Concept Of Metasurfaces Research Articles

Acoustic reflector remotely tunable by the acoustic radiation force

Specifically structured surface can be used to control acoustic waves reflection enabling wavefront shaping tailored for desired applications such as imaging or cloaking. With the development of metasurfaces, which enable changing the reflection of an incident acoustic wave at an interface using a sub-wavelength structure, it is possible to explore new frontiers of Snell-Descartes laws. However, the control over wave propagation is pre-established when using fixed structures, directing research towards metasurfaces reconfigurable in both time and space. This work proposes a metamaterial architecture using deformations of a fluid interface between two media through the acoustic radiation pressure (ARP). An analytical and numerical study is conducted to assess the phase shift on airborne acoustic wave induced by a unit cell featuring a Helmholtz resonator tuned by the water surface elevation induced by ARP. The possibility to program in real-time the phase shift is successfully tested experimentally for an incident wave of frequency 3400 Hz. The extension to a metasurface is introduced through the juxtaposition of 15 unit cell. A retroreflection effect is then numerically demonstrated, highlighting the effectiveness of the proposed approach. This concept of real-time and non-contact reconfigurable metasurface opens new possibilities for beam deflection, acoustic holography or information encoding.

A0 mode Lamb wave propagation in a nonlinear medium and enhancement by topologically designed metasurfaces for material degradation monitoring

This paper intends to provide an application example of using metamaterials for elastic wave manipulation inside a nonlinear waveguide. The concept of phase-gradient metasurfaces, in the form of artificially architectured structures/materials, is adopted in nonlinear-guided-wave-based structural health monitoring (SHM) systems. Specifically, the second harmonic lowest-order antisymmetric Lamb waves (2nd A0 waves), generated by the mutual interaction between primary symmetric (S) mode and antisymmetric (A) mode waves, show great promise for local incipient damage monitoring. However, the mixing strength is adversely affected by the wave beam divergence, which compromises the 2nd A0 wave generation, especially in the far field. To tackle this problem, a metasurface is designed to tactically enhance the 2nd A0 waves through manipulating the phases and amplitudes of both primary waves simultaneously. After theoretically revealing the features of the 2nd A0 wave generation in a weakly nonlinear plate, an inverse-design strategy based on topology optimization is employed to tailor-make the phase gradient while ensuring the high transmission of the primary waves, thus converting the diverging cylindrical waves into quasi-plane waves. The efficacy of the design is tested in a 2nd-A0-wave-based SHM system for material degradation monitoring. Results confirm that the manipulated S and A mode waves can propagate in a quasi-planar waveform after passing the surface-mounted metasurface. Changes in material properties inside a local region of the host plate can be sensitively captured through examining the variation of the 2nd A0 wave amplitude. The concept presented here not only showcases the potential of metamaterial-enhanced 2nd A0 waves for material degradation monitoring, but also illuminates the promising direction of metamaterial-aided SHM applications in nonlinear waveguides.

Few-layer bifunctional metasurfaces enabling asymmetric and symmetric polarization-plane rotation at the subwavelength scale

We introduce and numerically validate the concept of few-layer bifunctional metasurfaces comprising two arrays of quasiplanar subwavelength resonators and a middle grid (array of rectangular holes) that offer both symmetric and asymmetric transmissions connected, respectively, with symmetric and asymmetric polarization-plane rotation functionalities. The proposed structures are thinner than λ/7\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda /7$$\\end{document} and free of diffractions. Usually, the structure’s symmetry or asymmetry, i.e. unbroken or broken spatial inversion symmetries, are considered for metasurfaces as prerequisites of the capability of symmetric or asymmetric conversion of linearly polarized waves, respectively. Due to the achieved adjustment of the resonances enabling the rotation of the polarization plane simultaneously for both orthogonal polarizations of the incident wave, the symmetric polarization-plane rotation functionality can be obtained within one subwavelength band, whereas the asymmetric polarization-plane rotation functionality associated with the asymmetric transmission is obtained within another subwavelength band. This combination of the functionalities in one subdiffraction structure is possible due to the optimal choice of the grid parameters, since they may strongly affect the coupling between the two resonator arrays. Although normal incidence is required for the targeted bifunctionality, the variations of the incidence angle can also be exploited for the enrichment of the overall functional capability. Variations of the polarization angle give another important degree of freedom. The connection between the polarization-angle dependence of cross-polarized transmission and capability of symmetric and asymmetric polarization-plane rotation functionalities is highlighted. The feasible designs of the bifunctional metasurfaces are discussed.

