Research On Terahertz Functional Devices Based On Metasurface

Terahertz frequency is between far-infrared and microwave is a new frequency band in the electromagnetic spectrum that needs to be studied urgently.It has very broad application prospects in non-destructive testing,national defense security,radar imaging,life sciences,sensing,and electronic information.The development of terahertz optoelectronic functional devices will greatly promote the practical process of terahertz technology.Metasurface is an artificial two-dimensional periodic structure with a thickness less than the wavelength.The phase,amplitude,polarization,frequency and other characteristics of electromagnetic waves can be controlled by sub-wavelength microstructures,and it has received extensive attention in the academic community.In this paper,a series of terahertz devices with specific functions are designed based on the metasurface.The main contents are divided into the following three aspects.(1)A reflective ultra-broaband polarization converter based on the double split ring metasurface is designed,which can effectively convert incident linearly polarized electromagnetic waves into cross-polarized electromagnetic waves in the frequency from 2.04 THz to 5.33 THz with a relative bandwidth of 89% and the polarization conversion ratio(PCR)is over 90%.The proposed metasurface is insensitive to the polarization angle,and the physical mechanism is revealed in detail by the surface current distribution.In addition,the Pancharatnam-Berry phase concept is used to design multi-bit metasurface cell,and a metasurface array antenna is designed by using a 2-bit metasurface element combined with the ordered coding sequence,which can be used to achieve the reduction of radar cross section in an ultra-wideband and wide angle range.(2)A new polarization conversion metasurface based on dual E-type structure is proposed,and the multi-bit digital coding metasurface elements are designed by using Pancharatnam Berry phase principle.For the 1-bit coding metasurface,the redirection of reflected electromagnetic waves is achieved by designing different "0" and "1" coding sequences.For the 2-bit coding metasurface,we explore its application in RCS reduction by designing ordered coding sequences.For the 3-bit coding metasurface,two kinds of gradient-phase flat plates are designed to realize the orbital angular momentum electromagnetic beams,respectively.(3)To address the static incoherence of metal metasurface,a tunable multi-band perfect absorber based on graphene metasurface is carried out in this paper.The absorber was modeled and analyzed using the finite element method,and its main operating principles,geometric parameters and the effect of Fermi energy level on the absorption spectrum were investigated.The numerical simulation results show that the absorptivity of the designed absorber is 99.9%,99.9% and 98.5% at 2.02 thz,3.44 thz and 7.58 thz,respectively.The five near-perfect absorption peaks can be achieved by composite bilayer graphene metasurface.