Research On Terahertz Reflection-type Coding Metasurface Regulation

Terahertz wave has properties such as strong penetrability,high signal-to-noise ratio and high resolution,which plays an important role in many research and applications such as biomedicine,astronomical observation,wireless communication and imaging systems.With the development of terahertz technology,the demand for fast and efficient terahertz devices is more and more urgent.Metasurface overcomes the shortcomings of metamaterial,such as large volume and large loss,and shows great potential in electromagnetic wave regulation.After the concept of coding metamaterial was put forward,metasurface came into being,which ushered in a significant development.With the development of coding metasurfaces,various kinds of terahertz coding metasurface theories have been proposed,which can achieve beam control,low radar cross section and other functions.The traditional coding metasurface is generally composed of metal-medium,whose function is determined by the coding sequence of the unit structure in the metasurface.Once the coding sequence is determined,the corresponding function will be fixed.This coding metasurface is not tunable and has unique function,so it can not realize dynamic regulation.Therefore,the appearance of tunable coding metasurface solves this problem well.Based on the traditional coding metasurface,some functional materials are introduced into the tunable metasurface to realize different functions by changing the characteristics of functional materials.This thesis is mainly divided into two parts:one is the design of three pancharanam-Berry phase reflection-tyoe coding metasurfaces,which not only simplifies the process of unit structure design and optimization,but also improves the efficiency of the design.At the same time,the RCS performance of the target is reduced effectively.The other is to design three kinds of tunable reflection-type coding metasurfaces based on the traditional reflection-type metasurfaces.By introducing vanadium dioxide,liquid crystal and other functional materials,the tunable function of terahertz reflection beam control is realized by changing the characteristics of functional materials.The specific research contents are as follows:Terahertz reflection-type coding metasurface of"S"unit structure is propos.Through the arrangement of"S"different periodic coding sequences,two and four terahertz reflection waves can be flexibly controlled,and the terahertz random coding metasurface is designed by using the random coding sequence.Compared with the bare metal plate of the same size,the structure's terahertz coding metasurface reaches radar cross section(RCS)reduction values below-10d B in the range of 0.9THz to1.4THz and 1.65THz to 1.9THz.Terahertz reflection-type coding metasurface of"M"shape.Eight coding particles are obtained by rotating the"M"shape metal pattern.Various coding metasurfaces are designed by using regular or random arrangement method.The flexible control of two,four and six terahertz reflected waves is realized,and the RCS is effectively reduced.Compared with bare metal plate,the RCS of random coding metasurface is reduced below-10d B in the frequency range of 0.7THz to 1.45THz.Terahertz reflection-type coding metasurface embedded vanadium dioxide(VO₂)in the metal structure.At the frequency of 1.0THz,the dynamic tunability of the reflected terahertz beam is realized,and the backscatter energy is effectively reduced.When VO₂is in the metal state,the effect of beam control and reducing backscatter energy will gradually become worse with the increase of incident angle;when VO₂is in the insulating state,the terahertz beam under the vertical incidence will directly reflect to form a single beam with a elevation angle of 0°in the x-z plane.Compared with the same size bare metal plate,the RCS reduction of 1-bit random coding metasurface is-10d B in the range of 0.9THz to 1.9THz,and the effective bandwidth of RCS reduction can be reduced by the angle of oblique incidence.Terahertz reflection-type coding metasurface of open resonant frame structure.The 1-bit,2-bit and random coding metasurfaces are designed.By using the pre-designed coding sequence,the terahertz beam splitting control and RCS Reduction are realized.The maximum reduction is-15d B.Terahertz reflection-type coding metasurface of VO₂composite symmetric double L-shaped fusion structure.The 1-bit,2-bit and 3-bit coding metasurfaces are designed based on the metal insulation characteristics of VO₂and Pancharanam-Berry(PB)phase to realize the flexible control of reflected terahertz beam.When VO₂is in the metal state,the coding metasurface formed by different coding sequences can control two and four reflected terahertz waves;when VO₂is in the insulating state,the terahertz beam is directly reflected as a single beam pointing to the elevation angle of 0°in the x-z plane.Reflection-type coding metasurface of liquid crystal(LC)composite structure.A tunable liquid crystal coding metasurface is designed by using liquid crystal as a dielectric layer and applying bias voltage to change its dielectric constant.By changing the coding sequence in real time,the phase distribution of metasurface can be controlled dynamically,and the angle of reflected beam can be adjusted dynamically.