Experimental Demonstration of >230° Phase Modulation in Gate-Tunable Graphene-Gold Reconfigurable Mid-Infrared Metasurfaces

Thermal management broadband-emitting device based on VO2 applied in the mid-infrared band

Mid-infrared thermal radiation has attracted attention due to its wide range of applications. Compared to the static process of thermal emission, if thermal radiation can be dynamically controlled, it would be more suitable for practical applications. Herein, we designed a controllable thermal emitter based on phase change materials. When the temperature changes from low to high, VO2 transitions from a dielectric state to a metallic state, and its imaginary part of the dielectric constant significantly increases, leading to differences in emission characteristics. At low temperatures, the device is in a low dielectric state and resonates weakly with incident light. The main emission comes from the bottom of the grating structure, with an emissivity of 0.21. At high temperatures, the structure is in a high dielectric state, and multiple resonance modes are excited within the structure, such as cavity resonance and surface plasmon resonance, which increases the emissivity to 0.95 and achieves effective heat dissipation. Given its superior thermal management capabilities and stability, this design holds promise for applications in thermal imaging, infrared communication, and energy-efficient devices.

Electrically switchable 2N-channel wave-front control for certain functionalities with N cascaded polarization-dependent metasurfaces

Metasurfaces with tunable functionalities are greatly desired for modern optical system and various applications. To increase the operating channels of polarization-multiplexed metasurfaces, we proposed a structure of N cascaded dual-channel metasurfaces to achieve 2N electrically switchable channels without intrinsic loss or cross-talk for certain functionalities, including beam steering, vortex beam generation, lens, etc. As proof of principles, we have implemented a 3-layer setup to achieve 8 channels. In success, we have demonstrated two typical functionalities of vortex beam generation with switchable topological charge of l = -3 ~ +4 or l = -1 ~ -8, and beam steering with the deflection direction switchable in an 8×1 line or a 4×2 grid. We believe that our proposal would provide a practical way to significantly increase the scalability and extend the functionality of polarization-multiplexed metasurfaces. Although this method is not universal, it is potential for the applications of LiDAR, glasses-free 3D display, OAM (de)multiplexing, and varifocal meta-lens.

Reconfigurable Micro/Nano-Optical Devices Based on Phase Transitions: From Materials, Mechanisms to Applications

In recent years, numerous efforts have been devoted to exploring innovative micro/nano-optical devices (MNODs) with reconfigurable functionality, which is highly significant because of the progressively increasing requirements for next-generation photonic systems. Fortunately, phase change materials (PCMs) provide an extremely competitive pathway to achieve this goal. The phase transitions induce significant changes to materials in optical, electrical properties or shapes, triggering great research interests in applying PCMs to reconfigurable micro/nano-optical devices (RMNODs). More specifically, the PCMs-based RMNODs can interact with incident light in on-demand or adaptive manners and thus realize unique functions. In this review, RMNODs based on phase transitions are systematically summarized and comprehensively overviewed from materials, phase change mechanisms to applications. The reconfigurable optical devices consisting of three kinds of typical PCMs are emphatically introduced, including chalcogenides, transition metal oxides, and shape memory alloys, highlighting the reversible state switch and dramatic contrast of optical responses along with designated utilities generated by phase transition. Finally, a comprehensive summary of the whole content is given, discussing the challenge and outlooking the potential development of the PCMs-based RMNODs in the future.

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.

Dual-band and spectrally selective infrared absorbers based on hybrid gold-graphene metasurfaces

In this paper, we propose a dual-band and spectrally selective infrared (IR) absorber based on a hybrid structure comprising a patterned graphene monolayer and cross-shaped gold resonators within a metasurface. Rooted in full-wave numerical simulations, our study shows that the fundamental absorption mode of the gold metasurface hybridizes with the graphene pattern, leading to a second absorptive mode whose properties depend on graphene's electrical properties and physical geometry. Specifically, the central operation band of the absorber is defined by the gold resonators whereas the relative absorption level and spectral separation between the two modes can be controlled by graphene's chemical potential and its pattern, respectively. We analyze this platform using coupled-mode theory to understand the coupling mechanism between these modes and to elucidate the emergence and tuning of the dual band response. The proposed dual-band device can operate at different bands across the IR spectrum and may open new possibilities for tailored sensing applications in spectroscopy, thermal imaging, and environmental monitoring.

Optimization of Novel 2D Material Based SPR Biosensor Using Machine Learning

Biosensors are needed for today's health monitoring system for detecting different biomolecules. Graphene is a monolayer material that can be utilized to sense biomolecules and design biosensors. We have proposed a Graphene-Gold-Silver hybrid structure design based on Zinc Oxide which gives sensitive performance to detect hemoglobin biomolecules. The advanced biosensor designed based on this hybrid structure shows the highest sensitivity of 1000 nm/RIU which is far better concerning similar structure previously analyzed. The graphene-gold-silver hybrid structure is presented for its possible reflectance results and electric field results. The E-field results match well with the reflectance results given by the sensitive hybrid structure. The sensing biomolecules are presented above the structure where a combination of graphene-gold-silver hybrid structure improves the sensitivity to a great extent. The optimized parameters are obtained by applying variations in the physical parameters of the design. The machine learning algorithm employed for reflectance prediction shows a high prediction accuracy and can be utilized for simulation resource reduction. The proposed biosensor can be used in real-time hemoglobin monitoring.

Sub-1-Volt Electrically Programmable Optical Modulator Based on Active Tamm Plasmon

Reconfigurable optical devices hold great promise for advancing high-density optical interconnects, photonic switching, and memory applications. While many optical modulators based on active materials have been demonstrated, it is challenging to achieve a high modulation depth with a low operation voltage in the near-infrared (NIR) range, which is a highly sought-after wavelength window for free-space communication and imaging applications. Here, electrically switchable Tamm plasmon coupled with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is introduced. The device allows for a high modulation depth across the entire NIR range by fully absorbing incident light even under epsilon near zero conditions. Optical modulation exceeding 88% is achieved using a CMOS-compatible voltage of ±1 V. This modulation is facilitated by precise electrical control of the charge carrier density through an electrochemical doping/dedoping process. Additionally, the potential applications of the device are extended for a non-volatile multi-memory state optical device, capable of rewritable optical memory storage and exhibiting long-term potentiation/depression properties with neuromorphic behavior.

