and precluding effective use in metamaterials. This paper reports a general approach to developing tunable plasmonic metasurfaces with hydrogel meta-atoms at the subwavelength scale. Periodic arrays of hydrogel nanodots with continuously tunable diameters are fabricated on silver substrates, resulting in humidity-responsive surface plasmon polaritons (SPPs) at the nanostructure-metal interfaces. The peaks of the SPPs are controlled reversibly by absorbing or releasing water within the hydrogel matrix, the matrix-generated plasmonic color rendering in the visible spectrum. This work demonstrates that metasurfaces designed with these spatially patterned nanodots of varying sizes benefit applications in anti-counterfeiting and generate multicolored displays with single-nanodot resolution. Furthermore, this work shows system versatility exhibited by broadband beam-steering on a phase modulator consisting of hydrogel supercell units in which the size variations of constituent hydrogel nanostructures engineer the wavefront of reflected light from the metasurface.

Strong optical coupling in metallo-dielectric hybrid metasurfaces

Metasurfaces consisting of hybrid metal/dielectric nanostructures carry advantages of both material platforms. The hybrid structures can not only confine electromagnetic fields in subwavelength regions, but they may also lower the absorption losses. Such optical characteristics are difficult to realize in metamaterials with only metal or dielectric structures. Hybrid designs also expand the scope of material choices and the types of optical modes that can be excited in a metasurface, thereby allowing novel light matter interactions. Here, we present a metallo-dielectric hybrid metasurface design consisting of a high-index dielectric (silicon) nanodisk array on top of a metal layer (aluminum) separated by a buffer oxide (silica) layer. The dimensions of Si nanodisks are tuned to support anapole states and the period of the nanodisk array is designed to excite surface plasmon polariton (SPP) at the metal-buffer oxide interface. The physical dimensions of the Si nanodisk and the array periods are optimized to excite the anapole and the SPP at normal incidence of light in the visible-NIR (400-900 nm) wavelength range. Finite difference time domain (FDTD) simulations show that, when the nanodisk grating is placed at a specific height (∼200 nm) from the metal surface, the two modes strongly couple at zero detuning of the resonances. The strong coupling is evident from the avoided crossing of the modes observed in the reflectance spectra and in the spectral profile of light absorption inside the Si nanodisk. A vacuum Rabi splitting of up to ∼ 129 meV is achievable by optimizing the diameters of Si nanodisk and the nanodisk array grating period. The proposed metasurface design is promising to realize open cavity strongly coupled optical systems operating at room temperatures.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Meta-Optics-Empowered Switchable Integrated Mode Converter Based on the Adjoint Method

Monolithic integrated mode converters with high integration are essential to photonic integrated circuits (PICs), and they are widely used in next-generation optical communications and complex quantum systems. It is expected that PICs will become more miniaturized, multifunctional, and intelligent with the development of micro/nano-technology. The increase in design space makes it difficult to realize high-performance device design based on traditional parameter sweeping or heuristic design, especially in the optimal design of reconfigurable PIC devices. Combining the mode coupling theory and adjoint calculation method, we proposed a design method for a switchable mode converter. The device could realize the transmission of TE0 mode and the conversion from TE0 to TE1 mode with a footprint of 0.9 × 7.5 μm² based on the phase change materials (PCMs). We also found that the mode purity could reach 78.2% in both states at the working wavelength of 1.55 μm. The designed method will provide a new impetus for programmable photonic integrated devices and find broad application prospects in communication, optical neural networks, and sensing.

Nonvolatile multi-level adjustable optical switch based on the phase change material

For the advantages of the faster computation speed and lower energy consumption, all-optical computation has attracted great attention compared with the traditional electric computation method. Optical switches are the critical elementary units of optical computation devices. However, the traditional optical switches have two shortcomings, expending the outside energy to keep the switch state and the weak multi-level adjustable ability, which greatly restrict the realization of the large-scale photonic integrated circuits and optical spiking neural networks. In this paper, we use a subwavelength grating slot-ridge (SWGSR) waveguides on the silicon platform to design a nonvolatile multi-level adjustable optical switch based on the phase change material Ge₂Sb₂Te₅ (GST). Changing the phase state of GST can modulate the transmission of the optical switch, and the change of the optical transmittance of the optical switch is about 70%, which is much higher than that of previous optical switches. As no static power is required to maintain the phase state, it can find promising applications in optical switch matrices and reconfigurable optical spiking neural networks.