Broadband generation of a multi-polarization multi-beam using a receiving-transmitting metasurface

Generating multiple beams in distinct polarization states is promising in multi-mode wireless communication but still remains challenging in metasurface design. Here, we theoretically and experimentally demonstrate a concept of broadband receiving-transmitting metasurface and its application to the generation of multi-polarization multi-beam. By employing U-slot patch, an efficient receiving-transmitting element with full phase coverage is designed within a wide bandwidth. Based on this architecture, a methodology is proposed to generate dual spin-decoupled beams and then developed into the strategy of generating multiple beams at different linear polarizations. To verify our strategy, two lens antennas, respectively radiating dual-spin dual-beam and quad-polarization quad-beam, are devised. With multi-polarization multi-beam radiated, the two lens antennas are both with whole aperture efficiency above 40% within the bandwidth of 10.6–12.3 GHz (14.8%), firmly validating our strategy and design.

An Ultrathin, Fast-Response, Large-Scale Liquid-Crystal-Facilitated Multi-Functional Reconfigurable Metasurface for Comprehensive Wavefront Modulation.

The rapid advancement of prevailing communication/sensing technologies necessitates cost-effective millimeter-wave arrays equipped with a massive number of phase-shifting cells to perform complicated beamforming tasks. Conventional approaches employing semiconductor switch/varactor components or tunable materials encounter obstacles such as quantization loss, high cost, high complexity, and limited adaptability for realizing large-scale arrays. Here, a low-cost, ultrathin, fast-response, and large-scale solution relying on metasurface concepts combined together with liquid crystal (LC) materials requiring a layer thickness of only 5µm is reported. Rather than immersing resonant structures in LCs, a joint material-circuit-based strategy is devised, via integrating deep-subwavelength-thick LCs into slow-wave structures, to achieve constitutive metacells with continuous phase shifting and stable reflectivity. An LC-facilitated reconfigurable metasurface sub-system containing more than 2300 metacells is realized with its unprecedented comprehensive wavefront manipulation capacity validated through various beamforming functions, including beam focusing/steering, reconfigurable vortex beams, and tunable holograms, demonstrating a milli-second-level function-switching speed. The proposed methodology offers a paradigm shift for modulating electromagnetic waves in a non-resonating broadband fashion with fast-response and low-cost properties by exploiting functionalized LC-enabled metasurfaces. Moreover, this extremely agile metasurface-enabled antenna technology will facilitate a transformative impact on communication/sensing systems and empower new possibilities for wavefront engineering and diffractive wave calculation/inference.

Gradient High-Q Dielectric Metasurfaces for Broadband Sensing and Control of Vibrational Light-Matter Coupling.

Surface-enhanced infrared absorption spectroscopy (SEIRA) has emerged as a powerful technique for ultrasensitive chemical-specific analysis. SEIRA can be realized by employing metasurfaces that can enhance light-matter interactions in the spectral bands of molecular vibrations. Increasing sample complexity emphasizes the need for metasurfaces that can operate simultaneously at different spectral bands, both accessing rich spectral information over a broad band, and resolving subtle differences in the absorption fingerprints through narrow-band resonances. Here, a novel concept of resonance-gradient metasurfaces is introduced, where the required spectral selectivity is achieved via local high-quality-factor (high-Q) resonances, while the continuous coverage of a broad band is enabled by the gradual adjustment of the unit-cell dimensions along the planar structure. The highly tailorable design of the gradient metasurfaces provides flexibility for shaping the spectral sampling density to match the relevant bands of target analytes while keeping a compact device footprint. The versatility of the gradient metasurfaces is demonstrated through several sensing scenarios, including polymer mixture deconvolution, detecting a multistep bioassay, and identification of the onset of vibrational strong coupling regime. The proposed gradient-resonance platform significantly contributes to the rapidly evolving landscape of nonlocal metasurfaces, enabling applications in molecular detection and analysis of fundamental light-matter interaction phenomena.

