Search

Article

x

留言板

尊敬的读者、作者、审稿人, 关于本刊的投稿、审稿、编辑和出版的任何问题, 您可以本页添加留言。我们将尽快给您答复。谢谢您的支持!

姓名
邮箱
手机号码
标题
留言内容
验证码

Ultra-wideband linear polarization converter based on square split ring

Xu Jin Li Rong-Qiang Jiang Xiao-Ping Wang Shen-Yun Han Tian-Cheng

Ultra-wideband linear polarization converter based on square split ring

Xu Jin, Li Rong-Qiang, Jiang Xiao-Ping, Wang Shen-Yun, Han Tian-Cheng
PDF
HTML
Get Citation
  • 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.
      Corresponding author: Han Tian-Cheng, tchan123@swu.edu.cn
    [1]

    Beruete M, Navarro-Cia M, Sorolla M, Campillo I 2008 J. Appl. Phys. 103 053102

    [2]

    Xu J, Li T, Lu F F, Wang S M, Zhu S N 2011 Opt. Express 19 748

    [3]

    Shi H J, Ma H F, Jiang W X, Cui T J 2012 Phys. Rev. B 86 035103

    [4]

    Smith D R, Pendry J B, Wiltshire M C K 2004 Science 305 788

    [5]

    Lee H, Xiong Y, Fang N, Srituravanich W, Durant S, Ambati M, Sun C, Zhang X 2005 New J. Phys. 7 255

    [6]

    Schurig D, Mock J J, Justice B J, Cummer S A, Pendry J B, Starr A F, Smith D R 2006 Science 314 977

    [7]

    Landy N I, Sajuyigbe S, Mock J J, Smith D R, Padilla W J 2008 Phys. Rev. Lett. 100 207402

    [8]

    Ye Y, He S 2010 Appl. Phys. Lett. 96 203501

    [9]

    Menzel C, Helgert C, Rockstuhl C, Kley E B, Tunnermann A, Pertsch T, Lederer F 2010 Phys. Rev. Lett. 104 253902

    [10]

    Shi H, Zhang A, Zheng S, Li J, Jiang Y 2014 Appl. Phys. Lett. 104 034102

    [11]

    Wang S, Liu W, Wen G 2018 Sci. Rep. 8 3791

    [12]

    Hao J, Yuan Y, Ran L, Jiang T, Kong J, Chan C T, Zhou L 2007 Phys. Rev. Lett. 99 063908

    [13]

    Feng M, Wang J, Ma H, Mo W, Ye H, Qu S 2013 J. Appl. Phys. 114 074508

    [14]

    Pu M, Chen P, Wang Y, Zhao Z, Huang C, Wang C, Ma X, Luo X 2013 Appl. Phys. Lett. 102 131906

    [15]

    Cheng Y Z, Withayachumnankul W, Upadhyay A, Headland D, Nie Y, Gong R Z, Bhaskaran M, Sriram S, Abbott D 2014 Appl. Phys. Lett. 105 181111

    [16]

    Zhang L, Zhou P, Chen H, Lu H, Xie J, Deng L 2015 Appl. Phys. B 120 617

    [17]

    Chen H, Wang J, Ma H, Qu S, Xu Z, Zhang A, Yan M, Li Y 2014 J. Appl. Phys. 115 154504

    [18]

    Ding J, Arigong B, Ren H, Zhou M, Shao J, Lin Y, Zhang H 2014 Opt. Express 22 29143

    [19]

    Shi H, Li J, Zhang A, Wang J, Xu Z 2014 Opt. Express 22 20973

    [20]

    Wen X, Zheng J 2014 Opt. Express 22 28292

    [21]

    范亚, 屈绍波, 王甲富, 张介秋, 冯明德, 张安学 2015 物理学报 64 184101

    Fan Y, Qu S B, Wang J F, Zhang J Q, Feng M D, Zhang A X 2015 Acta Phys. Sin. 64 184101

    [22]

