| Terahertz wave can only produce very weak response when interacting with natural materials.This makes terahertz functional devices very scarce,which is also the main reason for the obstruction of the development of terahertz technology.With the proposal of metamaterials and metarsurfaces,terahertz functional devices have also been developed.In recent years,actively tunable metamaterials and metarsurfaces have attracted increasing attention because of their advantages of saving manufacturing cost and strong practicability.The phase transition feature of chalcogenide phase change material Ge2Sb2Te5（GST）provides a promising solution for the realization of reconfigurable THz elements.In this thesis,two actively tunable terahertz functional devices are designed by combining metarsurface and GST.The main contents are as follows:1.Bifunctional metasurface based on chalcogenide phase change material GST.Realizing two different functions on a simple structure has become a research hotspot.Broadband polarization converter and broadband absorber can be realized on the same metasurface.Utilizing the phase transition feature of GST,the metasurface can be switched between the two functions.When the conductivity of GST is 0 S/m,the function of metasurface is a broadband polarizer which can transform the incident x polarization into the reflected y polarization in 0.53-1.22 THz.And its polarization converted ratio is more than 90%in 0.6-1.15 THz.The metasurface will be a broadband absorber as the conductivity of GST is 1×105 S/m,and it can absorb more than 75%of incident light in 0.44-1.34 THz.Compared with previous studies,this metasurface has the advantages of simple structure,high efficiency and large working bandwidth.According to the features of phase change material GST,it also has nonvolatile advantages.In addition,its performance has been verified by simulation,experiment and theory.Through comparison,the three results have high fitting degree.The design provides a new idea for the switchable photonics technology in terahertz band.2.Switchable chiral metasurface based on phase change material GST.A chiral metasurface which can be switched between left-handed circularly polarized（LCP）mirror and right-hand circularly polarized（RCP）mirror is designed.The intensity of circular dichroism（CD）changed from 76%to-64%.It has the advantages of simple manipulation and nonvolatile.Then,six chiral unit cells with different rotation angles were constructed into a group by using Pancharatnam–Berry phase and generalized Snells law.It can realize the wavefront control of terahertz outgoing wave,that is,anomalous reflection.The design has important application prospects in terahertz wavefront control. |
