Gradient-index Metasurfaces Research Articles

Metasurface with two-dimensional gradient-index phase distribution constructed by rotation ellipse elements for backward RCS reduction

ABSTRACT In this letter, the realization of minimizing the scattering of the Ku band is investigated by designing a two-dimensional gradient-index metasurface to control the propagation of the incident waves. Through the gradient phase design, the maximum RCS reduction was observed at 14 GHz, the x-polarized plane wave exhibits an RCS reduction of 15.43 dB, while the y-polarized plane wave shows an RCS reduction of 13.13 dB compared to the same size of PEC. The proposed metasurface with low backward RCS shows the capability to scatter the incident plane wave in different directions at both normal and oblique incidence. According to equation 2, by calculating the variable phase distribution of large-scale arrays at different frequencies, the maximum reduction of RCS can be obtained at any frequencies. Therefore, this work effectively designs and suggests the research of low RCS metasurface at different frequencies.

Polarization-insensitive composite gradient-index metasurface array for microwave power reception

A composite gradient-index metasurface (C-GM) array for microwave power reception (MPR) operating at 5.8 GHz center frequency was introduced. The array is insensitive to the polarization of incident plane waves and possesses cosine gradient modulation characteristics, which can convert the incident plane waves into surface waves focused at the center of the array. The focused surface wave energy is then collected by an omnidirectional antenna and fed back to a single rectifier for radio frequency (RF) to direct current (DC) conversion, resulting in DC output. Simulation results show that the array can fully excite surface waves under different polarized plane wave excitations, and the omnidirectional antenna has good matching characteristics with the array, achieving an energy collection efficiency of 66.52%. The array was fabricated and measured, achieving an energy reception efficiency of 59.91% and an RF-DC conversion efficiency of 34.83%, which is consistent with the simulation results. The proposed C-GM array can capture higher electromagnetic power by simply increasing its size, without the need for additional rectifiers or complex matching networks, making it a flexible and efficient MPR solution for different application scenarios.

Gradient Index Metasurface Lens for Microwave Imaging.

This paper presents the design, simulation and experimental validation of a gradient-index (GRIN) metasurface lens operating at 8 GHz for microwave imaging applications. The unit cell of the metasurface consists of an electric-LC (ELC) resonator. The effective refractive index of the metasurface is controlled by varying the capacitive gap at the center of the unit cell. This allows the design of a gradient index surface. A one-dimensional gradient index lens is designed and tested at first to describe the operational principle of such lenses. The design methodology is extended to a 2D gradient index lens for its potential application as a microwave imaging device. The metasurface lenses are designed and analyzed using full-wave finite element (FEM) solver. The proposed 2D lens has an aperture of size 119 mm (3.17λ) × 119 mm (3.17λ) and thickness of only 0.6 mm (0.016λ). Horn antenna is used as source of plane waves incident on the lens to evaluate the focusing performance. Field distributions of the theoretical designs and fabricated lenses are analyzed and are shown to be in good agreement. A microwave nondestructive evaluation (NDE) experiment is performed with the 2D prototype lens to image a machined groove in a Teflon sample placed at the focal plane of the lens.

Efficient asymmetric acoustic transmission based on the combination of a binary metasurface and a gradient index metasurface

We design and demonstrate the asymmetric acoustic transmission based on the combination of a binary metasurface (BM) and a gradient index metasurface (GIM). It yields efficient asymmetric acoustic transmission for two opposite incident directions: nearly total transmission for positive incidence but nearly total reflection for negative incidence. The underlying mechanism is ascribed to the diffraction characteristics of the BM and the GIM. We exemplify the BM and the GIM by coating unit cells consisting of three-layers of acoustic labyrinthine metamaterials, which provide full phase control and total transmission. Numerical simulations agree well with the theoretical analysis.