Polarization-controlled varifocal metalens with a phase change material GSST in mid-infrared

Detection of aldehyde carbonyl radiation plays an essential role in guaranteeing the safety of fried food. However, the radiation of low-content aldehyde carbonyl is always weak and includes polarized light. Focusing the weak radiation with polarization-sensitive configurations provides an efficient way to improve the signal-to-noise ratio of detection. The advent of dynamic metasurfaces based on phase-change materials (PCMs) have demonstrated superiorities over their traditional counterparts in tunability and miniaturization. In this paper, we propose two reflected varifocal metasurfaces, which combine Ge₂Sb₂Se₄Te₁ (GSST) with two materials that have close optical constants with amorphous and crystalline GSST. The first one realizes a four-spot focal system with linearly-polarized incidence based on polarization multiplexing. It adds a new polarization degree of freedom compared with traditional varifocal metasurfaces. Compared with traditional spatial-multiplexing method, our second metasurface enables the independent control of the polarization and phase profiles of circularly-polarized light. Remarkably, it reduces energy loss and crosstalk. We believe the novel scenarios of combing GSST with similar materials provide a new direction for tunable metasurfaces based on PCMs.

Recent Advancement in Optical Metasurface: Fundament to Application

Metasurfaces have gained growing interest in recent years due to their simplicity in manufacturing and lower insertion losses. Meanwhile, they can provide unprecedented control over the spatial distribution of transmitted and reflected optical fields in a compact form. The metasurfaces are a kind of planar array of resonant subwavelength components that, depending on the intended optical wavefronts to be sculpted, can be strictly periodic or quasi-periodic, or even aperiodic. For instance, gradient metasurfaces, a subtype of metasurfaces, are designed to exhibit spatially changing optical responses, which result in spatially varying amplitudes of scattered fields and the associated polarization of these fields. This paper starts off by presenting concepts of anomalous reflection and refraction, followed by a brief discussion on the Pancharatanm-Berry Phase (PB) and Huygens' metasurfaces. As an introduction to wavefront manipulation, we next present their key applications. These include planar metalens, cascaded meta-systems, tunable metasurfaces, spectrometer retroreflectors, vortex beams, and holography. The review concludes with a summary, preceded by a perspective outlining our expectations for potential future research work and applications.

Strong Field Enhancement and Unidirectional Scattering Based on Asymmetric Nanoantenna

Dielectric-metal nanostructures have lately emerged as one of the most promising approaches to modulating light at the optical frequency. Their remarkable electric and magnetic resonances give them a one-of-a-kind ability to augment local field enhancements with negligible absorption losses. Here, we propose a hybrid metal-dielectric-metal (MDM) nanoantenna that contains a dimer of three-layers of shell nanoparticles. In addition, we only theoretically and numerically show the optical properties of the hybrid dimer nanoantenna. We found that the nanoantenna sustained unidirectional forward scattering with narrow beamwidth (30.9 deg) and strong scattering intensity (up to 5 times larger than the single MDM particle). Furthermore, when the hybrid asymmetric dimer was excited by the plane wave with different electric polarization directions, our findings revealed that the hybrid nanoantenna boosted the gap’s electric near-field while also supporting unidirectional forward scattering. Finally, we analyzed the hybrid dimer with substrates of different materials. It supported strong electric high-order moments along the z-axis and x-axis in gaps between MDM nanoparticles and between MDM nanoparticles and the Ge substrate, owing to the intense displacement currents inside of the dielectric layer. We found that the local electric field of this MDM hybrid dimer nanoantenna with Ge substrate was well improved and attained 3325 v/m.