Active Huygens’ metasurface based on in-situ grown conductive polymer

Abstract Active metasurfaces provide unique advantages for on-demand light manipulation at a subwavelength scale for emerging visual applications of displays, holographic projectors, optical sensors, light detection and ranging (LiDAR). These applications put stringent requirements on switching speed, cycling duration, electro-optical controllability, modulation contrast, optical efficiency and operation voltages. However, previous demonstrations focus only on particular subsets of these key performance requirements for device implementation, while the other performance metrics have remained too low for any practical use. Here, we demonstrate an active Huygens’ metasurface based on conductive polyaniline (PANI), which can be in-situ grown and optimized on the metasurface. We have achieved simultaneously on the active metasurface switching speed of 60 frame per second (fps), switching duration of more than 2000 switching cycles without noticeable degradation, hysteresis-free controllability over intermediate states, modulation contrast of over 1400 %, optical efficiency of 28 % and operation voltage range within 1 V. Such PANI-powered active metasurface design can be readily incorporated into other metasurface concepts to deliver high-reliability electrical control over its optical response, paving the way for compact and robust electro-optic metadevices.

Bianisotropic Metasurface for Spatial Separation of Multiple Harmonic Waves

An approach to tuning harmonic responses of surface impedance is introduced to spatially separate fundamental, third-harmonic, and fifth-harmonic waves by focusing on the physical implementation perspective. The concept of the bianisotropic metasurface is adopted to refract the wave toward different angles by assigning identical phase gradients at fundamental and third-harmonic frequencies. On the other hand, high-sheet impedance is realized by maintaining the in-phase condition at the fifth harmonic to bypass the incident wave without refraction. The target properties of each unit cell are implemented using I-shaped patterns based on a three-stacked-layer topology. Different sizes and intervals are applied for the I-shaped patterns, and only their horizontal lengths are tuned to achieve the desired sheet impedances. The feasibility of the proposed approach is validated for the fundamental frequency of 10 GHz and two of odd harmonics through far-and near-field measurements. The results confirm that the fabricated metasurface separates the superimposed harmonic waves toward 44 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> , 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> , and 0 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> with the measured refraction efficiencies of 65.6%, 67.2%, and 74% at 10, 30, and 50 GHz, respectively.

Highly efficient flexible structured metasurface by roll-to-roll printing for diurnal radiative cooling

An ideal radiative cooler requires accurate spectral control capability to achieve efficient thermal emission in the atmospheric transparency window (8–13 μm), low solar absorption, good stability, scalability, and a simple structure for effective diurnal radiative cooling. Flexible cooling films made from polymer relying on polymer intrinsic absorbance represent a cost-effective solution but lack accuracy in spectral control. Here, we propose and demonstrate a metasurface concept enabled by periodically arranged three-dimensional (3D) trench-like structures in a thin layer of polymer for high-performance radiative cooling. The structured polymer metasurface radiative cooler is manufactured by a roll-to-roll printing method. It exhibits superior spectral breadth and selectivity, which offers outstanding omnidirectional absorption/emission (96.1%) in the atmospheric transparency window, low solar absorption (4.8%), and high stability. Impressive cooling power of 129.8 W m−2 and temperature deduction of 7 °C on a clear sky midday have been achieved, promising broad practical applications in energy saving and passive heat dispersion fields.

Broadband absorption and asymmetric reflection of flexural wave by deep-subwavelength lossy elastic metasurface

The elastic metasurface with phase discontinuities has received much attention for realizing diverse wave steering applications. To date, the elastic metasurfaces capable of realizing the multi-reflection-enhanced absorption of flexural waves at a deep subwavelength scale have rarely been reported. Here, we propose a novel concept of deep-subwavelength lossy elastic metasurface (DLEM) composed of gradient lossy force-moment resonators with a small amount of loss. Based on the diffraction theorem, we provide an accurate approach for predicting diffracted flexural wave behaviors considering the coupling of flexural and longitudinal diffraction modes in a plate and DLEM. The underlying mechanism of high-efficiency broadband absorption and asymmetric reflection of flexural wave is theoretically revealed, which stems from multi-reflection-enhanced absorption of the n=0 and n=-1 order diffractions. For the composite subunit of the DLEM implemented by a lead block atop damping silicone rubber block, a simplified two degree-of-freedom (2DOF) mass-spring-damping model and the corresponding accurate model are both theoretically derived to reveal the deep-subwavelength mechanism of controlling the reflected wave phase. The experiments of the broadband wave absorption and asymmetric reflection are further carried out to validate the performance of the designed DLEMs. The proposed methodology opens a new perspective for broadband vibration suppression and wave manipulation of lossy elastic metasurface with a deep-subwavelength thickness.

Flexible Metasurface for Microwave-Infrared Compatible Camouflage via Particle Swarm Optimization Algorithm.