High-Efficiency Continuous-Flow Microwave Heating System Based on Asymmetric Propagation Waveguide

Continuous-flow microwave heating is widely used in chemical engineering and other fields. However, during microwave heating, the load’s electrical permittivity change will affect microwave heating efficiency. In this article, a WR430 asymmetric propagation waveguide is proposed based on gradient-index metasurfaces (GIMs), which can achieve high-efficiency continuous-flow heating with a large range of permittivity. First, asymmetric microwave propagation is realized in a WR430 3-D waveguide model, and the influence of metasurfaces index, thickness, and discrete element size is analyzed. Second, the asymmetric propagation waveguide is realized by fluted medium based on the permittivity equivalent method. The results show that the microwave inside the waveguide can only propagate along one direction at 2.4–2.6 GHz. Finally, the proposed waveguide is applied to a continuous-flow heating system. Compared with the single-mode cavity, multimode cavity, and waveguide water load, the proposed method can improve heating robustness and efficiency with different loads. Experiments show that the microwave absorption efficiency is stable and greater than 90% in the proposed system when the permittivity of the load changes drastically. The proposed system has a simple structure and strong applicability, which can replace the three-stub tuner and get rid of the circulator and water load for continuous-flow heating.

Asymmetric sandwich-like elements for bianisotropic acoustic metasurfaces

Conventional gradient metasurface suffers from low efficiency because of impedance mismatching, especially in large-angle manipulation. Here we propose an asymmetric sandwich-like material element to design bianisotropic metasurface for full-angle high-efficiency anomalous refractive manipulation. The proposed asymmetric element is composed of two different auxiliary layers attached to the core layer which can be viewed as a conventional gradient index metasurface, and we demonstrate analytically that these sandwich-like structures can exhibit a bianisotropic response and meet the normal impedance matching condition at both sides of the metasurface. Besides, this material design concept can be realized via the space-coiling structures. The availability of the element is first demonstrated by a nearly total refraction at an extreme refracted angle (up to 80°). Subsequently, these elements are used for the transition connection of different waveguides and dramatically improve the transmission performance. Our method brings new possibilities for the design and application of bianisotropic metasurfaces.

High-efficiency electrically direction-controllable spoof surface plasmon polaritons coupler

We propose a design of high-efficiency and direction-controllable spoof surface plasmon polaritons (SSPPs) coupler based on a Pancharatnam–Berry (PB) metasurface and a diode-controlled, linear-to-circular polarization conversion metasurface (PCM). The PB metasurface was designed to achieve high-efficiency excitation of SSPPs by manipulating the phase distribution. The PCM was placed above the PB metasurface at a certain distance, and the propagation direction of SSPPs could be controlled by changing the bias voltage of PCM. To validate the feasibility of the proposed design, a SSPPs coupler was fabricated and assessed. The experimental results were in good agreement with the simulation results. The conversion efficiency from the free space wave to SSPPs was obtained to be as high as 0.76 at 10 GHz, and the propagation direction of SSPPs became controllable under the normal incidence of x-polarized waves on the PCM. Compared to conventional devices such as prism, grating, and gradient-index metasurfaces, the proposed SSPPs coupler is more suitable for SSPPs excitation, thereby providing an interesting route toward developing plasmonic devices.

Planar microwave retroreflector based on transmissive gradient index metasurface

In this paper, a novel planar microwave retroreflector based on a transmissive gradient metasurface combined with a curved metal mirror is proposed and demonstrated. The transmissive metasurface can efficiently converge a wide-angle incident wave to a pre-designed curved metal mirror behind it with a proper distance, which acts as an effective reflective surface that can greatly enhance the backscattering of the incident wave with a wide-angle view. According to the full-wave simulations, the proposed metasurface retroreflector can perform an excellent retroreflective effect for incident microwaves of angle view between −30° and 30° range. A prototype was fabricated and the experimental results verify that the metasurface retroreflector can realize the monostatic radar cross section (RCS) enhancement with a continuous wide incident angle view from −30° to 30° at 10 GHz within a stable 3 dB RCS level. It is further demonstrated that the excellent wide-angle backscattering performance (absolute RCS enhancement value, operational bandwidth and/or incident angle view) of the proposed microwave metasurface retroreflector is competitive against the traditional trihedral corner reflector with comparable dimensions, thus opening up new possibilities to substitute the traditional bulky radar retroreflector by using a planar compact metasurface structure for microwave engineering. The presented microwave metasurface retroreflector is promising to develop into a low-profile, light weight and planar radar retroreflector which possesses tremendous RCS backscattering enhancement and wide-angle view operation range.

