Tunable Metasurface Research Articles

Liquid crystal-based wide-angle metasurface absorber with large frequency tunability and low voltage.

A tunable metasurface absorber (MA) based on polymer network liquid crystal is introduced in this paper. Despite the well-designed unit cell patterns, the proposed MA can achieve both large frequency tunability and wide-angle stability. Compared with traditional liquid crystal-based metasurfaces, the measured results suggest that the recovery time of the proposed structure was reduced by half. By applying an external voltage on the top electrode of the liquid crystal layer from 0 to a saturation voltage of 10 V, the absorption peak of the MA can be tuned from 112.7 GHz to 102.2 GHz, with a maximum frequency tunability of 9.3%, which is significantly higher than other proposed liquid crystal-based metasurfaces. Moreover, the proposed tunable absorber can maintain absorption greater than 90% with incident angles reaching up to 60° for both transverse electric and transverse magnetic polarizations. This design provides an efficient way for developing low-power consumption terahertz devices with large frequency tunability and wide-angle stability.

Broadside-coupling–enabled insulator-to-metal transition in a terahertz metasurface

We theoretically demonstrate stacked-dipole-resonators–based (broadside near-field coupling configuration) multilayer metasurfaces separated by a vanadium dioxide film to achieve stronger field confinement in the spacer (VO2) region. Under relatively intense terahertz excitation (20 Vm−1) assisted by larger area electric field confinement, insulator-to-metal transition (IMT) in VO2 spacer is realized resulting in frequency (dipole mode) and amplitude (Fano mode) tunable metasurfaces. Enhancement in probing THz field triggers much stronger field confinement inside the spacer layer leading to increased VO2 conductivity (responsible for IMT) through the Poole-Frankel effect. Such broadside coupled IMT-based terahertz metamaterials can help in realizing active meta devices for THz domain.

Design and Simulation of Tunable Infrared Absorber Based on Flexible Metasurface

Tunable metasurface plays a crucial role in the modulation of infrared. And the flexible substrate realizes the possibility for mechanical tuning without additional electronic equipment. Based on the nanoscale metasurface, a tunable infrared absorber is designed and simulated with a narrow-band absorption peak of 75.6% at 18.4 THz. A silver structure is embedded in the PDMS flexible substrate. Through the simulation of CST software and the analysis of the RLC model, it is evidenced that the infrared absorption spectrum of the metasurface appears an obvious redshift, with the increase of area. When the flexible metasurface is stretched by 132%, the frequency of absorption peak moves to 16.4 THz, which still stays above 75%. Based on stretchtuned metasurface, the absorber is expected to be applied in displacement sensing, infrared detection, and other fields.

A giantly chirality tunable terahertz metasurface based on 3D folded structure with vanadium dioxide

Compared with ordinary chiral materials, chiral metamaterials possess giant chiral optical response, which have been widely used in many optical applications, such as circular polarizers, chiral light imaging and asymmetric transmission. Here, a tunable 3D folded metasurface based on vanadium dioxide (VO2) is proposed to achieve giant chiral optical effect in the terahertz band. The 3D folded metasurface comprises of two folded antisymmetric rectangular split rings resonators (SRRs). By folding the SRRs at a certain angle, the mirror symmetric structure was broken. The results shows that the circular dichroism (CD) parameter reaches a maximum of 0.83 at the folding angle of 45° and the spin-selective transmission is up to 0.89. The designed 3D folded metasurface can operate in a wide incident angle range of 0–55°. What’s more, by adjusting the conductivity of VO2, the dynamic tuning of CD can be realized, achieving the switch between the “ON” and “OFF” states. The proposed 3D folded metasurfaces provides an approach to realize the giant CD, which shows potential application in many fields, such as chiral sensing, polarization control, spin optical communication, and optical imaging.

Ultrafast laser-induced integrated property–structure modulation of Ge2Sb2Te5 for multifunction and multilevel rewritable optical recording

Abstract In this paper, we report an approach for tuning the surface morphology and phase of Ge2Sb2Te5 (GST) by using an ultrafast laser in a one-step process. Four surface micro/nanostructures with specific phase states were sequentially formed by changing the pulse energy: the modified ripple structure, the completely crystallized structure, the ablated nanodots, and the ablated ripple structure. A high correlation existed between the surface micro/nanostructures and their property. Through integrated property–structure modulation, multifunctional optical recording could be achieved by using modified ripples with specific crystallized phase states. The geometric grating morphology caused by the volume shrinkage effect during crystallization enabled modified ripples to exhibit a structural color based on the grating’s diffraction effect. Moreover, the considerable change in the reflectivity of the crystallized area enabled easy grayscale identification. On the basis of the spatially resolved phase-transition threshold effect, the integrated modulation of the geometric nanograting proportion and degree of crystallization was conducted in multilevel states. Notably, different from the fixed ablated surface structures, the printed modified surface structures could be erased and rewritten by controlling its phase state. This paper presents a promising method for producing dynamic tunable metasurfaces, conducting optical anticounterfeiting, and achieving information storage.

