VO<sub>2 </sub>based terahertz anisotropic coding metasurface
Authors: Wang Jing-Li , Yang Zhi-Xiong , Dong Xian-Chao , Yin Liang , Wan Hong-Dan , Chen He-Ming , Zhong Kai ,   College of Electronic and Optical Engineering & College of Flexible Electronics (Future Technology), Nanjing University of Posts and Telecommunications, Nanjing 210023, China,   Bell Honors School, Nanjing University of Posts and Telecommunications, Nanjing 210023, China,   Key Laboratory of the Ministry of Education on Optoelectronic Information Technology, School of Precision Instrument and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China
Published: March 2023

Terahertz (THz) wave has the advantages of high resolution, large information capacity, easy beam focusing, etc, and can be used in the fields of communication, radar, detection and others. Firstly, as a two-dimensional artificial electromagnetic metamaterial, the coding metasurface is proposed in the microwave band. It uses the digital coding of the electromagnetic wave phase to adjust electromagnetic waves. Subsequently, as an important way to regulate THz, the metasurface extends to terahertz frequency band and becomes a research hotspot. In this paper, we design a coding metasurface based on vanadium dioxide (VO<sub>2</sub>) with anisotropic characteristics. It is composed of three layers, with a metal cross structure embedded in VO<sub>2</sub> at the top, polyimide in the middle, and pure metal at the bottom. The design of the cross shaped structure makes the coding metasurface unit anisotropic, which can provide complete and independent control of the orthogonally linearly polarized incident waves. The pure metal structure at the bottom can provide higher reflection amplitude for the incident wave. And VO<sub>2</sub> is introduced into the coding metasurface. As a phase change material, VO<sub>2</sub> can switch its properties between the insulating state and the metallic state, which further increases the flexibility of coding metasurface to regulate THz wave. Eight different coding metasurface units are designed in this work. They can be arranged according to a reasonable coding sequence to form a coding metasurface, which consisits of 20×20 metasurface units with an overall size of 2.4 mm × 2.4 mm. Its coding sequence will be changed with the phase of VO<sub>2</sub>, thus forming a corresponding 1 bit or 2 bit coding metasurface, and the generated beam form changes accordingly. The finite-difference time domain method is used for modeling and implementing simulation, and the results are as follows. The 1-THz orthogonal linearly polarized wave is vertically incident on the coding metasurface. When VO<sub>2</sub> is in the insulating state, the designed metasurface can be regarded as an anisotropic 2 bit coding metasurface to generate dual-polarization orbital angular momentum (OAM) vortex beams. The <i>x</i>-polarized vortex wave has an OAM mode number of 2, and the <i>y</i>-polarized vortex wave possesses an OAM mode number of 1. When VO<sub>2</sub> is in the metallic state, the designed metasurface can be regarded as an anisotropic 1 bit coding metasurface to generate dual-polarization symmetrical beams. Four reflected waves are generated by incident <i>x</i>-polarized waves, and two reflected waves are created by incident <i>y</i>-polarized waves. The proposed method of combining anisotropy material and phase change material realizes the function of generating multiple THz beams in different forms on the same metasurface. The present results provide a reference for the implementation of multi-functional coding metasurface that can be flexibly applied to multiple scenes.