Diffractive optical computing in free space

Structured optical materials create new computing paradigms using photons, with transformative impact on various fields, including machine learning, computer vision, imaging, telecommunications, and sensing. This Perspective sheds light on the potential of free-space optical systems based on engineered surfaces for advancing optical computing. Manipulating light in unprecedented ways, emerging structured surfaces enable all-optical implementation of various mathematical functions and machine learning tasks. Diffractive networks, in particular, bring deep-learning principles into the design and operation of free-space optical systems to create new functionalities. Metasurfaces consisting of deeply subwavelength units are achieving exotic optical responses that provide independent control over different properties of light and can bring major advances in computational throughput and data-transfer bandwidth of free-space optical processors. Unlike integrated photonics-based optoelectronic systems that demand preprocessed inputs, free-space optical processors have direct access to all the optical degrees of freedom that carry information about an input scene/object without needing digital recovery or preprocessing of information. To realize the full potential of free-space optical computing architectures, diffractive surfaces and metasurfaces need to advance symbiotically and co-evolve in their designs, 3D fabrication/integration, cascadability, and computing accuracy to serve the needs of next-generation machine vision, computational imaging, mathematical computing, and telecommunication technologies.

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.

Electrically Tunable Reflective Metasurfaces with Continuous and Full-Phase Modulation for High-Efficiency Wavefront Control at Visible Frequencies

All-dielectric optical metasurfaces can locally control the amplitude and phase of light at the nanoscale, enabling arbitrary wavefront shaping. However, lack of postfabrication tunability has limited the true potential of metasurfaces for many applications. Here, we utilize a thin liquid crystal (LC) layer as a tunable medium surrounding the metasurface to achieve a phase-only spatial light modulator (SLM) with high reflection in the visible frequency, exhibiting active and continuous resonance tuning with associated 2π phase control and uncoupled amplitude. Dynamic wavefront shaping is demonstrated by programming 96 individually addressable electrodes with a small pixel pitch of ∼1 μm. The small pixel size is facilitated by the reduced LC thickness, strongly suppressing cross-talk among pixels. This device is used to demonstrate dynamic beam steering with a wide field-of-view and high absolute diffraction efficiencies. We believe that our demonstration may help realize next-generation, high-resolution SLMs, with wide applications in dynamic holography, tunable optics, and light detection and ranging (LiDAR), to mention a few.

Heterodyne coherent detection of phase modulation in a mid-infrared unipolar device

Phase modulation is demonstrated in a quantum Stark effect modulator designed to operate in the mid-infrared at wavelength around 10 µm. Both phase and amplitude modulation are simultaneously resolved through the measurement of the heterodyne signal arising from the beating of a quantum cascade laser with a highly stabilized frequency comb. The highest measured phase shift is more than 5 degrees with an associated intensity modulation of 5 %. The experimental results are in full agreement with our model in which the complex susceptibility is precisely described considering the linear voltage dependent Stark shift of the optical resonance.

Mid-infrared integrated electro-optic modulators: a review

Integrated mid-infrared (MIR) photonics have various applications in optical fiber communication, spectral detection and identification, free-space communication, and light detection and ranging, etc. The MIR electro-optic (EO) modulator, which is one of the key components of MIR integrated photonic systems, has attracted a lot of research interests. In this paper, we review the reported integrated MIR EO modulators based on different modulation mechanisms and material platforms. The recent research progresses and challenges of MIR EO modulators are presented and discussed. The unique advantages and the corresponding applications of each type of MIR modulators are summarized as well. In the end, we provide our perspectives of a few areas in integrated MIR modulators that are worthy for research attention in future.

A universal metasurface antenna to manipulate all fundamental characteristics of electromagnetic waves

Metasurfaces have promising potential to revolutionize a variety of photonic and electronic device technologies. However, metasurfaces that can simultaneously and independently control all electromagnetics (EM) waves' properties, including amplitude, phase, frequency, polarization, and momentum, with high integrability and programmability, are challenging and have not been successfully attempted. Here, we propose and demonstrate a microwave universal metasurface antenna (UMA) capable of dynamically, simultaneously, independently, and precisely manipulating all the constitutive properties of EM waves in a software-defined manner. Our UMA further facilitates the spatial- and time-varying wave properties, leading to more complicated waveform generation, beamforming, and direct information manipulations. In particular, the UMA can directly generate the modulated waveforms carrying digital information that can fundamentally simplify the architecture of information transmitter systems. The proposed UMA with unparalleled EM wave and information manipulation capabilities will spark a surge of applications from next-generation wireless systems, cognitive sensing, and imaging to quantum optics and quantum information science.

Programmable Wavefront Control in the Visible Spectrum Using Low-Loss Chalcogenide Phase-Change Metasurfaces

All-dielectric metasurfaces provide unique solutions for advanced wavefront manipulation of light with complete control of amplitude and phase at sub-wavelength scales. One limitation, however, for most of these devices is the lack of any post-fabrication tunability of their response. To break this limit, a promising approach is employing phase-change materials (PCMs), which provide fast, low energy, and non-volatile means to endow metasurfaces with a switching mechanism. In this regard, great advancements have been done in the mid-infrared and near-infrared spectrum using different chalcogenides. In the visible spectral range, however, very few devices have demonstrated full phase manipulation, high efficiencies, and reversible optical modulation. In this work, a programmable all-dielectric Huygens' metasurface made of antimony sulfide (Sb2 S3 ) PCM is experimentally demonstrated, a low loss and high-index material in the visible spectral range with a large contrast (≈0.5) between its amorphous and crystalline states. ≈2π phase modulation is shown with high associated transmittance and it is used to create programmable beam-steering devices. These novel chalcogenide PCM metasurfaces have the potential to emerge as a platform for next-generation spatial light modulators and to impact application areas such as programmable and adaptive flat optics, light detection and ranging (LiDAR), and many more.

