Vanadium dioxide based terahertz dual-frequency multi-function coding metasurface
Authors: Wang Jing-Li , Dong Xian-Chao , Yin Liang , Yang Zhi-Xiong , 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 low photon energy, high resolution, large communication bandwidth, etc. It has broad application prospects in security detection, high-resolution imaging, high-speed communication, and other fields. In recent years, as a new way to control THz wave, THz metasurface functional devices have attracted extensive attention of researchers. In this work, vanadium dioxide (VO<sub>2</sub>), a phase change material, is introduced into the coding metasurface. By regulating a circularly polarized wave and the orthogonal linearly polarized waves independently, a multi-function coding metasurface that can work at dual frequency points is obtained. It is composed of three layers. The top layer is a metal-VO<sub>2</sub> composite structure. The middle is a polyimide dielectric layer. The bottom is a metal ground. Under certain conditions, the double split ring resonator (DSRR) and the cross structure in the top layer are relatively independent. Designing the coding sequences for them enable the coding metasurface to have multiple functions. The electromagnetic simulation software CST is used to establish model and conduct simulation, and the obtained results are as follows. When the VO<sub>2</sub> is in an insulating state and a circularly polarized wave at 0.34 THz is incident vertically, the characteristics of coding metasurface elements are mainly affected by the DSRR. The DSRR is rotated to meet the requirements of 3-bit Pancharatnam-Berry phase coding. The coding sequence is designed to generate vortex beams with the topological charge <i>l</i> = ±1 at a specific angle. The VO<sub>2</sub> state is changed into a metallic state, and the DSRR can be equivalent to a metal ring. When the orthogonal linearly polarized wave at 0.74 THz is incident vertically, the characteristics of coding metasurface elements are mainly affected by the cross structure. Because of its anisotropy, four different 2-bit coding metasurface elements can be obtained respectively by changing the length of the horizontal arm and the vertical arm. The design of appropriate coding sequences can reduce the radar cross section of the <i>x</i>-polarized wave and the beam splitting of the <i>y</i>-polarized wave, and the results have broadband characteristics. Multiple coding sequences can be designed by special characteristics of the coding metasurface, then various expected functions can be realized on the same metasurface. It solves the problem of single function of ordinary metasurface devices to a certain extent, and paves a novel way to the development of THz multi-function systems.