    Gao X, Han X, Cao W, Li H, Ma H, Cui T 2015 IEEE Trans. Antennas Propag. 63 3522

    [23]

    Zhao J, Cheng Y 2017 Optik 136 52

    [24]

    余积宝, 马华, 王甲富, 冯明德, 李勇峰, 屈绍波 2015 物理学报 64 178101

    Yu J B, Ma H, Wang J F, Feng M D, Li Y F, Qu S B 2015 Acta Phys. Sin. 64 178101

    [25]

    Zhao J, Cheng Y 2016 Appl. Phys. B 122 255

    [26]

    Ding F, Wang Z, He S, Shalaev V M, Kildishev A V 2015 ACS Nano 9 4111

    [27]

    Zhang L, Zhou P, Chen H, Lu H, Xie H, Zhang L, Li E, Xie J, Deng L 2016 Sci. Rep. 6 33826

    [28]

    Zhang L, Zhou P, Lu H, Zhang L, Xie J, Deng L 2016 Opt. Mater. Express 6 1393

    [29]

    Mei Z, Ma X, Lu C, Zhao Y 2017 AIP Adv. 7 125323

    [30]

    方振华, 罗春荣, 赵晓鹏 2017 光子学报 46 1216001

    Fang Z H, Luo C R, Zhao X P 2017 Acta Photon. Sin. 46 1216001

    [31]

    Khan M I, Fraz Q, Tahir F A 2017 J. Appl. Phys. 121 045103

    [32]

    Li S, Cao X, Xu L, Zhou L, Yang H, Han J, Zhang Z, Zhang D, Liu X, Zhang C, Zheng Y, Zhao Y 2016 Sci. Rep. 5 37409

    [33]

    Sui S, Ma H, Wang J, Feng M, Pang Y, Xia S, Xu Z, Qu S 2016 Appl. Phys. Lett. 109 014104

    [34]

    韩江枫, 曹祥玉, 高军, 李思佳, 张晨 2016 物理学报 65 044201

    Han J F, Cao X Y, Gao J, Li S J, Zhang C 2016 Acta Phys. Sin. 65 044201

    [35]

    Jia Y, Liu Y, Zhang W, Gong S 2016 Appl. Phys. Lett. 109 051901

  • 图 1  基本单元结构示意图 (a) 立体图; (b) 俯视图

    Figure 1.  Schematic demonstration of the unit cell: (a) 3D view; (b) top view.

    图 2  (a) 极化转换器的工作原理; (b) 反射波沿uv方向的振幅及相位差

    Figure 2.  (a) The working principle of the proposed polarization converter; (b) amplitudes and phase difference of reflected wave along u and v directions.

    图 3  (a) 交叉极化和共极化反射系数; (b) 极化转换率

    Figure 3.  (a) Reflection coefficients of cross-polarization and co-polarization; (b) polarization conversion ratio.

    图 4  上下金属表面的电流分布 (a) 7.64 GHz; (b) 16.94 GHz

    Figure 4.  Distributions of the surface current on the metallic surfaces: (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

    Figure 5.  (a) Reflection spectra for co-polarization; (b) reflection spectra for cross-polarization; (c) mean PCR with the change of incident angle.

    图 6  不同的结构参数对极化转换器的性能的影响

    Figure 6.  Influence of different geometric parameters on the performance of polarization converter.

    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

    Figure 7.  (a) Fabricated sample and measurement system; (b) measured result and simulation result of rxy; (c) measured result and simulation result of ryy.

    表 1  与其他宽带极化转换器的对比

    Table 1.  Comparison with other wideband polarization converters.

