Tunable Polarization Characteristics Of Terahertz Metasurfaces

Metasurfaces,which are two-dimensional metamaterials consisting of periodic subwavelength artificial structures with unique electromagnetic properties beyond naturally materials.Compared with metamaterials,the complexity and cost of fabrication metasurfaces are effectively reduced,and the losses are also significantly lessened.Since the optical properties of chiral substances in nature are relatively weak,chiral metasurfaces,which are formed by compounding chiral structures with metasurfaces,can enhance the chiroptical response by manipulating polarization,phase,and amplitude of electromagnetic waves,and has important research value in biology,chemistry,optoelectronics and other fields.In this dissertation,graphene is compounded with chiral metasurface based on its tunable properties,and several polarization manipulation characteristics of metasurfaces are studied by CST simulation software,which include circular dichroism(CD),polarization conversion.The brief content of this dissertation is as follows:In this dissertation,the relationship between graphene conductivity and external bias voltage and the basic principle of chirality are firstly analyzed,and then a dynamically tunable chiral metasurface device is designed.By optimizing the device geometric parameters,at 1.181 THz,the absorption difference for the left circularly polarized wave and the right circularly polarized wave reaches the maximum(0.85),That is,the circular dichroism is 0.85.The circular dichroism of the chiral metasurface can be dynamically tuned by adjusting the Fermi level of the graphene.Combined with the characteristics of the chiral metasurface,the initial unit structure of the chiral metasurface and its mirrored unit structure are arranged according to the quadrant to realize terahertz near-field digital imaging.Manipulating the Fermi level of graphene can dynamically change its imaging resolution in terahertz near-field digital imaging applications.Furthermore,a dual-function tunable chiral metasurface device operating in the terahertz band is designed to achieve different functions in the presence of both linearly polarized and circularly polarized incident waves.On the one hand,under the incidence of circularly polarized waves,the metasurface device achieves circular dichroism,which can reach 0.63 at 1.37 THz.On the other hand,under the incidence of linearly polarized waves,the metasurface device realizes the polarization conversion from linear polarization to circular polarization.By changing the Fermi level of graphene from 0.1 e V to 1 e V,the switching of circular dichroism “off” and “on” and the switching of polarization conversion“off” and “on” can be achieved.Changing the incident angle and polarization angle,the device can maintain circular dichroism and polarization conversion characteristics stably.