Design and demonstration of a wide-angle and high-efficient planar metasurface lens

In this paper, a gradient index metasurface lens (metalens) for planar wide-angle and high-efficient focusing is designed, fabricated and tested. By properly designing the ultra-thin double-layer metasurfaces with gradient meta-atoms, the proposed metalens can focus wide-angle incident wave efficiently working at transmission regime. The numerical and experimental results demonstrate that the metalens can focus oblique incident wave continuously up to 40°within half maximum focused field intensity of normal one on one side. Besides, the accurate focused spot mapping dependent on both the normal and oblique incident angle is established and analyzed quantitatively. The designed metalens demonstrate good achromatic capability with large spatial incident angle view. This work of wide-angle and high-efficient metalens enhances the performance of conventional ones which operate within narrow incident angle range and can opens up new ways to develop some novel ultra-thin devices like wide-angle scanning antenna and planar retro-reflector in microwave band.

Broadband high-quality airy beams via lossy acoustic gradient-index metasurfaces

In this study, we show that fine manipulation of sound waves can be achieved through lossy acoustic gradient-index metasurfaces by utilizing the energy losses. This work, provides a practical route for achieving independent and arbitrary modulation of the amplitude and phase of sound. We demonstrate this mechanism by producing broadband high-quality airy beams. Furthermore, we realize the multifocal focusing based on this lossy acoustic gradient-index metasurfaces. Our findings increase the freedom and flexibility of the control of sound transmission.

Extremely Asymmetrical Acoustic Metasurface Mirror at the Exceptional Point.

Previous research has attempted to minimize the influence of loss in reflection- and transmission-type acoustic metasurfaces. This Letter shows that, by treating the acoustic metasurface as a non-Hermitian system and by harnessing loss, unconventional wave behaviors that do not exist in lossless metasurfaces can be uncovered. Specifically, we theoretically and experimentally demonstrate a non-Hermitian acoustic metasurface mirror featuring extremely asymmetrical reflection at the exception point. As an example, the metasurface mirror is designed to have high-efficiency retroreflection when the wave comes from one side and near-perfect absorption when the wave comes from the opposite side. This work marries conventional gradient index metasurfaces with the exceptional point from non-Hermitian systems, and it paves the way for identifying new mechanisms and functionalities for wave manipulation.

Wavefront shaping in underwater with soft gradient-index metasurfaces at ultrasonic frequencies

Metasurfaces are planar metamaterials with a subwavelength thickness that allows wavefront shaping by introducing in-plane variations, namely, gradients, in the spatial wave response of these flat structures. Most of the recently reported acoustic metasurfaces were designed to control air-borne waves. Here, we report a new class of acoustic gradient-index (GRIN) metasurfaces engineered from soft graded-porous silicone rubber with a high and tunable acoustic index for broadband ultrasonic 3-D wavefront shaping in aqueous media. The functionalities of these soft flat lenses are illustrated through various experiments, which demonstrate beam steering and beam focusing, as well as vortex beam generation in free water. These new GRIN metasurfaces may have important applications in various domains using designed ultrasonic fields (biomedical imaging, industrial non-destructive testing, contactless particle manipulation), since their fabrication is very straightforward with common polymer science engineering. Y. Jin, R. Kumar, O. Poncelet, O. Mondain-Monval, and T. Brunet, Nat. Commun. 10, 143 (2019).