    Ref.[17] Ref.[19] Ref.[22] Ref.[25] Ref.[27] This work
    OBa/GHz 10.60—17.50 9.65—14.16 12.40—27.96 5.70—10.30 9.20—19.20 7.12—18.82
    RBb/% 49.0 37.9 77.1 57.5 70.4 90.0
    注: aoperating bandwidth (PCR > 90%), brelative bandwidth (PCR > 90%).
    DownLoad: CSV
  • [1]

    Beruete M, Navarro-Cia M, Sorolla M, Campillo I 2008 J. Appl. Phys. 103 053102

    [2]

    Xu J, Li T, Lu F F, Wang S M, Zhu S N 2011 Opt. Express 19 748

    [3]

    Shi H J, Ma H F, Jiang W X, Cui T J 2012 Phys. Rev. B 86 035103

    [4]

    Smith D R, Pendry J B, Wiltshire M C K 2004 Science 305 788

    [5]

    Lee H, Xiong Y, Fang N, Srituravanich W, Durant S, Ambati M, Sun C, Zhang X 2005 New J. Phys. 7 255

    [6]

    Schurig D, Mock J J, Justice B J, Cummer S A, Pendry J B, Starr A F, Smith D R 2006 Science 314 977

    [7]

    Landy N I, Sajuyigbe S, Mock J J, Smith D R, Padilla W J 2008 Phys. Rev. Lett. 100 207402

    [8]

    Ye Y, He S 2010 Appl. Phys. Lett. 96 203501

    [9]

    Menzel C, Helgert C, Rockstuhl C, Kley E B, Tunnermann A, Pertsch T, Lederer F 2010 Phys. Rev. Lett. 104 253902

    [10]

    Shi H, Zhang A, Zheng S, Li J, Jiang Y 2014 Appl. Phys. Lett. 104 034102

    [11]

    Wang S, Liu W, Wen G 2018 Sci. Rep. 8 3791

    [12]

    Hao J, Yuan Y, Ran L, Jiang T, Kong J, Chan C T, Zhou L 2007 Phys. Rev. Lett. 99 063908

    [13]

    Feng M, Wang J, Ma H, Mo W, Ye H, Qu S 2013 J. Appl. Phys. 114 074508

    [14]

    Pu M, Chen P, Wang Y, Zhao Z, Huang C, Wang C, Ma X, Luo X 2013 Appl. Phys. Lett. 102 131906

    [15]

    Cheng Y Z, Withayachumnankul W, Upadhyay A, Headland D, Nie Y, Gong R Z, Bhaskaran M, Sriram S, Abbott D 2014 Appl. Phys. Lett. 105 181111

    [16]

    Zhang L, Zhou P, Chen H, Lu H, Xie J, Deng L 2015 Appl. Phys. B 120 617

    [17]

    Chen H, Wang J, Ma H, Qu S, Xu Z, Zhang A, Yan M, Li Y 2014 J. Appl. Phys. 115 154504

    [18]

    Ding J, Arigong B, Ren H, Zhou M, Shao J, Lin Y, Zhang H 2014 Opt. Express 22 29143

    [19]

    Shi H, Li J, Zhang A, Wang J, Xu Z 2014 Opt. Express 22 20973

    [20]

    Wen X, Zheng J 2014 Opt. Express 22 28292

    [21]

    范亚, 屈绍波, 王甲富, 张介秋, 冯明德, 张安学 2015 物理学报 64 184101

    Fan Y, Qu S B, Wang J F, Zhang J Q, Feng M D, Zhang A X 2015 Acta Phys. Sin. 64 184101

    [22]

    Gao X, Han X, Cao W, Li H, Ma H, Cui T 2015 IEEE Trans. Antennas Propag. 63 3522

    [23]

    Zhao J, Cheng Y 2017 Optik 136 52

    [24]

    余积宝, 马华, 王甲富, 冯明德, 李勇峰, 屈绍波 2015 物理学报 64 178101

    Yu J B, Ma H, Wang J F, Feng M D, Li Y F, Qu S B 2015 Acta Phys. Sin. 64 178101

    [25]

    Zhao J, Cheng Y 2016 Appl. Phys. B 122 255

    [26]

    Ding F, Wang Z, He S, Shalaev V M, Kildishev A V 2015 ACS Nano 9 4111

    [27]

    Zhang L, Zhou P, Chen H, Lu H, Xie H, Zhang L, Li E, Xie J, Deng L 2016 Sci. Rep. 6 33826

