Research On Symmetry Protected Bound States In The Continuum Based On Bulk Dirac Semimetal Metasurface

Metasurface are two-dimensional metamaterials,which not only have the advantages of three-dimensional metamaterials,but also are more competitive in integration,preparation complexity and material loss than the latter.By artificially designing their structural patterns,metasurfaces can achieve properties and functions beyond those of natural materials.Combined with the increasingly mature micro-nano processing technology,metasurface devices can achieve optical regulation from visible light to terahertz(THz)wave,which brings a great opportunity for the development of high-performance functional devices urgently needed by THz technology.As a threedimensional graphene material,bulk Dirac semimetallic(BDS)has the characteristics of extremely high carrier mobility and adjustable electrical conductivity,which has great advantages in the development of terahertz active metasurface devices.In terms of the working mechanism of the metasurface,the bound states in the continuum(BIC)is outstanding in the field of photonics because of its infinite quality factor and field enhancement effect.Thz metasurface based on this mode will further improve the performance of the device.In this paper,the mechanism and application of symmetry protected BIC are studied,and two BDS metasurface based on symmetry protected BIC working in THz are designed.The main work of this paper is as follows:(1)The longitudinal dynamic conductivity of BDS is calculated by the random phase approximation method,and the equivalent dielectric constant of BDS is obtained according to the two-band model.Based on the method of Green's function expansion,the theoretical analysis of the symmetry protected BIC is carried out,and the conclusion is drawn that the quality factor has inverse square ratio to the asymmetric parameters.The transmission of the quasi-BIC is described by Fano's formula.(2)A BDS metasurface composed of dual split ring resonators was designed based on BIC.The metasurface supports polarization-dependent symmetry protected BIC mode,and the BIC degenerates into electromagnetically induced transparency(EIT)quasi BIC and Fano quasi BIC,respectively,according to the different polarization directions under oblique incidence.The two types of quasi-BIC can be interpreted as the coupling between two backpropagating waveguide modes and the coupling between two magnetic dipole modes with slightly mismatched phases,respectively.In addition,breaking the symmetry of the structure can make BIC degenerate into quasi-BIC.The quasi-BIC due to symmetry breaking can be interpreted as the coupling of the magnetic quadpole mode and the magnetic dipole mode at the x polarization,and the coupling of the two magnetic quadpole modes at the y polarization.In addition,by adjusting the Fermi energy of BDS,the modulation of the BIC resonance frequency is achieved.(3)An asymmetric elliptical stage BDS metasurface with symmetry protected BIC was designed.According to the different ways of breaking symmetry,two kinds of quasi-BIC supported by the metasurface of elliptical table with same long axis and same short axis are studied.Through the analysis of their transmission spectra,surface electric field and surface current vector distribution,the formation of the two quasi-BIC can be explained as the generation of the net electric dipole moment along the direction of electric field polarization caused by symmetry breaking.Then,by changing the BDS Fermi energy,the quasi-BIC frequency supported by the metasurface of the two configurations was also regulated.Finally,the potential application of the metasurface in refractive index sensor was explored,and the sensing sensitivity reached 87.8/RIU.