High-efficiency Anomalous Reflection Research Articles

Interlayer coupled dual-layer metagratings for broadband and high-efficiency anomalous reflection

Recent progress in metagratings highlights the promise of high-performance wavefront engineering devices, notably for their exterior capability to steer beams with near-unitary efficiency. However, the narrow operating bandwidth of conventional metagratings remains a significant limitation. Here, we propose and experimentally demonstrate a dual-layer metagrating, incorporating enhanced interlayer couplings to realize high-efficiency and broadband anomalous reflection within the microwave frequency band. The metagrating facilitated by both intralayer and interlayer couplings is designed through the combination of eigenmode expansion (EME) algorithm and particle swarm optimization (PSO) to significantly streamline the computational process. Our metagrating demonstrates the capacity to reroute a normally incident wave to +1 order diffraction direction across a broad spectrum, achieving an average efficiency approximately 90% within the 14.7 to 18 GHz range. This study may pave the way for future applications in sophisticated beam manipulations, including spatial dispersive devices, by harnessing the intricate dynamics of multi-layer metagratings with complex interlayer and intralayer interactions.

Near-perfect retroreflection of flexural waves via optimized elastic metagratings

The potential of elastic wave retroreflection is substantial for various applications such as structural identification and detection. However, realizing accurate retroreflection presents significant challenges owing to the need for high-efficiency anomalous reflection from boundaries. This study delves into the relatively less-explored field of flexural wave retroreflection in thin plates and proposes the use of metagratings for its realization. The present metagratings, which are built from unit cells created by possibly-overlapping circular holes, offer a simple fabrication process owing to their straightforward geometry. They hold an advantage over metasurfaces, which often struggle with wave steering because of their unmanageable higher-order diffraction modes. However, no systematic approach has been proposed so far to design metagratings for flexural wave retroreflection. To address this gap, we have established an optimization problem that identifies the optimal geometries and locations of the circular holes built into the proposed retroreflecting metagratings using a gradient-based optimizer. The design process involves diffraction grating theory and spatial fast Fourier transform. Both single and multiple retroreflections at various angles were considered and the numerical findings were substantiated by experimental results. The findings of this study imply that the investigated metagratings and design approaches have promising potential for more sophisticated wave manipulations.

Anomalous reflection with customized high-efficiency bandwidth.

Anomalous reflection from metasurfaces with 100% efficiency at optical frequencies was not achieved until an all-dielectric quasi-three-dimensional subwavelength structure was proposed. The desired nonlocal control of light waves is realized by designing phase responses of multilayer films at a single wavelength. However, a high-efficiency bandwidth is not controllable by designing only the phase response at a single wavelength. Here, we propose the use of a multilayer metasurface to achieve anomalous reflection with a customized high-efficiency bandwidth. The interference of the successive light waves scattered from the structure at multiple wavelengths is controlled by phase dispersion regulation of multilayer films. As a proof of concept, two sets of multilayer films with different phase dispersions were designed to realize broadband (∼110 nm) and narrowband (∼15 nm) anomalous reflections, both with an efficiency of over 80%. The results offer a general strategy to design high-efficiency anomalous reflection with arbitrary bandwidth and might stimulate various potential applications for metadevices.

Coiling-Up Space Metasurface for High-Efficient and Wide-angle Acoustic Wavefront Steering

The recent advent of acoustic metasurface displays tremendous potential with their unique and flexible capabilities of wavefront manipulations. In this paper, we propose an acoustic metagrating made of binary coiling-up space structures to coherently control the acoustic wavefront steering. The acoustic wave steering is based on the in-plane coherent modulation of waves in different diffraction channels. The acoustic metagrating structure with a subwavelength thickness is realized with 3D printed two coiling-up space metaunits. By adjusting structural parameters of the metaunits, the −1st-order diffraction mode can be retained, and the rest of the diffraction orders are eliminated as much as possible through destructive interference, forming a high-efficiency anomalous reflection in the scattering field. The anomalous reflection performance of the designed metagrating is achieved over a wide range of incident angles with high efficiency.

A Terahertz gradient metasurface based on hybridized dipole and quadrupole resonances

Metasurfaces have demonstrated a wide range of functions, including guided scattering and localization of electromagnetic waves, and they are a promising platform for replacing bulk components in the Terahertz frequency range. Nevertheless, precise local control of the scattered wave on a very subwavelength scale remains a challenge, with high magnitude and full 2π phase tuning being difficult to achieve with a single resonance structure. Here, we present a simple deeply subwavelength (λ/16) metamaterial unit cell based upon the coupling of dipole and quadrupole resonances, which achieves complete phase coverage as well as high reflection magnitude. We numerically demonstrate high efficiency anomalous reflection (81%) at an angle of 45°, avoiding almost all other spurious diffraction orders (0.15%). We also design a structure with a lower reflection angle of 25°, where efficiency is expected to degrade due to the presence of higher order Floquet modes. However, we observe that efficiency remains high (∼ 85%) with 0.58% spurious reflections. Hence, our design can be used in a variety of applications such as beam forming and scanning antennas, reflectarray antennas, phase shifters and beam splitters.

