Research On Terahertz Beam Modulation Based On Coding Phase Gradient Metasurface

Terahertz waves have broad application prospects in various fields such as communication,radar,imaging,detection,and biomedicine.The control of terahertz waves is crucial for terahertz technology.Traditional natural materials cannot respond to terahertz waves,while metasurfaces can flexibly manipulate terahertz waves.In this paper,a coding phase gradient metasurface is proposed,which combines the advantages of phase gradient metasurface and coding metasurface,providing more flexibility in controlling terahertz waves.To sum up,three kinds of coding phase gradient metasurfaces are designed in this paper.The specific research is as follows:This study leverages the PB geometric phase principle to design 1bit,2bit,and 3bit coding elements.Genetic algorithms are employed to optimize the coding sequences and obtain the best arrangement for each bits’ coding element.The coding phase gradient metasurface is obtained by arranging the coding elements according to the optimized coding sequence.The results show that the RCS reduction effect of 1bit,2bit and 3bit coding phase gradient metasurface is almost the same with the increase of bit number.The 1 bit coding phase gradient metasurface is adopted considering its design simplicity.It can achieve RCS reduction of more than 10 d B in the frequency range of0.87-1.725 THz,with a maximum reduction of 31.26 d B.Moreover,the performance of this metasurface remains stable for incident angles ranging from 0° to 30° degrees.Based on the PB geometric phase principle,2bit coding elements "00","01","10",and "11" are designed.The coding sequences that can generate terahertz vortex beams are designed,and the arrangement of the four coding elements is used to obtain vortex beams with modes l=1,l=2,and l=3,respectively.By skillfully combining the phase distributions of vortex beams of any two modes(such as l=1 and l=2 or l=1 and l=3)through the phase superposition principle,a coding phase gradient metasurface is obtained.Under terahertz linear polarization wave excitation,the coding phase gradient metasurface can simultaneously generate four different mode vortex beams in four different directions.Moreover,this metasurface has a high efficiency and uses fewer unit cells.A coding phase gradient metasurface with tunable terahertz scattering beams is designed based on graphene material.The structure is composed of a top rod-like structure,a polyimide medium layer embedded with a single-layer graphene structure,and a metal bottom plate.The Fermi level of graphene is regulated by applying voltage.When the Fermi level of graphene is 0e V,the 2bit coding elements "00","01","10" and "11" are designed based on the PB geometric phase principle.Additionally,three different coding sequences are designed to achieve tunable anomalous reflection,tunable diffuse reflection,and tunable multi-beam and multi-mode vortex beams.The proposed coding phase gradient metasurfaces have great potential application value in future THz radar stealth,terahertz wireless communication,and imaging fields,providing a reference for the flexible control of terahertz waves.