Multifunctional and Transformative Metaphotonics with Emerging Materials

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Perfect Control of Diffraction Patterns with Phase-Gradient Metasurfaces

Phase-gradient metasurfaces (PGMs) constitute an efficient platform for deflection of a beam in a desired direction. According to the generalized Snell's law, the direction of the reflected/refracted wave can be tuned by the spatial phase function provided by the PGMs. However, most studies on PGM focus only on a single diffraction order, that is, the incident wave can be reflected or refracted to a single target direction. Even in the case of multiple beams pointing in different directions, the beams are still in the same order mode, and the energy carried by different beams cannot be controlled. In addition, the energy ratio of multiple beams is generally uncontrollable. Here, we propose a general method to perfectly control diffraction patterns based on a multi-beam PGM. An analytical solution for arbitrarily controlling diffraction beams is derived through which the generation and energy distribution in high-order diffraction beams can be achieved. Three metasurfaces with different diffraction orders and energy ratios are designed and fabricated to demonstrate the proposed method. The efficiencies of diffraction for the desired channels are close to 100%. The simulated and measured far-field patterns are in good agreement with theoretical predictions, validating the proposed method that provides a new way to design multi-beam antennas and that has significance in wireless communication applications.

Reconfigurable and spectrally switchable perfect absorber based on a phase-change material

In this paper, we propose a lithography-free spectrally tunable prefect absorber based on an asymmetric Fabry-Perot cavity using Ge₂Sb₂Te₅ (GST), a phase-change material, as the cavity layer. The proposed device shows a maximum absorption of 99.7% at 1550 nm, at a particular angle of incidence and polarization when the phase of GST is in the amorphous state. The absorption spectrum is spectrally switched to longer wavelength when the phase of GST is transformed from amorphous to crystalline. The tuning range is about 866 nm, and the maximum absorption is maintained above 99% in the whole tuning range. The crystallinity ratio of GST is varied by applying voltage pulses of different amplitudes and durations. The electrothermal cosimulations show that the phase change is obtained in the whole GST layer. Furthermore, by reamorphization of GST, the absorption spectrum can be switched back, enabling a reconfigurable perfect absorber. This work shows a viable path toward achieving a tunable perfect absorber covering a 1550 nm communication wavelength window as well as an emerging optical communication window around 2 µm wavelength.

Electrically driven reprogrammable phase-change metasurface reaching 80% efficiency

Phase-change materials (PCMs) offer a compelling platform for active metaoptics, owing to their large index contrast and fast yet stable phase transition attributes. Despite recent advances in phase-change metasurfaces, a fully integrable solution that combines pronounced tuning measures, i.e., efficiency, dynamic range, speed, and power consumption, is still elusive. Here, we demonstrate an in situ electrically driven tunable metasurface by harnessing the full potential of a PCM alloy, Ge₂Sb₂Te₅ (GST), to realize non-volatile, reversible, multilevel, fast, and remarkable optical modulation in the near-infrared spectral range. Such a reprogrammable platform presents a record eleven-fold change in the reflectance (absolute reflectance contrast reaching 80%), unprecedented quasi-continuous spectral tuning over 250 nm, and switching speed that can potentially reach a few kHz. Our scalable heterostructure architecture capitalizes on the integration of a robust resistive microheater decoupled from an optically smart metasurface enabling good modal overlap with an ultrathin layer of the largest index contrast PCM to sustain high scattering efficiency even after several reversible phase transitions. We further experimentally demonstrate an electrically reconfigurable phase-change gradient metasurface capable of steering an incident light beam into different diffraction orders. This work represents a critical advance towards the development of fully integrable dynamic metasurfaces and their potential for beamforming applications.

The authors demonstrate an efficient platform for electrically driven reconfigurable metasurfaces by using Ge₂Sb₂Te₅ to realize non-volatile, reversible, multilevel, and fast optical modulation and wavefront engineering in the near-infrared spectral range.

Highly Unidirectional Radiation Enhancement Based on a Hybrid Multilayer Dimer

Dimers made of plasmonic particles support strong field enhancements but suffer from large absorption losses, while low-loss dielectric dimers are limited by relatively weak optical confinement. Hybrid dimers could utilize the advantages of both worlds. Here, we propose a hybrid nanoantenna that contains a dimer of core-dual shell nanoparticles known as the metal-dielectric-metal (MDM) structure. We discovered that the hybrid dimer sustained unidirectional forward scattering, which resulted in a nearly ideal Kerker condition in the frequency close to the resonance peak of the dimer due to enhancing the amplitude of the induced high-order electric multiples in the gap and effectively superimposing them with magnetic ones, which respond to the excitation of the plane wave in the dielectric layer of the dimer. Furthermore, when an electric quantum emitter is coupled to the dimer, our study shows that the optimal hybrid dimer simultaneously possesses high radiation directivity and low-loss features, which illustrates a back-to-front ratio of radiation 53 times higher than that of the pure dielectric dimer and an average radiation efficiency 80% higher than that of the pure metallic dimer. In addition, the unique structures of the hybrid hexamer direct almost decrease 75% of the radiation beamwidth, hence heightening the directivity of the nanoantenna based on a hybrid dimer.

