1.College of Energy, Soochow University, Suzhou 215006, China 2.Wenzheng College, Soochow University, Suzhou 215104, China 3.School of Physical Science and Technology, Soochow University, Suzhou 215006, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11974010) and the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (Grant No. 16KJB140013)
Received Date:26 July 2019
Accepted Date:03 October 2019
Available Online:07 December 2019
Published Online:05 January 2020
Abstract:How to effectively control the refraction, reflection, propagation and wavefront of dynamic waves or light has become one of hot research points in the field of optics. In the past few years, the concept of phase gradient metasurface has been proposed: it introduces a gradient of the phase discontinuity covering the entire angle 2π along the interface to provide an effective wave vector $\kappa $ and completely control the direction of outing wave. Therefore, the metasurface can possess many novel optical applications, such as holograms, metalenses, photonic spin Hall effect, etc. In this work, we design a simplified reflection-type optical metagrating. The results demonstrate that the metagrating can achive the function of two-channel retroreflection, that is, redirecting the incident wave back toward the source, with a nearly perfect conversion efficiency.The metagrating designed in this paper contains only two sub-cells with π reflection phase difference in period. The working wavelength (λ) of metagrating is fixed at 3 μm. The two sub-cells are filled with an impedance matching material (their material relative refractive indexes are n1 = 1 and n2 = 1.5 respectively and their thickness is d = 1.5 μm.).The period length range is 1.5 μm ≤ p ≤ 3 μm(considering reducing the reflection order). When the incident angle is ${\theta _{\rm{i}}}= \pm \arcsin [\lambda /(2p)]$, the absolute values of the incident angle and the reflected angle are equal, and then retroreflection occurs. When the wavelength is greater than the period ($\lambda \geqslant p$), the angle of retroreflection can be adjusted to any value ($\left| {{\theta _{\rm{i}}}} \right| \geqslant {\rm{3}}{{\rm{0}}^ \circ }$) by adjusting the period p. In this work, COMSOL MULTIPHYSICS software is used to simulate the retroreflection reflectivity and field pattern of the designed metagrating. The results verify the two-channel retroreflection property of the metagrating. In addition,as the angle of incidence changes from 30° to 60°, the efficiency of retroreflection at any incident angle can reach to more than 95%. When the incident angle is 75.4°, the metagrating still has an efficiency of 80% retroreflection. Therefore, the metagrating also achieves the function of high-efficiency retroreflection at a large-angle. Comparing with multiple sub-cells’ metasurface, the simplified metagrating with two sub-cells enables a similar function of retroreflection, but has many potential advantages, and can play an important role in high-efficiency sensing, imaging and communication. Keywords:subwavelength metallic metagrating/ metasurface/ retroreflection/ large-angle
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2.逆向反射超构光栅设计原理图1是设计的超构光栅的结构示意图及原理. 如图1(a)所示, 超构光栅中的灰色区域表示金属银, 蓝色和粉色区域表示具有周期性重复的两个结构单元, 并且这两个单元具有π的反射相位差. 图1(b)表示超构光栅一个周期的结构, 周期长度为p, 包含两个结构单元. 两个单元宽度均为w, 厚度均为d, 单元内填充不同的阻抗匹配材料, 材料折射率分别为${n_1}$和${n_{\rm{2}}}$, ${n_1}$和${n_{\rm{2}}}$满足${n_2} - {n_1} = \lambda /(4 d)$的关系. 由于x方向上超构光栅界面处满足切向动量守恒, 因此入射角和反射角需满足以下关系式[29]: 图 1 超构光栅的结构示意图 (a)逆向反射超构光栅的示意图, 其中红色和绿色箭头均表示回射, 蓝色箭头表示镜面反射; (b)超构光栅的结构单元示意图; (c)超构光栅入射和反射的等频图 Figure1. The structute of the metagrating: (a) The schematic of the retroreflection metagrating, wherein red and green arrows indicate retroreflection and blue arrows indicate specular reflection; (b) the diagram of metagrating with two sub-cells; (c) the iso-frequency contours of the incident wave and reflection wave for the metagrating.