Tunable Metasurfaces Based On Continuous Domain Bound States And Phase Change Material GST

Metamaterials refer to a kind of artificially constructed periodic structure,which has many properties not found in nature.By changing the element structure,period,material,etc.,it is possible to control the electromagnetic structure.Since von Neumann and Wegener first proposed the bound state in a continuous domain(BIC)in 1929,it has attracted much attention because of its radiation-free state with an infinitely high Q factor.Chirality refers to the property that a structure cannot coincide with its mirror mechanism by rotation or translation.The difference between the absorption,transmission or reflection of left circularly polarized light(LCP)and right circularly polarized light(RCP)of a chiral structure is called circular dichroism(CD).This paper mainly studies the design of all-dielectric high-Q metasurfaces in continuous domain bound states and the chiral structure based on phase change material GST.The main contents of this paper are as follows:Firstly,based on the continuous domain bound state,an all-medium high-Q metasurface is designed,which is composed of two asymmetric silicon cubes.By introducing the asymmetric parameter a and observing the transmission spectrum when a is increasing.When a is greater than 0,the structure has two Fano linear sharp resonances when a is not equal to 0,and its Q factors are as high as 10~7 and 10~6,respectively.Then,the peak disappears when a decreases to 0,which indicates the existence of quasi-BICs.Through the analyzed electric field and current density distribution,the magnetic dipole and magnetic quadrupole characteristics of BIC are revealed.The Q-factor of this structure can also be adjusted by changing the incidence Angle.This structure is easy to fabricate,which is of great significance for applications such as optical switching,optical sensing and modulation with high Q-factor Fano resonance.Then,a zigzag chiral structure composed of Z-shaped phase change material strips(ZGS)and L-shaped silicon strips(LSS)was designed,the absorption spectrum and CD spectrum of the structure under the excitation of RCP and LCP were shown,and the current distribution of the zigzag structure was analyzed under the two kinds of incident light.It was found that when the phase change material GST was in the crystalline state,the absorption difference between LCP and LCP was large.The CD is up to 0.88;when GST is in amorphous state,CD reaches 0.27.Therefore,by adjusting the external excitation,the CD value can be continuously adjusted by the range of 0.27to 0.88 when GST switches between the two phases.The analyzed electric field and current density distribution reveals that the main reason for this high CD value is the electric dipole and magnetic dipole resonance.This work uses phase change materials to improve the adjustability of CD,providing a new idea for designing actively tunable chiral metasurfaces.