Metamaterials have the powerful ability to freely control multiband electromagnetic (EM) waves through elaborately designed "artificial atoms" and are hence in the limelight in various fields. Typically, camouflage materials manipulate wave-matter interactions to achieve the desired optical properties,in particular, various techniques are used for multiband camouflage materials in both infrared (IR) and microwave (MW) ranges to overcome the scale difference between these bands. However, in the context of components required for microwave communications, simultaneous control of IR emission and MW transmission is required, which is challenging owing to differences in the wave-matter interactions in these two bands. Herein, the state-of-the-art concept of flexible compatible camouflage metasurface (FCCM) is demonstrated, which can manipulate IR signatures while maintaining MW selective transmission simultaneously. For achieving maximum IR tunability and MW selective transmission, it is performed optimization using the particle swarm optimization (PSO) algorithm. Consequently, the FCCM exhibits compatible camouflage performance with both IR signature reduction and MW selective transmission is demonstrated, with 77.7% IR tunability and 93.8% transmission achieved for a flat FCCM. Furthermore, the FCCM reached the 89.8% IR signature reduction effect even in curved situations.

Direction-Duplex Janus Metasurface for Full-Space Electromagnetic Wave Manipulation and Holography.

Janus metasurfaces, a category of two-faced two-dimensional (2D) materials, are emerging as a promising platform for designing multifunctional metasurfaces by exploring the intrinsic propagation direction (k-direction) of electromagnetic waves. Their out-of-plane asymmetry is utilized for achieving distinct functions selectively excited by choosing the propagation directions, providing an effective strategy to meet the growing demand for the integration of more functionalities into a single optoelectronic device. Here, we propose the concept of direction-duplex Janus metasurface for full-space wave control yielding drastically different transmission and reflection wavefronts for the same polarized incidence with opposite k-directions. A series of Janus metasurface devices that enable asymmetric full-space wave manipulations, such as integrated metalens, beam generators, and fully direction-duplex meta-holography, are experimentally demonstrated. We envision the Janus metasurface platform proposed here to open new possibilities toward a broader exploration of creating sophisticated multifunctional meta-devices ranging from microwaves to optical systems.

Controlling the light diffraction through a single subwavelength metallic slit via phase gradient

In this work, we show that the concept of phase gradient metasurfaces provides a versatile way to control the diffraction of light through small holes or slits. As an example, we consider a single subwavelength metallic slit surrounded by air grooves of gradient depth that induces the expected phase gradient. It is found that for normal incident light, the phase gradient can enable unidirectional excitation of surface plasmons, which flow directionally toward the slit, resulting in extraordinary optical transmission beyond that in conventional ways. Using this scheme, unidirectional radiation of an optical dipole located inside the slit can be obtained when different phase gradients are applied to both sides of the metal plate.

Optoelectronic Metasurface for Free-Space Optical–Microwave Interactions

Photon-electron interactions are essential for many areas such as energy conversion, signal processing, and emerging quantum science. However, the current demonstrations are typically targeted to fiber and on-chip applications and lack of study in wave space. Here, we introduce a concept of optoelectronic metasurface that is capable of realizing direct and efficient optical-microwave interactions in free space. The optoelectronic metasurface is realized via a hybrid integration of microwave resonant meta-structures with a photoresponsive material. As a proof of concept, we construct an ultrathin optoelectronic metasurface using photodiodes that is bias free, which is modeled and analyzed theoretically by using the light-driven electronic excitation principle and microwave network theory. The incident laser and microwave from the free space will interact with the photodiode-based metasurface simultaneously and generate strong laser-microwave coupling, where the phase of output microwave depends on the input laser intensity. We experimentally verify that the reflected microwave phase of the optoelectronic metasurface decreases as the incident laser power becomes large, providing a distinct strategy to control the vector fields by the power intensity. Our results offer fundamentally new understanding of the metasurface capabilities and the wave-matter interactions in hybrid materials.

Active Metasurface-Based Reconfigurable Polarization Converter With Multiple and Simultaneous Functionalities

We introduce a new concept of three-states polarization converting metasurface (TS-PCM) based on p-i-n diodes. The proposed structure can offer three distinct wave manipulation functionalities: linear polarization to orthogonal linear polarization (LP-OLP), linear polarization to circular polarization (LP-CP), and linear polarization to full reflection (LP-FR) state. Unlike previous structures, the multiple conversion features are simultaneously achieved in different frequency ranges, depending on the operating state of diode ( <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> or <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> ). Finally, a sample prototype was fabricated and the experimental results were verified with the simulated ones.