Gap-plasmon-driven spin angular momentum selection of chiral metasurfaces for intensity-tunable metaholography working at visible frequencies

AbstractTunable metasurfaces can replace conventional bulky active optical modules to realize practical flat optical devices such as lenses, LiDAR, holography, and augmented reality. However, tunable metasurfaces have generally been limited to switching between two distinct states. Here, we present liquid crystal (LC) integrated chiral metasurfaces, of which the metahologram intensity can be adjusted continuously between fully ‘on’ and ‘off’ states. The chiral metasurface consists of a gap-shifted split ring resonator (SRR), and exhibits spin angular momentum selection that reflects left-circularly-polarized light but perfectly absorbs right-circularly-polarized light (99.9%). The gap-shifted SRR realizes spin angular momentum selection using a metal–dielectric–metal multilayer structure and thereby induces a strong gap-plasmonic response, achieving the maximum calculated circular dichroism in reflection (CDR) of 0.99 at the wavelength of 635 nm. With the chiral metasurface, metaholograms are demonstrated with tunable intensities using LCs that change the polarization state of the output light using an applied voltage. With the LC integrated chiral metasurfaces, 23 steps of polarization are demonstrated for the continuous tuning of the holographic image intensity, achieving measured CDR of 0.91. The proposed LC integrated spin-selective chiral metasurface provides a new resource for development of compact active optical modules with continuously-tunable intensity.

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.

Graphene and metal hybridized terahertz metasurfaces toward tunable plasmon-induced transparency effects

Tunable metasurfaces have attracted much attention recently, owing to their capability of actively controlling electromagnetic waves. Here, we propose tunable graphene-metal hybridized terahertz (THz) metasurfaces exhibiting plasmon induced transparency (PIT) effects, driven alternately by metal and graphene resonators. The proposed meta-molecule comprises a central metal (graphene) strip placed orthogonally to two side graphene (metal) strips. Then, near-field coupling between the strips is probed by translating the central strip along the length of the side strips and the evolution of the PIT effect is investigated for orthogonal polarizations of probing THz. The coupled oscillator-based analytical model explains the coupling mechanism in these metasystems. We observe the strongest coupling and the most prominent PIT effect for edge-to-edge coupling (threshold position) between the central and side strips. Further, active PIT response is achieved by altering graphene's Fermi energy. These metasurfaces can contribute in designing compact and active metasurface-based hybrid devices for THz domain.

Thermally switchable and tunable metasurface with polarization independence based on VO2 phase transition

In this paper, a thermally tunable metasurface (TTMS) based on phase material vanadium dioxide (VO2) is designed and demonstrated, which consists of VO2 patterns, metallic resonators, dielectric substrates, and a gold reflective plate. Exhibiting ultra-broadband absorption from 4.72 THz to 9.88 THz with absorptivity exceeding 90% above the phase change temperature of VO2 and realizing single-band absorption when the VO2 resonators are in the insulator phase, the proposed TTMS possesses excellent low radar cross-section (RCS) performance as well as can be tailored by controlling the temperature of the phase material VO2. Furthermore, the devised TTMS is polarization-insensitive and can work well under a wide range of incident angles. The demonstrated tunable metasurface promises potential applications in the terahertz regime, such as modulating, radar stealth, and thermal detection.

Theoretical Design of Graphene-Based Bi-Functional Tunable Terahertz Metasurfaces

Tunable multifunctional metasurface has wide application such as optical electromagnetics and material science. In this paper, a terahertz (THz) metasurface based on double graphene split-ring resonators (GSRRs) are theoretically demonstrated, integrating dual-band absorption and plasmon-induced transparency (PIT) filtering effect. The structure is composed of a monolayer of graphene arrays with periodic patterns and a metal ground surface partitioned by a silicon dioxide dielectric layer. When the initial structure of unit cell is three-layer sandwich structure (bottom metal plate), its dual-frequency absorption spectra appears two peaks at 2.50 THZ and 3.38 THz, which are 99.98% and 97.94%, respectively. Then the mechanism of double band absorption is explained by analyzing the distribution of surface current and electric intensity of the absorbent material. When the initial arrangement of the cell is a double layer structure (without the bottom metal plate), the PIT effect will occur when the incident wave is y-polarized. And in a certain range to achieve more than 90% of the transmission. In addition, CST simulations demonstrate that the designed model supports changing the operating frequency by adjusting the Fermi energy of graphene The dual-function terahertz metasurface proposed in this work has broad application prospects in broadband communication, terahertz imaging and industrial sensors.

