VO2 based terahertz anisotropic coding metasurface

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 twodimensional 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 (VO2 ) with anisotropic characteristics. It is composed of three layers, with a metal cross structure embedded in VO2 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 VO2 is introduced into the coding metasurface. As a phase change material, VO2 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 20x20 metasurface units with an overall size of 2.4 mm x 2.4 mm. Its coding sequence will be changed with the phase of VO2 , 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 VO2 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 x-polarized vortex wave has an OAM mode number of 2, and the y-polarized vortex wave possesses an OAM mode number of 1. When VO2 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 x-polarized waves, and two reflected waves are created by incident y-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.