Beam-steering metasurfaces assisted coherent optical wireless multichannel communication system

The metasurface based beam-steering devices with the advantages of large steering angles, arbitrary channels and ultra-compactness have played an important role for data allocation and exchange in the optical wireless communication. However, the current metasurface based optical wireless communication systems are mainly on intensity modulation and direct detection (IM/DD), which shows a relative lower transmission capacity, lower received optical signal-to-noise ratio (OSNR) and complexity of system. In this study, a bidirectional multichannel optical wireless system enabled by a polarization independent metasurface with coherent modulation and reception is designed and experimentally demonstrated, which exhibits exclusive 100 Gbps coherent optical signals to multiusers with their own wavelengths, 2 m free space distance and field of view of 20° × 20°. In addition, the proposed system can support optical broadcasting system with capacity of 900 Gbps. The demonstrated metasurface assisted optical wireless communication system merges the optical coherent communication techniques and emerging concept of metasurface, which reduces the complexity and cost of the system while contributing a high transmission capacity, opening a new avenue for high performance optical wireless communications.

Electrically controllable optical switch metasurface based on vanadium dioxide

We report a voltage-tunable reflective gold wire grid metasurface on vanadium dioxide thin film, which consists of a metal-insulator-metal (MIM) structure. We excite surface plasmon polariton (SPP) modes on the gold surface by fabricating a one-dimensional structured gold wire grid. Joule heating of laser-induced graphene (LIG) can be controlled by the voltage at the bottom, allowing vanadium dioxide in the structure to complete the transition from the insulating state to the metallic state. The phase transition of vanadium dioxide strongly disrupts the plasmon modes excited by the gold wire grid above, thereby realizing a huge change in the reflection spectrum. This acts as a tunable metasurface optical switch with a maximum modulation depth (MD) of over 20 dB. We provide a more effective and simple method for creating tunable metasurfaces in the near-infrared band, which can allow metasurfaces to have wider applications in optical signal processing, optical storage, and holography.

Graphene Hybrid Metasurfaces for Mid-Infrared Molecular Sensors

We integrated graphene with asymmetric metal metasurfaces and optimised the geometry dependent photoresponse towards optoelectronic molecular sensor devices. Through careful tuning and characterisation, combining finite-difference time-domain simulations, electron-beam lithography-based nanofabrication, and micro-Fourier transform infrared spectroscopy, we achieved precise control over the mid-infrared peak response wavelengths, transmittance, and reflectance. Our methods enabled simple, reproducible and targeted mid-infrared molecular sensing over a wide range of geometrical parameters. With ultimate minimization potential down to atomic thicknesses and a diverse range of complimentary nanomaterial combinations, we anticipate a high impact potential of these technologies for environmental monitoring, threat detection, and point of care diagnostics.

Integrated metasurfaces for re-envisioning a near-future disruptive optical platform

Metasurfaces have been continuously garnering attention in both scientific and industrial fields, owing to their unprecedented wavefront manipulation capabilities using arranged subwavelength artificial structures. To date, research has mainly focused on the full control of electromagnetic characteristics, including polarization, phase, amplitude, and even frequencies. Consequently, versatile possibilities of electromagnetic wave control have been achieved, yielding practical optical components such as metalenses, beam-steerers, metaholograms, and sensors. Current research is now focused on integrating the aforementioned metasurfaces with other standard optical components (e.g., light-emitting diodes, charged-coupled devices, micro-electro-mechanical systems, liquid crystals, heaters, refractive optical elements, planar waveguides, optical fibers, etc.) for commercialization with miniaturization trends of optical devices. Herein, this review describes and classifies metasurface-integrated optical components, and subsequently discusses their promising applications with metasurface-integrated optical platforms including those of augmented/virtual reality, light detection and ranging, and sensors. In conclusion, this review presents several challenges and prospects that are prevalent in the field in order to accelerate the commercialization of metasurfaces-integrated optical platforms.

Recent advanced applications of metasurfaces in multi-dimensions

Unlike traditional optical components, which rely on the gradual accumulation of light along the optical path over a distance much larger than the wavelength to form a wavefront, metasurfaces manipulate light field properties on the wavelength thickness by specially arranging various meta-atoms. Due to the ease of integration and compact planar structure, metasurfaces play a key role in the light field manipulations. Here, we review the recent advances of metasurfaces in multi-dimensions, including light wavelength, polarization, orbital angular momentum(OAM), and angular response. Progress in these fields has brought new applications in areas such as imaging, display, communication, and information encryption, etc. Finally, we also discuss the challenges and prospects of metasurfaces applications.

Electrical Phase Modulation Based on Mid-Infrared Intersubband Polaritonic Metasurfaces

Electrically reconfigurable metasurfaces that overcome the static limitations in controlling the fundamental properties of scattered light are opening new avenues for functional flat optics. This work proposes and experimentally demonstrates electrically phase-tunable mid-infrared metasurfaces based on the polaritonic coupling of Stark-tunable intersubband transitions in semiconductor heterostructures and electromagnetic modes in plasmonic nanoresonators. In the applied voltage range of -3 to +3 V, the local phase tuning of the light reflects from the metasurface, which enables the electrical control of the polarization state and wavefront of the reflected wave. Electrical beam polarization control, electrical beam diffraction control, and electrical beam steering are experimentally demonstrated as applications for local phase tunability. The proposed electrically tunable metasurfaces can easily tune the operating wavelength and function at relatively low voltages, which will enable various applications in the mid-infrared region.

Excitonic Beam Steering in an Active van der Waals Metasurface

Two-dimensional transition metal dichalcogenides (2D TMDCs) are promising candidates for ultrathin active nanophotonic elements due to the strong tunable excitonic resonances that dominate their optical response. Here, we demonstrate dynamic beam steering by an active van der Waals metasurface that leverages large complex refractive index tunability near excitonic resonances in monolayer molybdenum diselenide (MoSe₂). Through varying the radiative and nonradiative rates of the excitons, we can dynamically control both the reflection amplitude and phase profiles, resulting in an excitonic phased array metasurface. Our experiments show reflected light steering to angles between -30° and 30° at different resonant wavelengths corresponding to the A exciton and B exciton. This active van der Waals metasurface relies solely on the excitonic resonances of the monolayer MoSe₂ material rather than geometric resonances of patterned nanostructures, suggesting the potential to harness the tunability of excitonic resonances for wavefront shaping in emerging photonic applications.