Near-Field Angular Scan Enhancement of Antenna Arrays Using Metasurfaces

In this publication, we continue our previous work on extending the angular scan range of phased arrays using metasurfaces. We consider in detail scan enhancement using a single metasurface lens in the near-field of a source and provide analytical expressions for source excitation to obtain a desired beam. We show that such a device suffers from the same directivity degradation as in the case of far-field lens placement. Moreover, we discuss in detail that this approach has limitations, and one cannot place the lens arbitrarily close to the source array. We then propose a two-lens near-field scan enhancer which is not subject to the same limitations, although still subject to the same directivity degradation. Finally, we propose a binary metasurface concept which theoretically achieves desired scan enhancement without directivity degradation. Some theoretical claims are then verified via full-field simulations.

Virtual metasurfaces: reshaping electromagnetic waves in distance

Metasurface has provided unprecedented freedoms in manipulating electromagnetic (EM) waves, exhibiting fascinating functions. Conventionally, these functions are implemented right on metasurfaces, where spatial modulations on EM wave amplitudes or phases are achieved by meta-atoms. This study proposes the concept of virtual metasurface (VM), which is formed by arrays of foci away from the entity metasurface. Unlike conventional metasurfaces, spatial modulations on the amplitudes or phases of EM waves occur in the air, with a focal length distance from the entity metasurface. As a proof of concept, we demonstrated a transmissive VM. The entity metasurface consists of transmissive focusing metasurface tiles (TFMTs) with the same focal length. Two TFMTs were designed with phase difference π to enable the most typical checkerboard configuration. The TFMTs were assembled to form the entity metasurface, whereas their foci formed the VM. Due to the π phase difference among adjacent foci, EM propagation along the normal direction was cancelled, leading to four tilted far-field beams. The concept of VM can be readily extended to higher frequencies from terahertz to optical regimes and may find wide applications in communication, camouflage, and other fields.

Decoupling of Dual-Polarized Antenna Arrays Using Non-Resonant Metasurface.

A non-resonant metasurface (NRMS) concept is reported in this paper to improve the isolation of dual-polarized and wideband large-scale antenna arrays. By properly designing the NRMS, it can perform stable negative permeability and positive permittivity along the tangential direction of the NRMS within a wide band, which can be fully employed to suppress the mutual couplings of large-scale antenna arrays. At the same time, the proposed NRMS can also result in positive permittivity and permeability along the normal direction of the NRMS, which guarantees the free propagation of electromagnetic waves from antenna arrays along the normal direction. For demonstration, a 4×4 dual-polarized antenna array loading with the proposed NRMS is designed to improve the isolations of the antenna array. The simulations demonstrate that the isolations among all ports are over 24 dB from 4.36 to 4.94 GHz, which are experimentally verified by the measured results. Moreover, the radiation patterns of antenna elements are still maintained after leveraging the proposed NRMS. Due to the simple structure of the proposed NRMS, it is very promising to be widely employed for massive MIMO antenna arrays.

Liquid crystal metasurfaces for versatile electrically tunable diffraction

ABSTRACTLiquid crystal (LC) metasurfaces self-assemble on polymer alignment layers patterned by focused ion beam inducing micrometre and submicrometre scale modulations of LC director imparting specific wavefront profiles to the transmitted light required for versatile optical functionalities. We report on the design, optimisation, fabrication and characterisation of LC metasurfaces performing as Dammann gratings distributing light uniformly in many diffraction channels. Comparing them with the previously reported diffracting and beam-steering LC metasurfaces, we demonstrate how the same approach can be applied to create photonic devices with qualitatively different diffraction properties switchable between diffracting and transparent states by low voltage within several milliseconds. Analysing the achievements, we elucidate the future potential of the LC metasurface concept and discuss particular application prospects.

Design of low profile, compact, high gain and broadband metasurface-based MIMO antenna for 5G/WiMAX/WLAN applications

ABSTRACT We have presented two-port MIMO antenna designs with low-profile material. Two different MIMO antenna designs are prepared and investigated to observe their effect on the gain, bandwidth, etc. The designs are prepared based on metasurface concepts with one design having a circular ring metasurface design and second having circular disk metasurface design. The designs are showing broadband response covering frequency range of 1.85 GHz to 4.27 GHz. The bandwidth of 2.320 GHz is achieved for this metasurface MIMO antenna design. The gain and directivity results show that highest gain of 5.88 dB and directivity of 6.39 dB are achieved with circular ring metasurface MIMO antenna design. Both metasurface MIMO antenna designs are fabricated, and measured results are also provided to verify the simulation results. The broadband and high gain design are applicable for 5G communication, WiMAX and WLAN applications.