    [28]

    Zhang L, Zhou P, Lu H, Zhang L, Xie J, Deng L 2016 Opt. Mater. Express 6 1393

    [29]

    Mei Z, Ma X, Lu C, Zhao Y 2017 AIP Adv. 7 125323

    [30]

    方振华, 罗春荣, 赵晓鹏 2017 光子学报 46 1216001

    Fang Z H, Luo C R, Zhao X P 2017 Acta Photon. Sin. 46 1216001

    [31]

    Khan M I, Fraz Q, Tahir F A 2017 J. Appl. Phys. 121 045103

    [32]

    Li S, Cao X, Xu L, Zhou L, Yang H, Han J, Zhang Z, Zhang D, Liu X, Zhang C, Zheng Y, Zhao Y 2016 Sci. Rep. 5 37409

    [33]

    Sui S, Ma H, Wang J, Feng M, Pang Y, Xia S, Xu Z, Qu S 2016 Appl. Phys. Lett. 109 014104

    [34]

    韩江枫, 曹祥玉, 高军, 李思佳, 张晨 2016 物理学报 65 044201

    Han J F, Cao X Y, Gao J, Li S J, Zhang C 2016 Acta Phys. Sin. 65 044201

    [35]

    Jia Y, Liu Y, Zhang W, Gong S 2016 Appl. Phys. Lett. 109 051901

  • [1] Deng Jun-Hong, Li Gui-Xin. Nonlinear photonic metasurfaces. Acta Physica Sinica, 2017, 66(14): 147803. doi: 10.7498/aps.66.147803
    [2] Yu Ji-Bao, Ma Hua, Wang Jia-Fu, Feng Ming-De, Li Yong-Feng, Qu Shao-Bo. High-efficiency ultra-wideband polarization conversion metasurfaces based on split elliptical ring resonators. Acta Physica Sinica, 2015, 64(17): 178101. doi: 10.7498/aps.64.178101
    [3] Pu Ming-Bo, Wang Chang-Tao, Wang Yan-Qin, Luo Xian-Gang. Subwavelength electromagnetics below the diffraction limit. Acta Physica Sinica, 2017, 66(14): 144101. doi: 10.7498/aps.66.144101
    [4] Zhang Li-Wei, Zhao Yu-Huan, Wang Qin, Fang Kai, Li Wei-Bin, Qiao Wen-Tao. Resonance properties of surface plasmon in the anisotropic metamaterial waveguide. Acta Physica Sinica, 2012, 61(6): 068401. doi: 10.7498/aps.61.068401
    [5] Zeng Li, Liu Guo-Biao, Zhang Hai-Feng, Huang Tong. An ultrawideband linear-to-circular polarization converter based on multiphysics regulation. Acta Physica Sinica, 2019, 68(5): 054101. doi: 10.7498/aps.68.20181615
    [6] Ma Xiao-Liang, Li Xiong, Guo Ying-Hui, Zhao Ze-Yu, Luo Xian-Gang. Meta-antenna: principle, device and application. Acta Physica Sinica, 2017, 66(14): 147802. doi: 10.7498/aps.66.147802
    [7] Long Yang, Ren Jie, Jiang Hai-Tao, Sun Yong, Chen Hong. Quantum spin Hall effect in metamaterials. Acta Physica Sinica, 2017, 66(22): 227803. doi: 10.7498/aps.66.227803
    [8] Yang Jiu-Long, Yuan Qing-Chen, Chen Run-Feng, Fang Han-Lin, Xiao Fa-Jun, Li Jun-Tao, Jiang Bi-Qiang, Zhao Jian-Lin, Gan Xue-Tao. Enhanced third-harmonic generation in silicon metasurface. Acta Physica Sinica, 2019, 68(21): 214207. doi: 10.7498/aps.68.20190789
    [9] Wang Mei-Ou, Xiao Qian, Jin Xia, Cao Yan-Yan, Xu Ya-Dong. Mid-infrared large-angle high-efficiency retroreflector based on subwavelenght metallic metagrating. Acta Physica Sinica, 2020, 69(1): 014211. doi: 10.7498/aps.69.20191144
    [10] Mo Man-Man, Wen Qi-Ye, Chen Zhi, Yang Qing-Hui, Li Sheng, Jing Yu-Lan, Zhang Huai-Wu. A polarization-independent and ultra-broadband terahertz metamaterial absorber studied based on circular-truncated cone structure. Acta Physica Sinica, 2013, 62(23): 237801. doi: 10.7498/aps.62.237801
  • Citation:
Metrics
  • Abstract views:  76
  • PDF Downloads:  4
  • Cited By: 0
Publishing process
  • Received Date:  27 February 2019
  • Accepted Date:  28 March 2019
  • Available Online:  16 August 2019
  • Published Online:  01 June 2019