Dynamic adjustable metalens based on a stretchable substrate with a double-layer metal microstructure

Based on the impedance-matching theory, a double-layer metal structure dynamical focusing cylindrical metalens with a stretchable substrate was designed at the operation frequency of 0.1 THz. The diameter, initial focal length, and NA of the metalens were 80 mm, 40 mm, and 0.7, respectively. The transmission phase of the unit cell structures could cover 0-2π by changing the size of the metal bars, and then the different unit cells were spatially arranged as the designed phase profile for the metalens. When the stretching range of the substrate was about 100%-140%, the focal length changed from 39.3 mm to 85.5 mm, the dynamic focusing range was about 117.6% of the minimum focal length, and the focusing efficiency decreases from 49.2% to 27.9%. Then, by rearranging the unit cell structures, a dynamically adjustable bifocal metalens was numerically realized. Using the same stretching ratio, compared to a single focus metalens, the bifocal metalens can provide a larger focal length control range.

Thermal metasurface with tunable narrowband absorption from a hybrid graphene/silicon photonic crystal resonance

We report the design of a tunable, narrowband, thermal metasurface that employs a hybrid resonance generated by coupling a tunable permittivity graphene ribbon to a silicon photonic crystal. The gated graphene ribbon array, proximitized to a high quality factor Si photonic crystal supporting a guided mode resonance, exhibits tunable narrowband absorbance lineshapes (Q > 10,000). Actively tuned Fermi level modulation in graphene with applied gate voltage between high absorptivity and low absorptivity states gives rise to absorbance on/off ratios exceeding 60. We employ coupled-mode theory as a computationally efficient approach to elements of the metasurface design, demonstrating an orders of magnitude speedup over typical finite element computational methods.

Dynamic Beam Steering and Focusing Graphene Metasurface Mirror Based on Fermi Energy Control

Beam steering technology is crucial for radio frequency and infrared telecommunication signal processing. Microelectromechanical systems (MEMS) are typically used for beam steering in infrared optics-based fields but have slow operational speeds. An alternative solution is to use tunable metasurfaces. Since graphene has gate-tunable optical properties, it is widely used in electrically tunable optical devices due to ultrathin physical thickness. We propose a tunable metasurface structure using graphene in a metal gap structure that can exhibit a fast-operating speed through bias control. The proposed structure can change beam steering and can focus immediately by controlling the Fermi energy distribution on the metasurface, thus overcoming the limitations of MEMS. The operation is numerically demonstrated through finite element method simulations.

Electrically programmable solid-state metasurfaces via flash localised heating

In the last decades, metasurfaces have attracted much attention because of their extraordinary light-scattering properties. However, their inherently static geometry is an obstacle to many applications where dynamic tunability in their optical behaviour is required. Currently, there is a quest to enable dynamic tuning of metasurface properties, particularly with fast tuning rate, large modulation by small electrical signals, solid state and programmable across multiple pixels. Here, we demonstrate electrically tunable metasurfaces driven by thermo-optic effect and flash-heating in silicon. We show a 9-fold change in transmission by <5 V biasing voltage and the modulation rise-time of <625 µs. Our device consists of a silicon hole array metasurface encapsulated by transparent conducting oxide as a localised heater. It allows for video frame rate optical switching over multiple pixels that can be electrically programmed. Some of the advantages of the proposed tuning method compared with other methods are the possibility to apply it for modulation in the visible and near-infrared region, large modulation depth, working at transmission regime, exhibiting low optical loss, low input voltage requirement, and operating with higher than video-rate switching speed. The device is furthermore compatible with modern electronic display technologies and could be ideal for personal electronic devices such as flat displays, virtual reality holography and light detection and ranging, where fast, solid-state and transparent optical switches are required.

Electrically tuned active metasurface towards metasurface-integrated liquid crystal on silicon (meta-LCoS) devices

Active metasurfaces add a new dimension to static metasurfaces by introducing tunability, and this has received enormous attention from industry. Although various mechanisms have been proposed over the past few years in literature, solutions with good practicality are limited. Liquid crystal (LC)-based active metasurface is one of the most promising approaches due to the well-established LC industry. In this paper, an electrically tunable active metasurface was proposed and experimentally demonstrated using photoaligned nematic LC. The good quality of the LC photoalignment on the metasurface was demonstrated. Tunable transmission was obtained for telecommunication C band and the modulation depth in transmission amplitude of 94% was realized for 1530 nm. Sub-millisecond response time was achieved at operating a temperature of 60°C. The progress made here presents the potential of LC-based active metasurfaces for fast-switching photonic devices at optical communication wavelengths. More importantly, this work lays the foundations for the next-generation liquid crystal on silicon (LCoS) devices that are integrated with metasurfaces (meta-LCoS).

Recent Advances in Reconfigurable Metasurfaces: Principle and Applications

Metasurfaces have shown their great capability to manipulate electromagnetic waves. As a new concept, reconfigurable metasurfaces attract researchers' attention. There are many kinds of reconfigurable components, devices and materials that can be loaded on metasurfaces. When cooperating with reconfigurable structures, dynamic control of the responses of metasurfaces are realized under external excitations, offering new opportunities to manipulate electromagnetic waves dynamically. This review introduces some common methods to design reconfigurable metasurfaces classified by the techniques they use, such as special materials, semiconductor components and mechanical devices. Specifically, this review provides a comparison among all the methods mentioned and discusses their pros and cons. Finally, based on the unsolved problems in the designs and applications, the challenges and possible developments in the future are discussed.