High-efficiency anomalous reflection of acoustic waves with a passive-lossless metasurface

We present the theoretical, numerical and experimental investigations on the high-efficiency anomalous reflection by a passive-lossless acoustic metasurface (AM). In contrast to the conventional AM, the direction of the reflected waves is controlled by the nonlinear local phase shifts in the passive-lossless AM rather than by the linear modulation. Closed-end tubes are designed to realize the passive-lossless AM. A considerable improvement of 31% in the overall efficiency is confirmed in the passive-lossless AM compared to that based on generalized reflection law when the incident and reflected angles are 0° and 75° respectively. Experimental results are in accord with finite element simulations and theoretical analyses. This advance in the proposed AM may pave a way for realizing more complex functionalities such as focusing, self-bending, and vortex generation.

Tunable graphene metasurfaces by discontinuous Pancharatnam–Berry phase shift

Metal–dielectric–graphene three-layer structures are proposed to improve the interaction of graphene micro- and nanostructures with incident waves, as the upper graphene cut-wire layer introduces a discontinuous Pancharatnam–Berry phase profile. A semi-analytical method based on the Jones calculus is conducted to investigate the conversion efficiency of cross-polarized light on this graphene metasurface for circularly polarized wave incidence, which predicts that the physical constraint (25%) of cross-coupling conversion efficiency for individual graphene layers can be overcome. Numerical simulations confirm the conclusion and demonstrate an efficiency as high as 60%. Based on this mechanism, high-efficiency anomalous reflection surfaces and flat focal mirrors are designed with the tunability of reflection angles and one order of magnitude improved focusing intensity. This method paves the way to engineering high-efficiency graphene metasurfaces for tunable electromagnetic wave manipulation.

Polarization-insensitive and broad-angle gradient metasurface with high-efficiency anomalous reflection

Polarization-insensitive metasurfaces are of great value in practical applications. In this paper, we present a polarization-insensitive reflective phase-gradient metasurface operating in optical communication band which has almost the same electromagnetic (EM) responses for both x-and y-polarized incident waves with high-efficiency anomalous reflection.The reflective metasurface employs a typical metal (Au)-insulator (SiO2)-metal (Au) structure, in which the top metal layer consists of periodic arrays of isotropic cross-shaped gold antennas, i.e. unit cells. The supercell of the metasurface is composed of five unit cells with their dimensions different from each other. The normally incident waves are reflected by the metal-grounded plane, but the reflection phases of both x-and y-polarized waves are controlled by changing the dimensions of their unit cells. Based on the finite-difference time-domain simulations, we investigate the polarization-dependent EM responses of this metasurface under the illumination of linearly polarized incident plane waves. Selecting carefully five cross-shaped gold antennas in different dimensions, we obtain polarization-insensitive metasurface with high-performance anomalous reflection in optical communication band.First, in order to investigate the polarization sensitivity of the proposed metasurface, we study the EM responses for x-and y-polarized incident waves, since arbitrary linearly-polarized EM waves can be separated into two orthogonally-polarized components. We find that the two orthogonally-polarized incident EM waves have almost the same phase and amplitude response with the phase nearly linearly changing from 0 to 2up within a supercell, hence a constant gradient of phase discontinuity is introduced and anomalous reflection will occur. We further analyze the reflected electric-field patterns and the far-field intensity distributions, from which we find that the reflected beams exhibit low-distortion wavefronts and the scattered light is predominantly reflected into the anomalous mode. As a consequence, high-efficiency anomalous reflection is realized, with a 70% reflectivity at the operating wavelength of 1480 nm. Furthermore, we look into the incident-angle dependence of the proposed metasurface, and find that the designed metasurface can exhibit polarization insensitivity within a broad incident angle ranging from -30 to 0.In conclusion, we propose a broad-angle polarization-insensitive reflective gradient metasurface with high-efficiency anomalous reflection, which has potential applications in optical communications, signal processing, displaying, imaging and other fields.

Achieving wideband polarization-independent anomalous reflection for linearly polarized waves with dispersionless phase gradient metasurfaces

In this letter, we proposed to achieve wideband polarization-independent anomalous reflection for linearly polarized (LP) waves based on the phase gradient metasurface (PGM) with a dispersionless phase gradient. A polarization-keeping metasurface reflector for circularly polarized (CP) waves was firstly presented. Based on the reflector, a reflective PGM was designed to realize dispersionless but adverse phase gradients for left- and right-handed circularly polarized (LCP and RCP, respectively) waves. Under normal incidence, high-efficiency anomalous reflection occurs for both LCP and RCP waves with opposite-signed reflection angles. Under the LP wave incidence, the reflected waves are separated into two beams due to the decomposition of LP waves into LCP and RCP waves. The anomalous reflection of LP waves is independent of the polarization angle. Both the experimental and simulated results are consistent with theoretical predictions, which convincingly verifies the wideband polarization-independent anomalous reflection of LP waves.