单层高效透射型相位梯度超表面的设计及实验验证

Design and experimental verification of single-layer high-efficiency transmissive phase-gradient metasurface

本文设计了一种单层高效透射型相位梯度超表面,并通过仿真和实验进行了验证.在圆极化波入射条件下,超表面单元的交叉极化转化率大于90%的频带范围为14-15.8 GHz.通过对单元的面内旋转可实现在保持高交叉极化透射幅度的前提下对交叉极化透射相位进行调控.基于6个旋转步进为300的超表面单元周期排布设计了一维相位梯度超表面,该超表面对左/右旋圆极化波分别形成方向相反的相位梯度,因此线极化波经过超表面后将会分离成两束对称传播的圆极化波15 GHz处的近场电场分布和远场归一化透射能量方向图的仿真结果表明,奇异透射角仿真值为33.50,与理论设计值(33.75°)符合得很好.仿真并测试了透射功率密度谱,结果表明在14.9-15.3GHz频带范围内垂直入射的线极化波被高效分离成两束圆极化波.相比于以往的透射型极化调制超表面,该超表面具有工作效率高、厚度薄、重量轻等优点,在电磁波传播和极化操控领域具有重要的应用价值.

Polarization characteristic is an important feature of electromagnetic（EM） wave.Manipulating polarization state and controlling propagation direction of EM wave by phase-gradient metasurface（PGM） have become a research hotspot in recent years.However,using transmissive PGM for polarization manipulation often suffers a low efficiency.To alleviate this problem,multilayered structure was utilized.However,it often suffered bulky volume and design complexity.Therefore,engineering a thin high-efficiency transmissive PGM with polarization manipulation is a pressing and challenging issue.In this paper,a single-layer high-efficiency transmissive PGM with cross-polarization conversion and anomalous refraction is designed.To illustrate the working mechanism,the PGM is comprehensively investigated through theoretical analysis,EM simulations and experimental measurements.The unit cell evolving from an electric-field-coupled resonator is carefully designed to exhibit a Pancharatnam-Berry phase gradient.Each rotated element irradiated separately by the normally-incident left-handed circularly polarized（LHCP） and right-handed circularly polarized（RHCP）waves is simulated in CST microwave studio.The results show that the cross-polarization transmission magnitude keeps over 0.9 and does not change as the rotation angle varies.Moreover,the phase shift is twice the rotation angles and the direction of refracted beam is opposite under the above two different polarizations.In addition,the cross-polarization conversion ratio is above 0.9 from 14 GHz to 15.8 GHz.On the premise of high transmission magnitude,the phase of the cross-polarized transmission can be freely manipulated via varying axis orientation.By spatially arranging six unit cells in rotation angle steps of 30°,a PGM with a phase difference of 60° between adjacent unit cells is designed.As is well known,linearly-polarized（LP） EM waves can be decomposed into LHCP and RHCP waves with equal amplitudes.Therefore,an LP wave through the PGM will be separated into two counterpropagating CP waves.The high-efficiency anomalous refraction of the PGM is verified from simulated near-field electric field distributions and far field normalized power patterns.The simulated refracted angle is 33.5°,which is in accordance with the theoretical designed value（33.75°）.Moreover,the transmissive power intensity spectrum under the normally-incident LP waves is simulated and measured.The simulated and measured results are in good agreement with each other,showing that the transmitted wave is perfectly split into two counterpropagating waves from 14.9 GHz to 15.3 GHz.Compared with the available transmissive PGMs,our proposed PGM features high efficiency and thin structure with only single layer,making the proposed PGM a promising alternative to manipulating propagation and polarization of EM waves.