Electrically tunable conducting oxide metasurfaces for high power applications

Active metasurfaces designed to operate at optical frequencies are flat optical elements that can dynamic, subwavelength-scale wavefront control of reflected or transmitted light. The practical and fundamental power-handling limits of active metasurfaces at high pulse energies and high average powers determine the potential applications for these emerging photonic components. Here, we investigate thermal performance limits of reflective gate-tunable conducting oxide metasurfaces illuminated with high power density laser beams, for both continuous wave (CW) and pulsed laser illumination. Our gate-tunable metasurfaces use indium tin oxide (ITO) as an active material, which undergoes an epsilon-near-zero (ENZ) transition under applied electrical bias. We experimentally show that under CW illumination, there is no significant change in the electrically tunable metasurface optical response for high irradiances ranging from 1.6 kW/cm2 to 9.1 kW/cm2 when the illuminating laser beam diameter is 7 μm. Even under an applied bias, when over 60% of the incoming light is absorbed in a 1 nm-thick charge accumulation layer within ITO, the local temperature rise in the metasurface is modest, supporting its robustness for high-power applications. Additionally, we theoretically show that in the ENZ regime, the metasurface reflectance can be increased by a factor of 10 by replacing the active ITO layer with cadmium oxide (CdO). Thus conducting oxide metasurfaces can tolerate the power densities needed in higher power applications, including free space optical communications, to light detection and ranging (LiDAR), as well as laser-based additive manufacturing.

Tunable plasmonics on epsilon-near-zero materials: the case for a quantum carrier model

The carrier density profile in metal-oxide-semiconductor (MOS) capacitors is computed under gating using two classical models - conventional drift-diffusion (CDD) and density-gradient (DG) - and a self-consistent Schrödinger-Poisson (SP) quantum model. Once calibrated the DG model approximates well the SP model while being computationally more efficient. The carrier profiles are used in optical mode computations to determine the gated optical response of surface plasmons supported by waveguides incorporating MOS structures. Indium tin oxide (ITO) is used as the semiconductor in the MOS structures, as the real part of its optical permittivity can be driven through zero to become negative under accumulation, enabling epsilon-near-zero (ENZ) effects. Under accumulation the predictions made by the CDD and SP models differ considerably, in that the former predicts one ENZ point but the latter predicts two. Consequently, the CDD model significantly underestimates perturbations in n _{e f f} of surface plasmons (by ∼4×) and yields incorrect details in surface plasmon fields near ENZ points. The discrepancy is large enough to invalidate the CDD model in MOS structures on ENZ materials under accumulation, strongly motivating a quantum carrier model in this regime.

Nano-electromechanical spatial light modulator enabled by asymmetric resonant dielectric metasurfaces

Spatial light modulators (SLMs) play essential roles in various free-space optical technologies, offering spatio-temporal control of amplitude, phase, or polarization of light. Beyond conventional SLMs based on liquid crystals or microelectromechanical systems, active metasurfaces are considered as promising SLM platforms because they could simultaneously provide high-speed and small pixel size. However, the active metasurfaces reported so far have achieved either limited phase modulation or low efficiency. Here, we propose nano-electromechanically tunable asymmetric dielectric metasurfaces as a platform for reflective SLMs. Exploiting the strong asymmetric radiation of perturbed high-order Mie resonances, the metasurfaces experimentally achieve a phase-shift close to 290^∘, over 50% reflectivity, and a wavelength-scale pixel size. Electrical control of diffraction patterns is also achieved by displacing the Mie resonators using nano-electro-mechanical forces. This work paves the ways for future exploration of the asymmetric metasurfaces and for their application to the next-generation SLMs.

This work experimentally demonstrates nano-electromechanically tunable asymmetric dielectric metasurfaces. The metasurfaces enable large phase tuning, high reflection, a wavelength-scale pixel size, and electrical control of diffraction patterns.

Dynamic and complete terahertz wavefront manipulation via an anisotropic coding metasurface

A reconfigurable anisotropic coding metasurface composed of a graphene layer and anisotropic Jerusalem-cross metallic layer is proposed for dynamic and complete multi-channel terahertz wavefront manipulation. By controlling the Fermi energy of graphene, continuous amplitude modulation is realized for the coding elements with certain phase responses. By arranging anisotropic phase coding elements with a specific coding sequence and changing the Fermi energy of graphene, the proposed metasurface can dynamically control multi-channel reflection beams with designed power distribution and simultaneously manipulate the scattering pattern from diffusion to mirror scattering under x- and y-polarized incidence, respectively. Compared with the dynamic phase modulation metasurface, such a tunable metasurface uses three degrees of freedom, including the polarization, phase, and amplitude responses to fully control the reflected wavefronts, which may have promising applications in tunable terahertz multi-functional holograms and multi-channel information communication.

Active and tunable nanophotonic metamaterials

Metamaterials enable subwavelength tailoring of light-matter interactions, driving fundamental discoveries which fuel novel applications in areas ranging from compressed sensing to quantum engineering. Importantly, the metallic and dielectric resonators from which static metamaterials are comprised present an open architecture amenable to materials integration. Thus, incorporating responsive materials such as semiconductors, liquid crystals, phase-change materials, or quantum materials (e.g., superconductors, 2D materials, etc.) imbue metamaterials with dynamic properties, facilitating the development of active and tunable devices harboring enhanced or even entirely novel electromagnetic functionality. Ultimately, active control derives from the ability to craft the local electromagnetic fields; accomplished using a host of external stimuli to modify the electronic or optical properties of the responsive materials embedded into the active regions of the subwavelength resonators. We provide a broad overview of this frontier area of metamaterials research, introducing fundamental concepts and presenting control strategies that include electronic, optical, mechanical, thermal, and magnetic stimuli. The examples presented range from microwave to visible wavelengths, utilizing a wide range of materials to realize spatial light modulators, effective nonlinear media, on-demand optics, and polarimetric imaging as but a few examples. Often, active and tunable nanophotonic metamaterials yield an emergent electromagnetic response that is more than the sum of the parts, providing reconfigurable or real-time control of the amplitude, phase, wavevector, polarization, and frequency of light. The examples to date are impressive, setting the stage for future advances that are likely to impact holography, beyond 5G communications, imaging, and quantum sensing and transduction.

