Research And Design Of Terahertz Phase Metasurface

Terahertz waves have important research value in detection and sensing,highprecision imaging,wireless communication applications,and national defense security due to their advantages such as wide bandwidth and high penetration.Metasurfaces,as two-dimensional artificial electromagnetic materials with subwavelength structures,can manipulate terahertz waves.With the development of terahertz technology,the demand for terahertz devices is increasing.In this thesis,based on the electromagnetic resonance response of the unit structure of the metasurface,controls the polarization and phase of the electromagnetic waves and propose and study the broadband polarization conversion metasurface and the reflective tunable phase metasurface to realize terahertz wavefront control.The main research contents of this thesis are as follows:1.A reflective broadband linear-to-circular polarization conversion metasurface is proposed.Based on the relationship between the amplitude and phase of the reflection coefficient of the metasurface structure and the surface current distribution,the linear polarization to circular polarization conversion is analyzed theoretically.The designed metasurface achieves the conversion from linear polarization to circular polarization in the broadband frequency range of 2.5THz-3.4THz,with a relative bandwidth of 30.5%for the 3d B axis ratio.And the metasurface can still ensure polarization conversion with an axis ratio less than 3d B in the incident angle range of 0°～30°.2.A terahertz broadband wavefront control metasurface based on geometric phase is constructed.In the frequency band of 2.35THz-3.65 THz,the cross-polarization reflection coefficient is higher than-1d B,and the polarization conversion rate(PCR)is greater than 90%.Utilizing the characteristics of the geometric phase,a 2 π phase coverage is achieved in the working frequency band to manipulate the cross-circularly polarized reflected electromagnetic waves.At the frequency point of 2.45 THz,the effects of anomalous reflection and one-dimensional focusing are achieved.3.A terahertz reconfigurable metasurface based on metal-graphene is designed to dynamically control the reflection phase by adjusting the Fermi energy level of graphene.Based on the transmission phase principle,a 2-bit coding metasurface is constructed.Different encoding sequences are combined with the metasurface based on the Fourier convolution principle to achieve flexible control of the anomalous beam deflection of the reflected wave.The simulation results of deflection angle are consistent with the theoretical ones.And on its basis,a varifocal metalens is constructed,which shows good focusing function in the upper half-space.