Ultra-wideband linear polarization converter based on square split ring

    Corresponding author: Han Tian-Cheng, tchan123@swu.edu.cn
  • 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

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.

    • 电磁波的极化是指电场矢量在垂直于传播方向的平面中的振荡方向[1]. 电磁波的极化态在卫星通信、雷达探测以及立体显示成像等方面具有重要的应用价值, 因此对电磁波极化态的调控是科学研究中的一个重要方向. 传统的电磁极化调控技术主要是通过法拉第效应和双折射晶体来实现[2,3], 对材料的厚度有一定要求(导致体积大), 且不具有宽带调制特性(导致频带窄).

      近年来, 超构材料受到了人们的广泛关注, 它具有自然界材料所不具备的物理特性, 因此能实现许多奇异的电磁现象和功能器件, 如负折射现象、超透镜、隐形斗篷和完美吸收器等[4-7]. 同时, 超构材料也被广泛用于电磁波的极化调控, 主要分为透射型和反射型极化转换器. 透射型极化转换器普遍存在效率低、频带窄的问题[8-11]. 与透射型极化转换器相比, 反射型结构在能量利用率上有明显优势, 基于H形结构单元的反射型极化转换器很好地验证了这一点[12], 但工作带宽窄的问题限制了其实际应用. 为了拓宽极化转换器的工作带宽, 研究人员提出了许多新奇的结构, 主要分为三种类型: 第一种是基于单层超构表面结构[13-31], 在拓宽工作频带和提高转换效率方面已取得了显著效果, 但仍有改进空间; 第二种是基于优化算法的拓扑结构[32,33], 该方法虽然能够有效地拓宽极化转换的带宽, 但无法满足在整个工作频带内的极化转换率均高于90%; 第三种是基于多层超构表面结构[34,35], 该方法将不可避免地增大样品的体积. 因此, 探索结构简单、易于加工、转换效率高、工作频带宽的高性能极化转换器仍是目前研究极化态操控的主要方向.

      本文利用一种简单的方形开口环超构表面结构设计了超宽带线性极化转化器, 在反射模式下能够将入射波的极化方向旋转90°. 全波仿真和实验测试结果表明, 该结构在7.12—18.82 GHz的超宽频带内极化转换率高于90%, 相对带宽达到90%. 对比已有的极化转换器, 所提出的结构更为简单从而易于加工, 并且具有转换效率高和工作频带宽的优势, 因此易于集成到相关的应用设备中.

    2.   结构设计及原理
    • 本文设计的极化转换器的单元结构如图1(a)所示, 由顶层的方形开口金属环、中间的介质层和底层的金属反射板组成. 其中, 金属均是厚度为17 μm的铜, 其电导率σ = 5.8 × 107 S/m, 介质层是厚度为3.5 mm的F4B-2, 其介电常数εr = 2.65, 损耗角正切tanδ = 0.002. 单元结构的几何参数如图1(b)所示, 其中单元周期p = 10 mm, 方形金属环的边长a = 6.8 mm, 环的宽度w = 0.5 mm, 环的开口宽度g = 1.0 mm.

      Figure 1.  Schematic demonstration of the unit cell: (a) 3D view; (b) top view.