Zinc oxide (ZnO) hybrid metasurfaces exhibiting broadly tunable topological properties

Extreme light confinement observed in periodic photonic structures, such as the vortex singularities in momentum (k) space, has been associated with their topological nature. Consequently, by exploiting and tuning their topological properties, optical metasurfaces have been demonstrated as an attractive platform for active photonics. However, given the fact that most active media under external excitations can only provide limited refractive index change, the potential advancements offered by the topological character of active metasurfaces have remained mostly unexplored. Zinc oxide (ZnO), which has recently exhibited optically-induced extraordinarily large permittivity modulations at visible and near-infrared frequencies, is an excellent active material for dynamic metasurfaces exhibiting strong tuning. This work demonstrates that a hybrid metasurface consisting of an array of ZnO nanodisks on a silver backplane displays broadly tunable topological properties. In particular, by performing k-space scattering simulations using measured pump-fluence-dependent material properties of ZnO, we study in detail the light reflection from the hybrid metasurface. Our results validate that the large k-space topology tuning of the metasurface can result in enormously strong polarization manipulation of near-infrared light in the vicinity of the topological features. The observed polarization switching effect is highly sensitive to the polarization and wavelength of an incident wave, owing to the symmetry and dispersion characteristics of the proposed system. Our study indicates that leveraging a combination of the extraordinary material properties and the k-space topology, hybrid metasurfaces based on ZnO may open new avenues for creating all-optical switchable metadevices.

Optical nonreciprocity via transmissive time-modulated metasurfaces

The frequency mixing property of time-modulated metasurfaces, attributed to the well-known phenomenon of temporal photonic transition, has led to several exotic functionalities in the last lustrum. Based on this concept, we demonstrate the possibility of achieving nonreciprocal responses in the near-infrared regime via combining a time-modulated platform and a static high-Q metasurface. In particular, the temporal metasurface is designed to up-convert the incident tone to the first higher-order harmonic, while the static platform is implemented to establish a filtering behavior with respect to the incident frequency. It is shown that while the receiver port acquires the transmitted signal in the forward direction, the amount of received power becomes negligible under the time-reversal scenario, which indicates the presented configuration exhibits different optical responses from opposite directions. In addition, the role of operating wavelength and the modulation frequency on the power isolation level are investigated, and it is demonstrated that by appropriate selection, the isolation level can reach -30 dB. Since this is the first time a nonreciprocal response is obtained in the near-infrared regime via a pure temporal modulation, we believe the idea of this paper can be of utmost importance in various applications, such as tunable optical isolators.

Nanostructured In3SbTe2 antennas enable switching from sharp dielectric to broad plasmonic resonances

Phase-change materials (PCMs) allow for non-volatile resonance tuning of nanophotonic components. Upon switching, they offer a large dielectric contrast between their amorphous and crystalline phases. The recently introduced "plasmonic PCM" In3SbTe2 (IST) additionally features in its crystalline phase a sign change of its permittivity over a broad infrared spectral range. While optical resonance switching in unpatterned IST thin films has been investigated before, nanostructured IST antennas have not been studied, yet. Here, we present numerical and experimental investigations of nanostructured IST rod and disk antennas. By crystallizing the IST with microsecond laser pulses, we switched individual antennas from narrow dielectric to broad plasmonic resonances. For the rod antennas, we demonstrated a resonance shift of up to 1.2 µm (twice the resonance width), allowing on/off switching of plasmonic resonances with a contrast ratio of 2.7. With the disk antennas, we realized an increase of the resonance width by more than 800% from 0.24 µm to 1.98 µm while keeping the resonance wavelength constant. Further, we demonstrated intermediate switching states by tuning the crystallization depth within the resonators. Our work empowers future design concepts for nanophotonic applications like active spectral filters, tunable absorbers, and switchable flat optics.

Enabling Active Nanotechnologies by Phase Transition: From Electronics, Photonics to Thermotics

Phase transitions can occur in certain materials such as transition metal oxides (TMOs) and chalcogenides when there is a change in external conditions such as temperature and pressure. Along with phase transitions in these phase change materials (PCMs) come dramatic contrasts in various physical properties, which can be engineered to manipulate electrons, photons, polaritons, and phonons at the nanoscale, offering new opportunities for reconfigurable, active nanodevices. In this review, we particularly discuss phase-transition-enabled active nanotechnologies in nonvolatile electrical memory, tunable metamaterials, and metasurfaces for manipulation of both free-space photons and in-plane polaritons, and multifunctional emissivity control in the infrared (IR) spectrum. The fundamentals of PCMs are first introduced to explain the origins and principles of phase transitions. Thereafter, we discuss multiphysical nanodevices for electronic, photonic, and thermal management, attesting to the broad applications and exciting promises of PCMs. Emerging trends and valuable applications in all-optical neuromorphic devices, thermal data storage, and encryption are outlined in the end.

Light-Matter Interactions in Hybrid Material Metasurfaces

This Review focuses on the integration of plasmonic and dielectric metasurfaces with emissive or stimuli-responsive materials for manipulating light-matter interactions at the nanoscale. Metasurfaces, engineered planar structures with rationally designed building blocks, can change the local phase and intensity of electromagnetic waves at the subwavelength unit level and offers more degrees of freedom to control the flow of light. A combination of metasurfaces and nanoscale emitters facilitates access to weak and strong coupling regimes for enhanced photoluminescence, nanoscale lasing, controlled quantum emission, and formation of exciton-polaritons. In addition to emissive materials, functional materials that respond to external stimuli can be combined with metasurfaces to engineer tunable nanophotonic devices. Emerging metasurface designs including surface-functionalized, chemically tunable, and multilayer hybrid metasurfaces open prospects for diverse applications, including photocatalysis, sensing, displays, and quantum information.

Thickness-Dependent Drude Plasma Frequency in Transdimensional Plasmonic TiN

Plasmonic transdimensional materials (TDMs), which are atomically thin metals of precisely controlled thickness, are expected to exhibit large tailorability and dynamic tunability of their optical response as well as strong light confinement and nonlocal effects. Using spectroscopic ellipsometry, we characterize the complex permittivity of ultrathin films of passivated plasmonic titanium nitride with thicknesses ranging from 1 to 10 nm. By measuring passivated TiN, we experimentally distinguish between the contributions of an oxide layer and thickness to the optical properties. A decrease in the Drude plasma frequency and increase in the damping in thinner films is observed due to spatial confinement. We explain the experimental trends using a nonlocal Drude dielectric response theory based on the Keldysh-Rytova (KR) potential that predicts the thickness-dependent optical properties caused by electron confinement in plasmonic TDMs. Our experimental findings are consistent with the KR model and demonstrate quantum-confinement-induced optical properties in plasmonic transdimensional TiN.

