Research Of Terahertz Polarization Conversion Technology Based On Metasurface

With the rapid development of terahertz technology,the terahertz source and detection technology are getting more and more mature.However,the development of terahertz band functional devices and related technologies is slow.In this paper,the polarization conversion properties and principle of terahertz polarization converter based on metal metasurface and graphene metasurface are studied.A terahertz reflective polarization converter based on metal metasurface is designed and measured,which is composed of a metal layer with periodic holes,a dielectric spacer and a metal plane layer,three-layer structure.The polarization conversion curve appears in the reflection spectrum of the structure,and the curve peaks at the frequencies of 0.279 THz and 0.332 THz,with the reflectance of 88.9%and 69.7%,and the polarization conversion ratio of 99.7%and 95.6%,respectively.The surface current distribution characteristics of the metal film indicate that magnetic resonances exist in the medium,which induce an electric field perpendicular to the polarization direction of the incident wave.By analyzing the characteristics of electric field distribution,it is shown that the terahertz wave excites surface plasmon resonances at the edge of L-shaped metal hole and propagates in the reflective structure.The principle of polarization conversion is demonstrated by the vector decomposition of the incident wave electric field and the comparison of the reflection spectrum and electric field distribution.By changing the parameters of the structure,the effects of the parameters on the polarization conversion characteristics are studied.The electric field distribution shows that metal hole and metal strip have different physical mechanism.Finally,the samples of the designed terahertz reflective polarization converter are fabricated and the samples are tested at a single trequency point by using the reflective test path constructed by the terahertz BWO system.The experimental results show that the polarization conversion ratio can achieve 61.9%.A terahertz reflective polarization converter based on graphene metasurface is designed,which is composed of a single-layered graphene with periodic holes,a dielectric spacer and a metal layer,three-layer structure.The polarization conversion curve appears in the reflection spectrum of the structure,and the curve peaks at 2.65 THz and 3.58 THz,and the reflectance is 66.4%and 56.0%,respectively,and the polarization conversion ratio is 96.6%and 89.9%,respectively.The characteristics of the electric field distribution perpendicular to the graphene surface indicate that the terahertz waves at the two peaks have different surface plasmon resonance modes on the graphene surface,and also indicate the transmission process of terahertz waves in the graphene-based reflective structure.By studying the effect of changing the chemical potential of graphene on the polarization conversion properties,it is shown that the polarization conversion properties of the structure are adjustable.A kind of polarization switcher is designed,which can switch the polarization of the reflected wave by adjusting the bias voltage.The influence of the structure parameters on the polarization conversion properties is analyzed,which provides a reference for the further optimization of the device parameters.