Plasmonic metasurface Luneburg lens

We present a new design of a plasmonic Luneburg lens made from a gradient-index metasurface that was constructed with an array of nanometer-sized holes in a dielectric thin film. The fabricated structure consists of a planar lens with a diameter of 8.7 μm composed of a rectangular array of holes with a periodicity of 300 nm. The experimental characterization includes leakage-radiation microscopy imaging in the direction and frequency space. The former allows for characterization of the point spread function and phase distribution, whereas the latter grants access to qualitative measurements of the effective mode indices inside the plasmonic lens. The experimental results presented here are in good agreement with the expected average performance predicted by the numerical calculations. Nevertheless, the robustness of the characterization techniques presented here is also exploited to determine deviations from the design parameters.

Design and studies on gradient index metasurfaces for broadband polarization-independent, subwavelength, and dichroic focusing.

This paper presents designs for and simulation studies on planar gradient index metasurfaces for polarization-independent and dichroic subwavelength focusing for broadband applications. Polarization-independent lenses are designed based on dielectric (Si3N4 and TiO2) gradient nanopillars. Dichroic metalenses are designed based on gradient aluminum nanohelices for helicity-dependent focusing of circularly polarized light. The helical shape is considered due to its sensitivity to circularly polarized light of a specific handedness depending upon the orientation of the helices in the lattice; this may help in 3D imaging. In the designed metalenses, the variation in the spatial dimension (fill factor) is in a gradient manner, which leads to directional bending of electromagnetic waves, and strong coupling between the bent electromagnetic waves leads to subwavelength focusing over the high numerical aperture. The designed metasurface can be materialized through multibeam interference using a combination of n plane beams and a nondiffracting Bessel beam of either zeroth or first order presented through the simulated irradiance profile and a proposed single-step experimental setup. The designed TiO2-based metalens focuses the incident arbitrary or plane polarized light to a spot sized 0.314λ, at a wavelength of 635 nm, that is based on Si3N4, enabling polarization-independent subwavelength focusing over a broad (436-810 nm) wavelength range. Realization of these lenses will enable polarization-independent high-numerical-aperture focusing and super-resolution real-time imaging of biological samples.

Fabrication and experimental demonstration of a hybrid resonant acoustic gradient index metasurface at 40 kHz

Over the past few years, acoustic gradient index metasurfaces (GIMs) have been actively studied for the numerous wave control capabilities that they facilitate. Previous research, however, has primarily focused on GIMs that operate in the audible frequency range, due to the difficulties in fabricating such intricate structures at the millimeter and submillimeter scales, for ultrasonic applications. In this work, we design, fabricate, and experimentally demonstrate the working of a hybrid resonant acoustic gradient index metasurface for airborne ultrasound at 40 kHz. The fabrication of such a GIM is made possible by projection microstereolithography, an emerging additive manufacturing technique. Numerical simulations were conducted to verify the metasurface design, and experiments were performed to corroborate these simulations. The stronger dissipation associated with airborne ultrasound is highlighted in this paper. The experimental demonstration of such a metasurface for airborne ultrasound could further its prospects as a candidate for miniaturized acoustic devices.

Subwavelength Sound Focusing and Imaging Via Gradient Metasurface-Enabled Spoof Surface Acoustic Wave Modulation

Although focusing sound waves beyond the diffraction limit offers possibilities for many applications, such as acoustic detection, sensing, and imaging, experimental realization remains a challenge. This study offers a straightforward way of superfocusing sound, through modulating spoof surface acoustic waves along a gradient-index metasurface. The same system also allows subwavelength acoustic imaging. Intriguingly, the enhanced and confined sound field is measurable even away from the metasurface, allowing ``visualization'' of the entire sound-field evolution process at subwavelength scales. This capability suggests an enhanced platform for observing many phenomena in wave physics.