      极化转换器的工作原理如图2(a)所示. 当y极化的电磁波垂直入射时, 其电场矢量可以沿u方向和v方向分解, 即可表示为${{{E}}_{\rm{i}}} = {{u}}{E_{{\rm{i}}u}}{{\rm{e}}^{{\rm{j}}\phi }} + $${{v}}{E_{{{\rm i}} v}}{{\rm{e}}^{{\rm{j}}\phi }}$, 经过极化转换器反射后, 其反射波可以表示为${{{E}}_{\rm{r}}} = {{u{}}r_u}{E_{{\rm{i}}u}}{{\rm{e}}^{{\rm{j}}(\phi + {\varphi _u})}} + {{v}}{r_v}{E_{{\rm{i}}v}}{{\rm{e}}^{{\rm{j}}(\phi + {\varphi _v})}}$, 其中rurv分别表示电磁波在uv方向上的反射系数. 由于所提出的超构表面具有各向异性, 这将导致经超构表面反射后的电磁波在uv方向上出现相位差(Δφ = |φuφr|). 当rurv且相位差Δφ ≈ 180°时, 沿u方向或者沿v方向的电场方向将会反向, 最终合成的电场方向就会沿着x方向, 即入射的y极化波经超构表面反射后其电场将被旋转90°成为x极化波. 基于有限元方法对提出的极化转换器进行仿真, 得到的uv方向上的反射系数rurv以及两者之间的相位差Δφ图2(b)所示. 从图中可以看出, 反射系数rurv近似相等且接近于1, 在7.12—18.82 GHz频率范围内, 相位差Δφ接近180°, 特别是在7.56, 11.84, 17.10和18.61 GHz四个频点处, 相位差Δφ = 180°.

      Figure 2.  (a) The working principle of the proposed polarization converter; (b) amplitudes and phase difference of reflected wave along u and v directions.

    3.   结果与讨论
    • 定义共极化反射系数rxx = |Exr|/|Exi|和ryy = |Eyr|/|Eyi|, 交叉极化反射系数ryx = |Eyr|/|Exi|和rxy = |Exr|/|Eyi|, 其中, 下标i, r分别表示入射波和反射波, 下标x, y表示电磁波的极化方向. 由于所提出的极化转换结构沿对角线方向对称, 共极化反射系数rxxryy相等, 交叉极化反射系数rxyryx相等. 因此, 本文仅考虑y极化波入射的情形. 为了验证所提出的超表面具有超宽带极化转换特性, 本文采用基于有限元法的频域仿真进行模拟. 得到的反射系数如图3(a)所示, 可以看出, 在7.12—18.82 GHz频带内, 交叉极化反射系数rxy大于0.945, 同时共极化反射系数ryy小于0.30. 这意味着在此频段入射的y极化波经超构表面反射后转换成了x极化波. 在7.64, 12.00, 16.94和18.66 GHz四个频点处, 交叉极化反射系数几乎等于1, 共极化反射系数等于0, 意味着入射的y极化波全部转换成了x极化波. 为了更直观地展示所提出的超构表面的极化转换性能, 图3(b)给出了该结构的极化转换率 (polarization conversion ratio, PCR), 其中PCR = rxy2/(rxy2 + ryy2). 可以看出, 在7.12—18.82 GHz频率范围内, PCR始终超过90%, 相对带宽达到90% (PCR > 90%), 并且在7.64, 12.00, 16.94和18.66 GHz四个频点处PCR接近100%.

      Figure 3.  (a) Reflection coefficients of cross-polarization and co-polarization; (b) polarization conversion ratio.

      为了进一步探究本文结构实现宽带极化转换的物理机理, 选取7.64和16.94 GHz两个谐振频点观察结构上下金属表面的电流分布, 如图4所示. 在频率为7.64 GHz时, 上下金属表面的电流方向相反, 意味着形成了磁谐振. 在频率为16.94 GHz时, 上下金属表面的电流方向相同, 意味着形成了电谐振. 本文结构会形成四个谐振频点, 通过合理的参数优化调节这四个谐振点的间距, 从而实现宽带极化转换[22].

