1.School of Physical Science and Technology, Southwest University, Chongqing 400715, China 2.College of Electronic Engineering, Chengdu University of Information Technology, Chengdu 610225, China 3.Research Center of Applied Electromagnetics, Nanjing University of Information Science and Technology, Nanjing 210044, China
Fund Project:Project supported by the Chongqing Research Program of Basic Research and Frontier Technology, China (Grant No. cstc2018jcyjA0572).
Received Date:27 February 2019
Accepted Date:28 March 2019
Available Online:01 June 2019
Published Online:05 June 2019
Abstract: Polarization state of electromagnetic wave has important applications in satellite communication, radar detection, and stereoscopic display imaging. Therefore, the control of polarization state of electromagnetic wave is an important direction in scientific research. The traditional method of manipulating the polarization state is mainly realized based on Faraday effect and birefringent crystal, which has a certain requirement for the material thickness (leading to large volume), and does not have broadband characteristics (leading to narrow band). Recently, metamaterial with subwavelength meta-atoms, has achieved many exotic phenomena and functionalities that cannot be found in nature. As an important branch of metamaterial-based devices, polarization converter has attracted great attention and achieved significant progress. However, most of them cannot realize ultra-broadband, high-efficiency, wide-angle, and simple geometry simultaneously. In this paper, a linear polarization converter based on a square split ring metasurface is proposed. Due to the anisotropic structure, the amplitudes of the reflected electric field along the two diagonal lines are equal, and their phase difference is 180°. As a result, the polarization direction of the incident wave can be rotated 90°. The simulation results show that the polarization conversion ratio (PCR) is higher than 90% in a frequency range from 7.12 to 18.82 GHz, which means that the relative bandwidth reaches 90%. The significant bandwidth expansion is attributed to the four electromagnetic resonances generated in a square-split-ring unit. We investigate the influence of geometric parameters on PCR in detail. We also examine the performance of the proposed structure under oblique incidence. It has little effect on the co-polarization and cross-polarization reflection coefficients when the incident angle is changed from 0° to 45°. Even if the incident angle reaches 45°, the mean PCR remains above 80%. The PCRs of the four electromagnetic resonant points are all close to 100%. Finally, we fabricate and measure the proposed polarization converter that contains $30\times30$ unit cells. The experimental results are in good agreement with the simulation results, and thus validating the design. In conclusion, we propose both theoretically and experimentally a linear polarization converter that possesses ultra-broadband, high-efficiency, wide-angle, and simple geometry simultaneously. The proposed scheme can be extended to terahertz and even optical frequencies. Keywords:polarization converter/ metamaterials/ metasurface/ ultra-broadband/ anisotropy
最后, 考察单元的几何尺寸对极化转换器性能的影响, 图6给出了极化转换率随不同结构参数的变化情况. 从图6(a)可以看出, 随着a的减小, 工作频带变窄且向高频移动. 从图6(b)可以看出, w过大或过小都会引起谐振频点的位置发生变化, 从而导致极化转换效率的下降. 从图6(c)可以看出, g过大导致带宽略微变窄, g过小会引起谐振频点的个数发生变化, 从而导致极化转换效率的下降. 从图6(d)可以看出, t过大或过小都会引起谐振频点发生变化(包括位置和个数), 从而导致极化转换效率的下降. 从整个分析可以看出, 无论结构的尺寸参数变大或变小, 极化转换器的工作带宽或工作效率都有所变差. 因此, a = 6.8 mm, w = 0.5 mm, g = 1.0 mm, t = 3.5 mm是最优的一组结构参数. 图 6 不同的结构参数对极化转换器的性能的影响 Figure6. Influence of different geometric parameters on the performance of polarization converter.
4.实验验证为了实验验证所设计的极化转换器, 采用印刷电路板工艺加工样品. 如图7(a)所示, 样品的尺寸为300 mm × 300 mm, 由30 × 30个基本结构单元组成. 样品测试在微波暗室进行, 测试系统如图7(a)所示, 两个标准的喇叭天线作为收发信号源连接到矢量网络分析仪(N5230A), 并保持两个喇叭天线在同一高度, 将样品放置在两个喇叭天线正下方, 周围围绕吸波材料以避免周围环境的干扰. 其中一个喇叭发射y极化波入射到样品表面, 另一个喇叭分别接收经样品表面反射后的y极化波和x极化波, 计算可得到相应的交叉极化和共极化反射系数. 图 7 (a) 加工样品与测试系统; (b) rxy测试结果与仿真结果; (c) ryy测试结果与仿真结果 Figure7. (a) Fabricated sample and measurement system; (b) measured result and simulation result of rxy; (c) measured result and simulation result of ryy.
图 1 基本单元结构示意图 (a) 立体图; (b) 俯视图
图 2 (a) 极化转换器的工作原理; (b) 反射波沿u和v方向的振幅及相位差
图 3 (a) 交叉极化和共极化反射系数; (b) 极化转换率
图 4 上下金属表面的电流分布 (a) 7.64 GHz; (b) 16.94 GHz
图 5 (a) 共极化反射谱; (b) 交叉极化反射谱; (c) 平均极化转换效率随入射角的变化
图 6 不同的结构参数对极化转换器的性能的影响
图 7 (a) 加工样品与测试系统; (b) rxy测试结果与仿真结果; (c) ryy测试结果与仿真结果