Research On The Terahertz Vortex Beam Based On Coding Metasurface

The unique electromagnetic properties of terahertz waves make it have essential application prospects in the fields of wireless communication,security inspection imaging,biological sensing,environmental monitoring,radar detection,et al.With the rapid development of wireless communications in recent years,spectrum resources have become increasingly scarce.Vortex beams that carry orbital angular momentum have the characteristics of orthogonality,divergence,stability,security,and multi-dimensional quantum entanglement,which make them have significant application value in improving the capacity of communication systems and spectrum utilization.Therefore,how to generate efficient vortex beams in the terahertz band is the basis for its application.It is a very effective way to generate vortex beams in the terahertz band by using coding metasurfaces.It can not only solve the disadvantages of traditional methods such as large volume and unfavorable integration but also reduce the manufacturing cost,which is of great significance for the development of terahertz technology.This thesis mainly conducts simulation and experimental research on the generation of vortex beams based on coding metasurfaces.The main work completed includes the following aspects:(1)The vortex beam modal reconfigurable dual-band coding metasurface terahertz wave response characteristics based on vanadium dioxide(VO₂)are simulated.The simulation results show that when VO₂ is in the insulated state,the metasurface can transform the linearly polarized incident wave in the 0.45-0.7 THz band into a vortex beam with the topological charge of-1 through transmission;when VO₂ changes to the metallic state,the metasurface can convert the circularly polarized incident wave in the0.95-1.2 THz band into vortex beam with the topological charge of+2 through reflection.Therefore,the dynamic regulation of the vortex beams modal can be realized in two frequency bands,and the generated vortex beams have high mode purity and small divergence angle.(2)A VO₂-based terahertz dual-band multifunctional coding metasurface is constructed,which has a multi-layer structure and can provide multiple degrees of freedom for regulating terahertz waves through VO₂ state changes.The simulation results show that when VO₂ is in the insulated state and the frequency is between 0.21-0.31 THz,the incident x-polarized wave along with the-z-direction and the incident y-polarized wave along the+z-direction can be converted into vortex beams with opposite modes,and the produced vortex beams have good mode purity and slight divergence angle;when VO₂ is in the metallic state and the frequency is between 0.32-0.41 THz,the incident y-polarized wave along the+z-direction is reflected by the metasurface and form two symmetrical beams,the beam deflection angle is basically the same with the theoretical predictions.(3)A dual-band multifunctional coding metasurface array structure is proposed to control the transmission and reflection wavefronts of K and Ka bands in the whole space.The simulation results show that through specific coding sequences,the coding metasurface can achieve RCS reduction and anomalous reflection for incident waves in different polarization directions in the Ka-band;the incident wave in the K-band is converted into a vortex beam with a topological charge of+2 through transmission.The experimental results are basically consistent with the simulation results,which verify the reliability of the proposed multifunctional coding metasurface design method.Several dual-band coding metasurfaces proposed in this thesis provide new ideas and methods for the design of terahertz wavefront control devices,which is of great significance to promote the development of application systems such as terahertz communication and imaging.