Fabrication and measurement of a hybrid resonant gradient index metasurface at 40kHz

Over the past decade, gradient index metasurfaces(GIMs) have been voraciously studied for the numerous wave control capabilities that they facilitate. In this regard, a hybrid structure consisting of shunted Helmholtz resonators and a straight channel is often chosen as building blocks of the metasurface. Prior research, however, has primarily focused on GIMs that operate in the audible frequency range, due to the difficulties in fabricating such intricate structures at the millimeter and sub-millimeter scales, for ultrasonic applications. In this paper, we design, fabricate and experimentally realize a gradient index metasurface for airborne ultrasound at 40kHz. The fabrication of such a GIM is made possible by projection micro-stereolithography, an emerging additive manufacturing technique capable of micro-scale, high aspect-ratio features over a wide area. Simulations were first conducted to verify the metasurface design. Experiments were subsequently performed to corroborate the simulations and theory. The challenges faced by the thermoviscous effects, their usefulness in certain applications and optimal designs for minimal dissipation are discussed.

Flat acoustics with soft gradient-index metasurfaces

There is an important demand for the manufacture of materials allowing a precise control of the propagation of acoustic waves. This depends, among other things, on the speed of propagation of the waves in the material, which is expressed by the so-called acoustic index: n = C L,ref / C L,mat where C L,ref and C L,mat are the propagation velocities of acoustic waves in a reference medium (water for us) and in the material, respectively. We have recently shown that silicone-based porous polymer materials possess exceptionally high and scalable propagation indices, which can be tuned by a precise control of the polymer's mechanical properties and porosity [1]. Our materials are obtained using an emulsion templating method coupled with a drying process that will be described in my presentation. Using this experimental approach, we were able to produce materials with an acoustic index gradient allowing, among other things, the development of a flat acoustic lens [2]. I will present the synthesis and the characterization of these materials as well as their acoustic properties. [1] A. Kovalenko, M. Fauquignon, T. Brunet, and O. Mondain-Monval, “Tuning the sound speed in macroporous polymers with hard or soft matrix,” Soft Matter 13, 4526–4532 (2017). [2] Y. Jin, R. Kumar, O. Poncelet, O. Mondain-Monval, and T. Brunet, “Flat acoustics with soft gradient-index metasurfaces”, Nat. Commun. (to be published).

Optimization and uncertainty quantification of gradient index metasurfaces [Invited

The design of intrinsically flat two-dimensional optical components, i.e., metasurfaces, generally requires an extensive parameter search to target the appropriate scattering properties of their constituting building blocks. Such design methodologies neglect important near-field interaction effects, playing an essential role in limiting the device performance. Optimization of transmission, phase-addressing and broadband performances of metasurfaces require new numerical tools. Additionally, uncertainties and systematic fabrication errors should be analysed. These estimations, of critical importance in the case of large production of metaoptics components, are useful to further project their deployment in industrial applications. Here, we report on a computational methodology to optimize metasurface designs. We complement this computational methodology by quantifying the impact of fabrication uncertainties on the experimentally characterized components. This analysis provides general perspectives on the overall metaoptics performances, giving an idea of the expected average behavior of a large number of devices.

Flat acoustics with soft gradient-index metasurfaces

Recently, metasurfaces have been proven to be effective and compact devices for the design of arbitrary wavefronts. Metasurfaces are planar metamaterials with a subwavelength thickness that allows wavefront shaping by introducing in-plane variations, namely, gradients, in the spatial wave response of these flat structures. Here we report a new class of acoustic gradient-index (GRIN) metasurfaces engineered from soft graded-porous silicone rubber with a high acoustic index for broadband ultrasonic three-dimensional wavefront shaping in water. The functionalities of these soft flat lenses are illustrated through various experiments, which demonstrate beam steering and beam focusing, as well as vortex beam generation in free space. These new GRIN metasurfaces may have important applications in various domains using designed ultrasonic fields (biomedical imaging, industrial non-destructive testing, contactless particle manipulation), since their fabrication is very straightforward with common polymer science engineering.