Single-layered phase-change metasurfaces achieving efficient wavefront manipulation and reversible chiral transmission

Efficient control of the phase and polarization of light is of significant importance in modern optics and photonics. However, traditional methods are often accompanied with cascaded and bulky designs that cannot fulfill the ongoing demand for further integrations. Here, a single-layered metasurface composed of nonvolatile phase-change material Ge₂Sb₂Se₄Te₁ (GSST) is proposed with tunable spin-orbit interactions in subwavelength scale. According to the spin-dependent destructive or constructive interference, asymmetric transmission for circularly polarized incidence (extinction ratio > 8:1) can be achieved when GSST is in an amorphous state. Moreover, when GSST changes to crystalline state, reversed chiral transmission (extinction ratio > 12:1) can be observed due to the existence of intrinsic chirality. In addition, as the average cross-polarized transmitted amplitude is larger than 85%, arbitrary wavefront manipulations can be achieved in both states simultaneously based on the theory of Pancharatnam-Berry phase. As a proof of concept, several functional metasurface devices are designed and characterized to further demonstrate the validation of our design methodology. It is believed that these multifunctional devices with ultrahigh compactness are promising for various applications including chiroptical spectroscopy, EM communication, chiral imaging, and information encryption.

Design of hybrid narrow-band plasmonic absorber based on chalcogenide phase change material in the infrared spectrum

Structures absorbing electromagnetic waves in the infrared spectral region are important optical components in key areas such as biosensors, infrared images, thermal emitters, and special attention is required for reconfigurable devices. We propose a three-dimensional metal-dielectric plasmonic absorber with a layer of PCM's (Phase Change Materials). The phase shift effects of PCMs are numerically analyzed, and it is possible to obtain a shifting control of the resonant absorption peaks between the amorphous and crystalline states using the Lorentz-Lorenz relation. By using this empirical relation, we analyzed the peak absorption shift at intermediate phases between the amorphous and the crystalline. The geometric parameters of the structure with the PCM layer in the semi-crystalline state were adjusted to exhibit strong absorption for normal incidence. The effects of the oblique incidence on the absorption for the TM and TE polarization modes were also analyzed. Our results demonstrate that PCMs have great potential for reconfigurable nanophotonic devices.

Dynamically tunable third-harmonic generation with all-dielectric metasurfaces incorporating phase-change chalcogenides

Subwavelength nonlinear optical sources with high efficiency have received extensive attention, although strong dynamic controllability of these sources is still elusive. Germanium antimony telluride (GST) as a well-established phase-change chalcogenide is a promising candidate for the reconfiguration of subwavelength nanostructures due to the strong non-volatile change of the index of refraction between its amorphous and crystalline states. Here, we numerically demonstrate an electromagnetically-induced-transparency-based silicon metasurface actively controlled with a quarter-wave asymmetric Fabry-Perot cavity incorporating GST to modulate the relative phase of incident and reflected pump beams. We demonstrate a giant third-harmonic generation (THG) switch with a modulation depth as high as ∼70dB at the resonant band. We also demonstrate the possibility of multi-level THG amplitude modulation for the fundamental C-band by controlling the crystallization fraction of GST at multiple intermediate states. This study shows the high potential of GST-based fast dynamic nonlinear photonic switches for real-world applications ranging from communications to optical computing.

Transmission and reflection bi-direction terahertz encoding metasurface with a single structure

Most reported metasurfaces operate in single propagation direction mode (either transmissive mode or reflective mode), which hamper practical application. Here, we proposed a bi-directional operation coding metasurface based on a phase change material of a vanadium dioxide (VO₂) assisted metasurface. It can realize a dynamically invertible switch between the transmissive mode or reflective mode in the terahertz regime by changing the external ambient temperature. The proposed structure consists of a silicon column, polyimide dielectric substrate layer, and VO₂ film bottom layer. When VO₂ is in dielectric state, the designed metasurface possesses the functions of transmission beam splitting and deflection and generates a transmission vortex beam. When VO₂ is in metallic state, the proposed metasurface exhibits many functions such as reflection beam splitting, deflection, radar scattering surface (RCS) reduction and reflection vortex beam generation. The proposed metasurface can solve transmissive and reflective bi-direction terahertz encoding regulation. This scheme provides a new method to realize multi-function terahertz devices.