On the Study of Advanced Nanostructured Semiconductor-Based Metamaterial

Tunable metamaterials belonging to the class of different reconfigurable optical devices have proved to be an excellent candidate for dynamic and efficient light control. However, due to the consistent optical response of metals, there are some limitations aiming to directly engineer electromagnetic resonances of widespread metal-based composites. The former is accomplished by altering the features or structures of substrates around the resonant unit cells only. In this regard, the adjusting of metallic composites has considerably weak performance. Herein, we make a step forward by providing deep insight into a direct tuning approach for semiconductor-based composites. The resonance behavior of their properties can be dramatically affected by manipulating the distribution of free carriers in unit cells under an applied voltage. The mentioned approach has been demonstrated in the case of semiconductor metamaterials by comparing the enhanced propagation of surface plasmon polaritons with a conventional semiconductor/air case. Theoretically, the presented approach provides a fertile ground to simplify the configuration of engineerable composites and provides a fertile ground for applications in ultrathin, linearly tunable, and on-chip integrated optical components. These include reconfigurable ultrathin lenses, nanoscale spatial light modulators, and optical cavities with switchable resonance modes.

Dynamic and Active THz Graphene Metamaterial Devices

In recent years, terahertz waves have attracted significant attention for their promising applications. Due to a broadband optical response, an ultra-fast relaxation time, a high nonlinear coefficient of graphene, and the flexible and controllable physical characteristics of its meta-structure, graphene metamaterial has been widely explored in interdisciplinary frontier research, especially in the technologically important terahertz (THz) frequency range. Here, graphene’s linear and nonlinear properties and typical applications of graphene metamaterial are reviewed. Specifically, the discussion focuses on applications in optically and electrically actuated terahertz amplitude, phase, and harmonic generation. The review concludes with a brief examination of potential prospects and trends in graphene metamaterial.

Two-dimensional beam steering with tunable metasurface in infrared regime

Tunable metasurfaces can change the optical properties of incident light at will such as amplitude, phase, and polarization in a time-dependent fashion. Ultrafast switching speed and the ability for the pixel size reduction of the tunable metasurface can allow various applications such as light detection and ranging, interferometric sensors, and free space optical communications, to name a few. Although there have been successful demonstrations of the wavefront shaping using the tunable metasurface, the implementation of the two-dimensional metasurface pixel array that can be individually addressed in the optical frequency regime still remains challenging. Here, we present the experimental demonstration of the two-dimensional beam steering with the metasurface array by the binary phase grating in the infrared regime. The metasurface unit cell is composed of metal-dielectric-oxide structure with the indium tin oxide as an active layer, which is modulated by using the top fan-out electrodes. The metasurface array is two-dimensionally pixelated and has the phase change above 137° in the infrared regime.

Predictable infrared dual-band narrow-band absorber for infrared detection

Dual-band infrared absorbers have received a great deal of attention for their potential applications in the field of sensing and detection. In this paper, we proposed a composite model consisting of Platinum nano-cylinder and micro-ring column stacked on top of Si₃N₄and Platinum films. The effect of geometrical parameters on spectral absorption was explored by finite difference in time domain methods, and the results revealed that there were narrow perfect absorption peaks in each of the two atmospheric window bands due to the magnetic polaritons. Meanwhile, the quantitative relationship of resonance wavelength and geometrical parameters were predicted by LC equivalent circuits. In addition, graphene was added to the structure to dynamically adjust the resonance wavelength by varying the Fermi level. The combination of graphene and microstructure achieved full coverage detection of wavelengths in the atmospheric window range. This dual-band absorber has potential applications in infrared detection because of its good absorption properties and its tunability.

High resolution multispectral spatial light modulators based on tunable Fabry-Perot nanocavities

Spatial light modulators (SLMs) are the most relevant technology for dynamic wavefront manipulation. They find diverse applications ranging from novel displays to optical and quantum communications. Among commercial SLMs for phase modulation, Liquid Crystal on Silicon (LCoS) offers the smallest pixel size and, thus, the most precise phase mapping and largest field of view (FOV). Further pixel miniaturization, however, is not possible in these devices due to inter-pixel cross-talks, which follow from the high driving voltages needed to modulate the thick liquid crystal (LC) cells that are necessary for full phase control. Newly introduced metasurface-based SLMs provide means for pixel miniaturization by modulating the phase via resonance tuning. These devices, however, are intrinsically monochromatic, limiting their use in applications requiring multi-wavelength operation. Here, we introduce a novel design allowing small pixel and multi-spectral operation. Based on LC-tunable Fabry-Perot nanocavities engineered to support multiple resonances across the visible range (including red, green and blue wavelengths), our design provides continuous 2π phase modulation with high reflectance at each of the operating wavelengths. Experimentally, we realize a device with 96 pixels (~1 μm pitch) that can be individually addressed by electrical biases. Using it, we first demonstrate multi-spectral programmable beam steering with FOV~18° and absolute efficiencies exceeding 40%. Then, we reprogram the device to achieve multi-spectral lensing with tunable focal distance and efficiencies ~27%. Our design paves the way towards a new class of SLM for future applications in displays, optical computing and beyond.

Broadband achromatic polarization-insensitive metalens in the mid-wave infrared range

Infrared imaging is widely used in astronomical observation, medical diagnosis, and military applications. In recent years, metasurface technology has provided an unparalleled platform for the development of miniaturized and integrated infrared imaging systems. However, metasurfaces normally have inevitable chromatic aberration due to the high phase dispersion of the building blocks, which makes broadband achromatic infrared imaging difficult to realize. In this paper, we propose a polarization-insensitive metalens with a numerical aperture of 0.38 that can eliminate chromatic aberration for unpolarized incidences with the wavelength ranging from 3 to 5 µm. The simulated results show that within the design bandwidth, the proposed device achieves near-diffraction limit focusing and can increase the fill factor of infrared focal plane array pixels by 2.3 times, from 11.1% to 36.4%, with an excellent optical crosstalk performance of about 2.72%. Our work may pave the way for the practical application of achromatic metalenses in mid-wave infrared imaging equipment.

