Mid-infrared large-angle high-efficiency retroreflector based on subwavelenght metallic metagrating
Authors: Wang Mei-Ou , Xiao Qian , Jin Xia , Cao Yan-Yan , Xu Ya-Dong ,   College of Energy, Soochow University, Suzhou 215006, China,   Wenzheng College, Soochow University, Suzhou 215104, China,   School of Physical Science and Technology, Soochow University, Suzhou 215006, China
Published: December 2020

<sec>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 2<i>π</i> along the interface to provide an effective wave vector <inline-formula><tex-math id="M190">\begin{document}$\kappa $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191144_M190.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191144_M190.png"/></alternatives></inline-formula> and completely control the direction of outing wave. Therefore, the metasurface can possess many novel optical applications, such as holograms, metalenses, photonic spin Hall effect, etc. In this work, we design a simplified reflection-type optical metagrating. The results demonstrate that the metagrating can achive the function of two-channel retroreflection, that is, redirecting the incident wave back toward the source, with a nearly perfect conversion efficiency.</sec><sec>The metagrating designed in this paper contains only two sub-cells with <i>π</i> reflection phase difference in period. The working wavelength (<i>λ</i>) of metagrating is fixed at 3 μm. The two sub-cells are filled with an impedance matching material (their material relative refractive indexes are <i>n</i><sub>1</sub> = 1 and <i>n</i><sub>2</sub> = 1.5 respectively and their thickness is <i>d</i> = 1.5 μm.).The period length range is 1.5 μm ≤ <i>p</i> ≤ 3 μm(considering reducing the reflection order). When the incident angle is <inline-formula><tex-math id="M1">\begin{document}${\theta _{\rm{i}}}= \pm \arcsin [\lambda /(2p)]$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191144_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191144_M1.png"/></alternatives></inline-formula>, the absolute values of the incident angle and the reflected angle are equal, and then retroreflection occurs. When the wavelength is greater than the period (<inline-formula><tex-math id="M2">\begin{document}$\lambda \geqslant p$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191144_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191144_M2.png"/></alternatives></inline-formula>), the angle of retroreflection can be adjusted to any value (<inline-formula><tex-math id="M3">\begin{document}$\left| {{\theta _{\rm{i}}}} \right| \geqslant {\rm{3}}{{\rm{0}}^ \circ }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191144_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191144_M3.png"/></alternatives></inline-formula>) by adjusting the period <i>p</i>. 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.</sec>