Reprogrammable optical metasurfaces by electromechanical reconfiguration

Metasurfaces, with artificially designed ultrathin and compact optical elements, enable versatile manipulation of the amplitude, phase, and polarization of light waves. While most of the metasurfaces are static and passive, here we propose a reprogrammable metasurface based on the state-of-art electromechanical nano-kirigami, which allows for independent manipulation of pixels at visible wavelengths through mechanical deformation of the nanostructures. By incorporating electrostatic forces between the top suspended gold nano-architectures and bottom silicon substrate, out-of-plane deformation of each pixel and the associated phase retardation are independently controlled by applying single voltage to variable pixels or exerting programmable voltage distribution on identical pixels. As a proof-of-concept demonstration, the metasurfaces are digitally controlled and a series of tunable metasurface holograms such as 3D dynamic display and ultrathin planar lenses are achieved at visible wavelengths. The proposed electromechanical metasurface provides a new methodology to explore versatile reconfigurable and programmable functionalities that may lead to advances in a variety of applications such as hologram, 3D displays, data storage, spatial light modulations, and information processing.

Near-IR reconfigurable 1D Ag grating Fabry-Perot absorber hybridized with phase-change material GSST

Chalcogenide phase-change materials (PCMs) offer a unique feature that can be used to dynamically control the response of the photonic devices and achieve fast, nonvolatile, reversible, multilevel, and specific optical modulation. The phase-change material Ge₂Sb₂Se₄Te₁ (GSST) has recently received a lot of attention due to the large index contrast between its amorphous and crystalline states with significantly low optical loss in the optical to near-IR spectrum. In this paper, we propose a tunable and reconfigurable hybrid PCM plasmonic nanostructure composed of a spacer layer of GSST sandwiched between a Ag back reflector and a 1D Ag Fabry-Perot grating structure. We use the finite element method (FEM) to numerically calculate the light absorption, absorption contrast, and figure of merit of the plasmonic nanostructure for both the amorphous and crystalline state of the GSST. Our calculations show that with constant structural variation the observed multimode absorption is drastically modified when the GSST undergoes a phase change from the amorphous to the crystalline state. The absorption contrast spectrum, which is defined as the absorption difference between the amorphous and crystalline state of GSST, shows four extrema modes between 70% and 89%. The figure of merit spectrum shows two large values of 44.39 and 37.78 at the 1502 nm and 2063 nm wavelengths, respectively. We also address the observed modes in the absorption contrast spectrum through spatial representation of the enhanced electric field distribution at their corresponding wavelengths. We show how the phase change in the GSST spacer can control the coupling between the optical cavity modes and the Ag surface plasmon resonance modes in the cavities and GSST spacer strip boundaries. The findings in this paper may open new avenues toward the design of next-generation photonic systems such as thermal emission controllers, sensors, ranging holograms, modulators and optical detection devices.

Quo Vadis, Metasurfaces?

The full manipulation of intrinsic properties of electromagnetic waves has become the central target in various modern optical technologies. Optical metasurfaces have been suggested for a complete control of light-matter interaction with subwavelength structures, and they have been explored widely in the past decade for creating next-generation multifunctional flat-optics devices. The current studies of metasurfaces have reached a mature stage where common materials, basic optical physics, and conventional engineering tools have been explored extensively for various applications such as light bending, metalenses, metaholograms, and many others. A natural question is where the future research on metasurfaces will be going: Quo vadis, metasurfaces? In this Mini Review, we provide perspectives on the future developments of optical metasurfaces. Specifically, we highlight recent progresses on hybrid metasurfaces employing low-dimensional materials and discuss biomedical, computational, and quantum applications of metasurfaces, followed by discussions of challenges and foreseeing the future of metasurface physics and engineering.

