Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 51675530)
Received Date:07 April 2020
Accepted Date:11 July 2020
Available Online:03 December 2020
Published Online:05 December 2020
Abstract:In recent years, due to their features nonexistent in natural matirials, the perfect absorbers based on metasurfaces have become a hot research point. Although great progress has been made, the absorbers with spin-selection are rarely reported. However, the absorbers with spin-selection have more widespread applications in chiral sensors and satellite communication. Therefore, a spin-selection absorber based on the metasurface with modified square split-ring structure is proposed. Firstly, the theoretical conditions for generating the spin-selection absorption are analyzed theoretically, and then the qualified metasurface unit cell is designed under the guidance of the theory. We design an asymmetric modified square split-ring resonator to break both the n-fold (n>2) rotational symmetry and mirror symmetry. The unit cell is composed of three layers, i.e. the top layer, which is a modified square split-ring, the middle layer, which is an FR4 dielectric plate with a thickness of 4 mm, and the bottom layer, which is an all-metal plate acting as the reflecting incident wave. In order to obtain the optimal performance, the designed meta-atom is optimized by CST Microwave Studio, a well-known commercial full wave simulation software.The numerical simulation results show that the unit cell can selectively absorb the right-handed circularly polarized waves and reflect left-handed circularly polarized waves at 7.2 GHz. A maximum absorption rate for the absorption of right-handed circularly polarized (RCP) waves reaches a value higher than 90%, while the absorption rate of the other spin state is kept lower than 19%. In addition, to meet the need of practical applications, the absorption performance is also further investigated under different oblique incident angles, with the wave vectors confined in the x-z plane and y-z plane, respectively. Finally, to further understand the mechanism of spin-selection absorber, the surface current distributions are also simulated for LCP and RCP wave, respectively. The different surface current distributions are obtained for incident LCP and RCP wave, which is a solid evidence for spin-selection absorption. This paper offers a reference for the generation of spin-selection absorber. The proposed method not only is suitable for microwave region, but also can be extended to higher frequencies, and hopefully it can be widely used in the field of communication. Keywords:metasurface/ absorber/ spin-selection
3.单元设计根据以上理论分析的结论, 为打破n重(n > 2)旋转对称和镜像对称, 本文在原始开口环的基础上(如图1(a)和图1(b))提出了一个改进的方形开口环结构, 如图1(c), (d)所示. 该超表面单元由三层结构组成, 顶层是厚度为0.105 mm的金属结构, 中间层是FR4介质板, 介电常数为4.2, 电正切损耗为0.025, 最底层为金属背板, 用来反射入射的电磁波. 该超表面工作在反射体系下, 优化后的结构参数为: p = 10 mm, a = 6.5 mm, g = 0.6 mm, w = 0.4 mm, l = 2 mm, d = 4 mm. 整个超表面单元的总体厚度为4.105 mm. 图 1 单元结构示意图 (a), (c) 顶视图; (b), (d) 侧视图 Figure1. Schematic of the unit cell: (a), (c) Top view; (b), (d) side view.
4.单元的仿真和实验验证采用CST MICROWAVE STUDIO (2018)软件对单元的反射系数进行仿真. 仿真的边界条件设置如图2(a)所示. 在x和y方向采用unit cell边界条件, 由于单元工作于反射体系下, 所以在Zmin方向采用electric(Et = 0)边界条件, 而Zmax方向采用open(add space)边界. 整个仿真采用圆极化波进行激励, 仿真结果如图2(b)所示. 原始单元的边界条件和仿真结果如图2(c)和图2(d)所示. 图 2 仿真边界条件设置和反射系数仿真结果 (a), (c) 边界条件设置; (b), (d) 反射系数仿真结果 Figure2. Setting of the simulation boundaries and the simulated results of reflection coefficient: (a), (c) Setting of the simulation boundaries; (b), (d) the simulated results of reflection coefficient.
式中${R_{ + - }}$(${R_{ - + }}$)表示交叉极化反射, ${R_{ - - }}$(${R_{ + + }}$)表示共极化反射. 计算的吸收率谱线如图3所示. 图 3 LCP和RCP波的吸收率谱线 Figure3. The absorption spectra of LCP and RCP wave.
