摘要:
偏振不敏感超表面在实际应用中具有重要意义，本文提出了一种光通信波段的、对偏振不敏感的异常反射式梯度超表面，这种超表面对于x-偏振和y-偏振入射光都能够实现高效率的异常反射，表现出偏振不敏感特性，为解决传统反射式超表面的偏振敏感性问题提供了一种新途径。它采用金属(Au)-绝缘层(SiO2)-金属(Au)结构，超表面的超晶胞由五个各向同性的、尺寸不同的十字形基本结构单元组成。仿真结果表明，这种超表面结构对不同线偏振入射平面光波有几乎相同的相位和振幅响应；合理的选取五个基本结构单元的尺寸，在一个超晶胞内实现了2π相位的覆盖，反射光波阵面畸变小，而且反射光都集中到异常反射级次，在工作波长1480 nm处具有较高的异常反射率(～70%)。此外，这种结构的超表面在?30?—0?的宽入射角度范围内都具有偏振不敏感的异常反射特性。在光通信、光信号处理、显示成像等领域具有潜在的应用前景。
Abstract:
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-e?ciency 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 2π 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 wave-fronts and the scattered light is predominantly reflected into the anomalous mode. As a consequence, high-e?ciency 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-e?ciency anomalous reflection, which has potential applications in optical communications, signal processing, displaying, imaging and other fields.
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