Multi-functional tunable ultra-broadband water-based metasurface absorber with high reconfigurability

Promising microwave ultra-broadband water-based metasurface absorbers have attracted increasing attention due to their novel applications in EM radiation prevention, stealth technology, and energy harvesting. In this paper, the proposed water-based metasurface absorber, composed of mushroom-shaped and layer-shaped water with resin shell, can reach over 90% polarization-insensitive and wide-angle absorption from 7.2 to 100 GHz with a relative bandwidth of 173%. Moreover, the proposed absorber is reconfigurable to control the water content, salinity, and type of inside liquid for tunable absorption and infrared radiation with thermally tunable absorption. Since the experimental and simulated ultra-broadband absorption originates from multiple resonances concentrated on the top, the ground-free all-dielectric design is achieved. Furthermore, the proposed curved absorber is fabricated and demonstrated for omnidirectional absorption. The water-based metasurface absorber shows great potential in achieving both microwave and infrared radiation camouflage for practical shaped structures in both civil and military domains.

Liquid crystal-powered Mie resonators for electrically tunable photorealistic color gradients and dark blacks

Taking inspiration from beautiful colors in nature, structural colors produced from nanostructured metasurfaces have shown great promise as a platform for bright, highly saturated, and high-resolution colors. Both plasmonic and dielectric materials have been employed to produce static colors that fulfil the required criteria for high-performance color printing, however, for practical applications in dynamic situations, a form of tunability is desirable. Combinations of the additive color palette of red, green, and blue enable the expression of further colors beyond the three primary colors, while the simultaneous intensity modulation allows access to the full color gamut. Here, we demonstrate an electrically tunable metasurface that can represent saturated red, green, and blue pixels that can be dynamically and continuously controlled between on and off states using liquid crystals. We use this to experimentally realize ultrahigh-resolution color printing, active multicolor cryptographic applications, and tunable pixels toward high-performance full-color reflective displays.

Actively Tunable Metasurfaces via Plasmonic Nanogap Cavities with Sub-10-nm VO2 Films.

Actively tunable optical materials integrated with engineered subwavelength structures could enable novel optoelectronic devices, including reconfigurable light sources and tunable on-chip spectral filters. The phase-change material vanadium dioxide (VO2) provides a promising solid-state solution for dynamic tuning; however, previous demonstrations have been limited to thicker and often rough VO2 films or require a lattice-matched substrate for growth. Here, sub-10-nm-thick VO2 films are realized by atomic layer deposition (ALD) and integrated with plasmonic nanogap cavities to demonstrate tunable, spectrally selective absorption across 1200 nm in the near-infrared (NIR). Upon inducing the phase transition via heating, the absorption resonance is blue-shifted by as much as 60 nm. This process is reversible upon cooling and repeatable over more than ten temperature cycles. Dynamic, ultrathin VO2 films deposited by ALD, as demonstrated here, open up new potential architectures and applications where VO2 can be utilized to provide reconfigurability including three-dimensional, flexible and large-area structures.

Hydrodynamic metasurface for programming electromagnetic beam scanning on the Azimuth and elevation planes

The development of multifunctional and reconfigurable metasurfaces capable of manipulating electromagnetic waves has created new opportunities for various exciting applications. Extensive efforts have been applied to exploiting active metasurfaces with properties that can be controlled by externally controlling active components. However, previous approaches have poor switch isolation, power handling limitations due to nonlinear effects, and complex biasing networks. Therefore, dynamically tunable metasurfaces have become a burgeoning field in many research areas. This paper reports a hydrodynamic metasurface (HMS) that can be programmed to realize electromagnetic beam scanning on the azimuth and elevation planes. The proposed HMS platform incorporates four micropumps, each controlling four metasurface elements via microfluidic channels, built into the HMS base. The proposed platform regulates microfluidic flow through micropumps, causing irregularities in incident wave transmission phase. An HMS was built as a proof of concept, and far-field scanning experiments were performed. Numerical and experimental results verify the feasibility of electromagnetic beam scanning using a hydrodynamic metasurface. This work advances metasurface research, with very high potential for wide-ranging application and a promising route for replacing bulky cascading active components.

Full 2\u03c0 tunable phase modulation using avoided crossing of resonances

Active metasurfaces have been proposed as one attractive means of achieving high-resolution spatiotemporal control of optical wavefronts, having applications such as LIDAR and dynamic holography. However, achieving full, dynamic phase control has been elusive in metasurfaces. In this paper, we unveil an electrically tunable metasurface design strategy that operates near the avoided crossing of two resonances, one a spectrally narrow, over-coupled resonance and the other with a high resonance frequency tunability. This strategy displays an unprecedented upper limit of 4π range of dynamic phase modulation with no significant variations in optical amplitude, by enhancing the phase tunability through utilizing two coupled resonances. A proof-of-concept metasurface is justified analytically and verified numerically in an experimentally accessible platform using quasi-bound states in the continuum and graphene plasmon resonances, with results showing a 3π phase modulation capacity with a uniform reflection amplitude of ~0.65.