Research of Gate-Tunable Phase Modulation Metasurfaces Based on Epsilon-Near-Zero Property of Indium-Tin-Oxide

In this paper, we proposed a reflection phase electrically tunable metasurface composed of an Au/Al2O3/ITO/Au grating structure. This antenna array can achieve a broad phase shift continuously and smoothly from 0° to 320° with a 5.85 V applied voltage bias. Tunability arises from field-effect modulation of the carrier concentrations or accumulation layer at the Al2O3/ITO interface, which excites electric and magnetic resonances in the epsilon-near-zero region. To make the reflected phase tuning range as wide as possible, some of the intensity of the reflected light is lost due to the excited surface plasmon effect. Simulation results show that the effect of optimal phase modulation can be realized at a wavelength range of 1550 nm by modulating the carrier concentration in our work. Additionally, we utilized an identical 13-unit array metasurface to demonstrate its application to the beam steering function. This active optical metasurface can enable a new realm of applications in ultrathin integrated photonic circuits.

Active optical metasurfaces: comprehensive review on physics, mechanisms, and prospective applications

Optical metasurfaces with subwavelength thickness hold considerable promise for future advances in fundamental optics and novel optical applications due to their unprecedented ability to control the phase, amplitude, and polarization of transmitted, reflected, and diffracted light. Introducing active functionalities to optical metasurfaces is an essential step to the development of next-generation flat optical components and devices. During the last few years, many attempts have been made to develop tunable optical metasurfaces with dynamic control of optical properties (e.g., amplitude, phase, polarization, spatial/spectral/temporal responses) and early-stage device functions (e.g., beam steering, tunable focusing, tunable color filters/absorber, dynamic hologram, etc) based on a variety of novel active materials and tunable mechanisms. These recently-developed active metasurfaces show significant promise for practical applications, but significant challenges still remain. In this review, a comprehensive overview of recently-reported tunable metasurfaces is provided which focuses on the ten major tunable metasurface mechanisms. For each type of mechanism, the performance metrics on the reported tunable metasurface are outlined, and the capabilities/limitations of each mechanism and its potential for various photonic applications are compared and summarized. This review concludes with discussion of several prospective applications, emerging technologies, and research directions based on the use of tunable optical metasurfaces. We anticipate significant new advances when the tunable mechanisms are further developed in the coming years.

Electrically tunable metasurface by using InAs in a metal-insulator-metal configuration

The ability of modulating optical properties at a lateral subwavelength scale is of crucial importance due to its potential applications for wide-angle holographic displays, optical communications, and interferometric sensors. Here, we present an electrically tunable metasurface whose optical properties can be element-wise controlled at the lateral subwavelength scale in the mid-infrared wavelength regime. Our proposed device facilitates an n-doped InAs layer as a dynamic-tunable layer, and the charge carrier concentration inside the InAs layer is tailored by external gate biases. This InAs active layer is sandwiched between top aluminum strip antennas and a bottom gold substrate, forming the metal-insulator-metal configuration. The change of the charge carrier concentration gives rise to modulation of the amplitude and phase of reflected light in a mid-infrared regime. Numerical investigations show the reflectivity contrast of 44%P with biases of -2.5-0 V and the phase change of 236° with biases of -15 V to +15 V at the wavelength of ∼5 μm. Versatile wavefront shaping such as beam focusing with Fresnel Zone plate and beam steering with saw-tooth phase grating is also provided.

Far-Infrared Near-Field Optical Imaging and Kelvin Probe Force Microscopy of Laser-Crystallized and -Amorphized Phase Change Material Ge3Sb2Te6

Chalcogenide phase change materials reversibly switch between non-volatile states with vastly different optical properties, enabling novel active nanophotonic devices. However, a fundamental understanding of their laser-switching behavior is lacking and the resulting local optical properties are unclear at the nanoscale. Here, we combine infrared scattering-type scanning near-field optical microscopy (SNOM) and Kelvin probe force microscopy (KPFM) to investigate four states of laser-switched Ge₃Sb₂Te₆ (as-deposited amorphous, crystallized, reamorphized, and recrystallized) with nanometer lateral resolution. We find SNOM to be especially sensitive to differences between crystalline and amorphous states, while KPFM has higher sensitivity to changes introduced by melt-quenching. Using illumination from a free-electron laser, we use the higher sensitivity to free charge carriers of far-infrared (THz) SNOM compared to mid-infrared SNOM and find evidence that the local conductivity of crystalline states depends on the switching process. This insight into the local switching of optical properties is essential for developing active nanophotonic devices.

Switchable polarization manipulation at the telecom wavelength based on L-shaped hybrid Au-VO2 nanoholes

We propose to achieve switchable polarization manipulation at the telecom wavelength at nanoscale based on L-shaped plasmonic nanoholes in an Au-VO₂ film. The L-shaped nanohole acts as a quarter-wave plate or a half-wave plate owing to the phase differences between different plasmon resonant modes, which is controlled by the insulator or metallic phases of VO₂. In addition, by changing the structure and removing the bottom Au layer, a switchable full-/quarter-wave plate can be achieved when VO₂ transits from the insulating state to the metallic state. Furthermore, we vary the geometrical parameters of the L-shaped hole to tune its resonant spectra and achieve a switchable full-wave plate/polarizer. The multifunctional switchable polarization manipulation abilities together with large bandwidths enable the proposed structures promising applications in nanophotonics and integrated optics.

Electro-optic polymer and silicon nitride hybrid spatial light modulators based on a metasurface

Spatial light modulators (SLMs) are important for various applications in photonics, such as near-infrared imaging, beam steering and optical communication. After decades of advances, current commercial devices are typically limited to kilohertz modulating speeds. To realize higher operating speeds, an electro-optic (EO) polymer and silicon nitride hybrid SLM has been demonstrated in this work. We utilize a specially designed metasurface to support a relatively high quality resonance and simultaneously confine most of the incident light in the active EO polymer layer. Combing with the high EO coefficient of the polymer, a clear modulation at 10 MHz with a driving voltage of Vp-p=±10 V has been observed in the proof-of-concept device. Our first-generation device leaves vast room for further improvement and may open an attractive route towards compact SLM with an RF modulation higher than 100 GHz.