Self-stabilizing curved metasurfaces as a sail for light-propelled spacecrafts

Laser-driven spacecrafts are promising candidates for explorations to outer space. These spacecrafts should accelerate to a fraction of the speed of light upon illumination with earth-based laser systems. There are several challenges for such an ambitious mission that needs to be addressed yet. A matter of utmost importance is the stability of the spacecraft during the acceleration. Furthermore, the spacecraft sails should effectively reflect the light without absorptive-overheating. To address these requirements, we propose the design of a lightweight, low-absorbing, high-reflective, and self-stabilizing curved metasurface made from c-Si nanoparticles. A method to determine the stability is presented and, based on the multipole expansion method, the rotational stability of the curved metasurfaces is examined and the optimal operating regime is identified. The curvature is shown to be beneficial for the overall stability of the metasurface. The validity of the method is verified through numerical simulations of the time evolution of the trajectory of an identified metasurface. The results show that curved metasurfaces are a promising candidate for laser-driven spacecrafts.

ITO-based microheaters for reversible multi-stage switching of phase-change materials: towards miniaturized beyond-binary reconfigurable integrated photonics

Inducing a large refractive-index change is the holy grail of reconfigurable photonic structures, a goal that has long been the driving force behind the discovery of new optical material platforms. Recently, the unprecedentedly large refractive-index contrast between the amorphous and crystalline states of Ge-Sb-Te (GST)-based phase-change materials (PCMs) has attracted tremendous attention for reconfigurable integrated nanophotonics. Here, we introduce a microheater platform that employs optically transparent and electrically conductive indium-tin-oxide (ITO) bridges for the fast and reversible electrical switching of the GST phase between crystalline and amorphous states. By the proper assignment of electrical pulses applied to the ITO microheater, we show that our platform allows for the registration of virtually any intermediate crystalline state into the GST film integrated on the top of the designed microheaters. More importantly, we demonstrate the full reversibility of the GST phase between amorphous and crystalline states. To show the feasibility of using this hybrid GST/ITO platform for miniaturized integrated nanophotonic structures, we integrate our designed microheaters into the arms of a Mach-Zehnder interferometer to realize electrically reconfigurable optical phase shifters with orders of magnitude smaller footprints compared to existing integrated photonic architectures. We show that the phase of optical signals can be gradually shifted in multiple intermediate states using a structure that can potentially be smaller than a single wavelength. We believe that our study showcases the possibility of forming a whole new class of miniaturized reconfigurable integrated nanophotonics using beyond-binary reconfiguration of optical functionalities in hybrid PCM-photonic devices.

Subwavelength grating waveguide filter based on cladding modulation with a phase-change material grating

Subwavelength engineering and utilizing phase-change materials with large contrast in their optical properties have become powerful design tools for integrated silicon photonics. Reversible phase-transition of phase-change materials such as Ge₂Sb₂Te₅ (GST) provide a new degree of freedom and open up the possibility of adding new functionalities to the designed devices. We present an optical filter based on a silicon subwavelength grating (SWG) waveguide evanescently coupled to phase-change material loading segments arranged periodically around the SWG core. The effect of the GST loading segments' geometry and their distance from the SWG core on the filter's central wavelength and bandwidth are studied with three-dimensional finite-difference time-domain simulations. The employment of GST in the structure adds a switching functionality with an extinction ratio of 28.8 dB. We also examine the possibility of using the proposed structure as a reconfigurable filter by controlling the partial crystallization of the GST offering a blueshift of more than 4 nm.

Liquid crystal integrated metadevice for reconfigurable hologram displays and optical encryption

The ultimate goal of metasurface research in recent years is to apply metasurface to reality applications and improve the performance compared to its counterpart, namely conventional optical elements with the same function. Inspired by the application of electrically addressing spatial light modulator (EA-SLM) and based on the binary holographic algorithm, here we propose a reconfigurable metadevice integrated with the nematic liquid crystal (NLC). The smart metadevice directly uses the subwavelength antennas as the main contributor to the phase accumulation instead of the NLC layer. By applying different electrical modulation patterns on the NLC, the metadevice can realize the function of dynamic holographic display as traditional SLMs but features in smaller size, higher resolution and lager field of view. In addition, we improved the existing computer-generated hologram algorithm to generate three holograms with quantitative correlation and also propose a new optical encryption method based on our metadevice. The encryption method needs four elements in total to decrypt and can fully meets the requirements of the various encrypted content. We believe such metadevice paves the way for the new generation of micro-optical display and optical encryption devices.