Transmission-Reflection-Integrated Multifunctional Continuously Tunable Metasurfaces for Decoupled Modulation of Acoustic Waves

The effective modulation of acoustic fields is the most important property of acoustic metasurfaces. The realization of full-space wavefront control can significantly enhance the functionality of metasurfaces; however, the existing solutions to this problem are limited by the coupled modulations of the transmitted and reflected wavefronts. In this study, we demonstrate the possibility of controlling transmitted and reflected acoustic wavefronts in a decoupled manner with a passive structure. Simulated analyses of the parameter dependences of the transmission and reflection phases reveal that these phases can be combined arbitrarily within a range of structural parameters. Meanwhile, tunable designs increase the flexibility and simplicity of the modulation of acoustic waves. Using such a tunable structure, a transmission-reflection-integrated (TRI) metasurface is designed. By applying a single TRI metasurface, multiple independent functions are simultaneously realized in the transmitted and reflected regions, which is further confirmed by pancratic multifocal focusing (performed both experimentally and theoretically) and holographic imaging simulations. The simulated, calculated, and experimental data obtained demonstrate efficient wavefront control and excellent functional-integration performance of the TRI metasurface. In this paper, we propose a decoupled method for the simultaneous manipulation of reflected and transmitted acoustic waves, which can enhance the spatial utilization and functionality of acoustic devices.

Designing fast and efficient electrically driven phase change photonics using foundry compatible waveguide-integrated microheaters

Phase change chalcogenides such as Ge2Sb2Te5 (GST) have recently enabled advanced optical devices for applications such as in-memory computing, reflective displays, tunable metasurfaces, and reconfigurable photonics. However, designing phase change optical devices with reliable and efficient electrical control is challenging due to the requirements of both high amorphization temperatures and extremely fast quenching rates for reversible switching. Here, we use a Multiphysics simulation framework to model three waveguide-integrated microheaters designed to switch optical phase change materials. We explore the effects of geometry, doping, and electrical pulse parameters to optimize the switching speed and minimize energy consumption in these optical devices.

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.

Tunable SERS Enhancement via Sub-nanometer Gap Metasurfaces.

Raman sensing is a powerful technique for detecting chemical signatures, especially when combined with optical enhancement techniques such as using substrates containing plasmonic nanostructures. In this work, we successfully demonstrated surface-enhanced Raman spectroscopy (SERS) of two analytes adsorbed onto gold nanosphere metasurfaces with tunable subnanometer gap widths. These metasurfaces, which push the bounds of previously studied SERS nanostructure feature sizes, were fabricated with precise control of the intersphere gap width to within 1 nm for gaps close to and below 1 nm. Analyte Raman spectra were measured for samples for a range of gap widths, and the surface-affected signal enhancement was found to increase with decreasing gap width, as expected and corroborated via electromagnetic field modeling. Interestingly, an enhancement quenching effect was observed below gaps of around 1 nm. We believe this to be one of the few studies of gap-width-dependent SERS for the subnanometer range, and the results suggest the potential of such methods as a probe of subnanometer scale effects at the interface between plasmonic nanostructures. With further study, we believe that tunable sub-nanometer gap metasurfaces could be a useful tool for the study of nonlocal and quantum enhancement-quenching effects. This could aid the development of optimized Raman-based sensors for a variety of applications.

On-Chip Optical Beam Manipulation with an Electrically Tunable Lithium-Niobate-on-Insulator Metasurface.

Photonic integrated circuits (PICs) have garnered increasing attention because of their high efficiency in information processing. Recently, lithium niobate on insulator (LNOI) has become a new platform for PICs with excellent properties. Several tunable devices such as on-chip tunable devices that utilize the electric-optic effect of LN have been reported. However, an on-chip electrically tunable beam modulator that can focus or deflect the wave has not yet been developed. In this study, we designed an electrically tunable LNOI metasurface for on-chip optical beam manipulation. With a carefully designed local phase profile, we realized the tunable focusing and reflection functions on the chip. As the bias voltage varies, the focusing length can be shifted up to 19.9 μm (~13λ), whereas the focusing efficiency remains greater than 72%. A continuously tunable deflection can also be achieved efficiently within a range of 0–45°. The beam modulator enhances the ability to manipulate light on LNOI chips, which is expected to promote the development of integrated on-chip photonics.