Research On Metasurface-based Tunable Terahertz Polarizer

Terahertz(THz)waves have great application potential in the fields of communication,security,medical imaging and other fields.To promote the development and application of terahertz technology,the development of various terahertz functional devices is needed urgently.Among them,the metasurface-based THz polarizers makes up for a series of defects and deficiencies of the traditional optical polarizer and the three-dimensional metamaterial polarizer,and has become a hot spot in current research.And with the advent of materials such as Dirac semimetals(DSs)and vanadium dioxide(VO2),the problems of uncontrollable dielectric constant and metal loss of traditional metal materials have been overcome,and the application capabilities of metasurface-based THz polarizers have been improved furtherly.DSs is called three-dimensional graphene,and the surface conductivity can be adjusted by applying external voltage and other methods,dynamically.As a phase change material,VO2 can achieve insulator-to-metal phase transition through external excitation such as temperature,and its conductivity can change by four orders of magnitude.These two materials have obvious advantages and great potential in the research of terahertz functional devices based on metasurfaces.The main research contents of this paper are as follows:1.A tunable THz narrow band cross polarization converter based on DSs was studied.The structure is composed of the top double V-shaped structure of DSs,the silicon dielectric layer and the bottom metal plate.The simulation results show that the polarization conversion rate(PCR)is as high as 99%,and the polarization angle reaches 88.17° at the resonant frequency of 1.252THz,which realizes cross polarization conversion of linearly polarized waves.The dynamically tunable of polarization conversion performance can be achieved by changing the Fermi level of DSs.The effects of structural parameters and incident angle of incident waves on the polarization conversion performance were also studied.2.A broadband polarizer with dynamically tunable polarization conversion performance based on DSs was studied.Its structure is composed of a top double-split ring,a middle dielectric layer and a perfect electrical conductor(PEC)bottom plate.This polarizer has a PCR of more than 80%in the working frequency band of 1.36-2.07THz.The tunability of the working bandwidth and PCR can be achieved by changing the Fermi level of DSs.The influence of the structural parameters on the polarization conversion performance was studied,and the optimal thickness of the intermediate medium is 27?m.3.A dual-function polarizer based on the hybrid matasurfaces of vanadium dioxide(VO2)and Dirac semimetals(DSs)was studied.The structure includes a double U-shaped Dirac semimetals on the top layer,a silica layer and a vanadium dioxide layer in the middle,and a PEC wire grid on the bottom layer.Due to the characteristics of VO2 for insulator-to-metal phase transition,it can switch between the function of transmission polarization conversion and reflection polarization conversion by changing the conductivity of VO2,without reoptimizing the structure.Our polarizer achieved a transmission polarization conversion ratio(PCR)of more than 90%over the frequency range 4.29-6.39 THz when used as dielectric.Whereas in the metal phase,it achieved reflection-type polarization conversion with PCR nearly 100%within a frequency width of 1.32 THz.The tunability of polarization performance was achieved by adjusting the conductivity of vanadium dioxide and the Fermi level of Dirac semimetals under the two types function of polarization conversion,and the influence of structural parameters on polarization performance was studied.