      Figure 4.  Distributions of the surface current on the metallic surfaces: (a) 7.64 GHz; (b) 16.94 GHz.

      接下来考察本文结构在电磁波斜入射情况下的表现. 图5(a)图5(b)分别给出了共极化反射率|ryy|2和交叉极化反射率|rxy|2随入射角的变化情况. 可以看出, 在斜入射情况下, 除了少数频点以外, 整体上共极化反射率保持在0.1以下, 交叉极化反射率保持在0.9以上, 这意味着该结构在斜入射下依然保持较高的极化转换效率. 随着入射角增大, 交叉极化反射率在低频段和高频段的特定频点处急剧下降, 低频段在9.6 GHz附近且不随入射角变化, 高频段在18 GHz附近(入射角为15°时)并随入射角增大向低频偏移. 出现此现象有两点原因: 其一是对应频点的共极化反射率有所增大; 其二是对入射波的吸收增大. 例如, 当入射角为20°时, 在9.6 GHz处的|rxy|2和|ryy|2分别为17.53%和49.15%, 因此吸收率高达33.32%. 为了更加全面地评估极化转换效率随入射角的变化情况, 定义平均极化转换率(mean PCR)为$\overline {{\rm{PCR}}} =\displaystyle\frac{1}{N}\sum\limits_{i = 1}^N {{\rm{PC}}{{\rm{R}}_i}} $, 其中N表示在工作频带内的采样频点数目. 图5(c)给出了在7.12—18.82 GHz范围内平均极化转换率随入射角度的变化. 可以看出, 即使入射角达到45°, 平均极化转换率仍然保持80%以上, 表明本文结构具有较好的宽入射角特性.

      Figure 5.  (a) Reflection spectra for co-polarization; (b) reflection spectra for cross-polarization; (c) mean PCR with the change of incident angle.

      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.

      Figure 6.  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.

      Figure 7.  (a) Fabricated sample and measurement system; (b) measured result and simulation result of rxy; (c) measured result and simulation result of ryy.

      图7(b)图7(c)为测试结果和仿真结果的对比, 二者符合较好. 测试结果表明, 在7—17 GHz频带内, 共极化反射系数都低于–20 dB; 在17.0—18.8 GHz频带内, 共极化反射系数都低于–10 dB. 实验测得的谐振频率与模拟结果略有出入, 原因可能是以下两点: 一是样品的加工精度不够; 二是在数值模拟中, 周期性边界条件使得结构沿xy方向无限延伸, 而实际制备的样品尺寸有限.

      表1为本文提出的结构与以往报道的宽带极化转换器的对比结果, 指标包括工作带宽(operating bandwidth)和相对带宽(relative bandwidth). 从表1可以看出, 相比于其他反射型极化转换器, 本文提出的结构在工作带宽上更有优势.

      Ref.[17] Ref.[19] Ref.[22] Ref.[25] Ref.[27] This work
      OBa/GHz 10.60—17.50 9.65—14.16 12.40—27.96 5.70—10.30 9.20—19.20 7.12—18.82
      RBb/% 49.0 37.9 77.1 57.5 70.4 90.0
      注: aoperating bandwidth (PCR > 90%), brelative bandwidth (PCR > 90%).

      Table 1.  Comparison with other wideband polarization converters.

    5.   结 论
    • 设计并制备了一种基于方形开口环结构的反射型极化转换器, 全波仿真和实验测试结果证实, 该结构可在7.12—18.82 GHz的较宽频带内将入射波的极化方向旋转90°, 相对带宽达到90%, 在工作频带内极化转换率超过了90%, 在四个谐振频点处的转换效率几乎达到100%. 这种极化转换器在卫星通信、雷达探测以及立体显示成像等众多领域中具有广阔的应用前景. 此外, 本文提出的设计方法可拓展到太赫兹甚至光波段.

Reference (35)

Catalog

    /

    返回文章
    返回