从图3可以清晰地看到, 在7.2 GHz处RCP波的吸收率达到了90%以上, 而LCP波的吸收率在19%以下. 这表明入射的RCP波被高效地吸收, 而几乎所有的LCP波被反射. 在实际的应用当中, 入射的电磁波往往以不同的角度斜入射在物体表面上, 因此讨论了单元对入射角度的敏感性. 在这里分别研究了入射波矢在xoz和yoz平面内的斜入射吸波性能. 计算结果如图4所示. 图 4 两种不同情况下不同入射角度的LCP和RCP波的吸收率谱线 (a), (c) RCP波吸收谱线; (b), (d) LCP波吸收谱线 Figure4. The absorption spectra of LCP and RCP wave under different incident angles with the wave vectors confined in the x-z plane and y-z plane, respectively: (a), (c) The absorption spectra of LCP wave; (b), (d) the absorption spectra of RCP wave.
从仿真结果可以看出, 对于入射波矢在xoz面内的情况, 当入射角从0°变化到45°的时候, 虽然 RCP 波的吸收率稍微有点降低, 但是仍然维持在了77%以上, 而LCP波的吸收率在30%以下. 然而, 对于入射波矢在yoz面的情况, RCP波的吸收率随着角度的增加而逐渐降低, 但是仍保持着60%以上的吸收率. 为进一步揭示物理机理, 对谐振频率7.2 GHz处超表面单元的表面电流和能量损耗分布进行仿真. 由于金属结构的非对称性, LCP波和RCP波将在结构表面引起不同的电流分布和能量损耗. 从图5的仿真结果可以很清楚地看到, RCP波入射能够引起很强的表面电流和能量损耗, 从而实现旋向选择吸波. 图 5 在谐振频率7.2 GHz处的表面电流和能量损耗密度分布 (a)右旋波圆极化波入射下电流分布; (b)左旋圆极化波入射下电流分布; (c)右旋圆极化波入射下能量损耗; (d)左旋圆极化波入射下能量损耗 Figure5. Surface current distributions on the unit cell at 7.2 GHz: (a) Surface current under the incidence of RCP wave; (b) surface current under the incidence of LCP wave; (c) energy loss under the incidence of RCP wave; (d) energy under the incidence of LCP of wave.
为了进一步证明该方案的可行性, 采用所设计的单元加工一个包含$20 \times 20$个单元的超表面进行实验验证. 样品的实物图和测试环境如图6所示. 图 6 (a)超表面样品; (b)测试环境设置 Figure6. (a) Fabricated prototype of the proposed metasurface; (b) experimental setup.
如图6(b)所示, 在测试的过程中, 将两个工作在4—8 GHz的圆极化喇叭作为发射和接收天线连结在矢量网络分析仪的两个端口上. 喇叭与样品之间的距离满足远场条件, 确保入射在样品表面的电磁波为平面波. 图7呈现了测试和仿真结果的对比. 图 7 两种不同情况下对于不同入射角度的LCP和RCP波的仿真和测试的吸收率谱线 (a), (c) RCP波吸收谱线; (b), (d) LCP波吸收谱线 Figure7. The simulated and measured absorption spectra of LCP and RCP wave under different incident angles with the wave vectors confined in the x-z plane and y-z plane, respectively: (a), (c) The absorption spectra of LCP wave; (b), (d) the absorption spectra of RCP wave.
从图7的结果中可以看出, 测试结果和仿真结果几乎是一致的, 微小的偏差主要是样品加工过程中不可避免的误差. 所设计的单元不仅可以在微波段实现旋向的选择吸波, 而且可以很容易地扩展到高频段. 将结构参数降低到微米级别进行了如图8所示的THz频段仿真. 具体的结构参数如下: p = 10 μm, a = 6.2 μm, g = 0.6 μm, w = 0.4 μm, l = 2 μm, d = 2.5 μm. 仿真结果表明, 在15.5 THz处右旋波达到了90%以上的吸收率, 而左旋波吸收率不足20%, 这充分证明了该方案可以很容易扩展到高频段的可行性. 接下来, 将本文结果与近年来的工作进行对比, 对比结果如表1所列. 从表1中可以看到, 本文的方法在吸收率和入射角度方面表现出了较优异的性能.
表1不同方法的对比 Table1.The comparison of different approaches.
图 8 在THz频段反射系数和吸收率谱线仿真结果 (a)反射系数; (b)吸收率谱线 Figure8. The simulated results of reflection coefficient and absorption spectra at frequency band of THz: (a) The reflection coefficient; (b) the absorption spectra.