Mechanism And Modulation Of Quasi-bound States In The Continuum In Graphene-dielectric Metasurfaces

Optical metasurface is a subwavelength periodic array structure that can enhance the interaction between light and matter.It is a logical extension of metamaterials in practical applications.Metasurface has incomparable control ability to electromagnetic wave,it can accurately and efficiently control the polarization,amplitude,phase,polarization mode and propagation mode of electromagnetic waves.Combined with the theory of the bound state in the continuum（BIC）,photonic structures with adjustable high Q factor have been realized,which greatly enhance the interaction between light and matters and increase the efficiency of light use.Graphene is a two-dimensional array of carbon atoms arranged in a honeycomb lattice,and its optical response is characterized by the surface conductivity controlled by the chemical potential（Fermi energy）,which determines its importance in photonics and optoelectronics.However,the intrinsic light absorption coefficient of graphene in the near-infrared band is only2.3%due to the atomic-thickness nature of two-dimensional materials,which has little interaction with light,thus limiting the performance of graphene devices.This paper mainly discusses the graphene-dielectric metasurface to enhance the interactions between light and matters based on the bound states in the continuum theory.（1）A graphene-dielectric（split rings）metasurface is proposed to enhance the interaction between light and graphene.The dielectric metasurface consists of periodically arranged silicon（Si）split rings placed on the silica（Si O₂）substrate,which supports a symmetry-protected bound state in the continuum（BIC）.When perturbation is introduced into the system,that is,the symmetry of the metasurface is reduced,the BIC will transform into the quasi-BIC with a high Q-factor.As the graphene layer is integrated with the dielectric metasurface,on the one hand,the absorption rate of graphene can be enhanced from 2.3%to the theoretical maximum absorption rate of50%.On the other hand,a bandwidth-tunable optical absorber is realized by optimizing the Fermi level of graphene and the asymmetric parameters of the metasurface to satisfy the critical coupling condition,with a bandwidth tuning range of two orders of magnitude.By varying the Fermi energy of graphene,the quasi-BIC resonance can be effectively modulated and the max transmission intensity difference is up to 81%and a smaller asymmetry parameter will lead to better modulation performance.Our results may provide theoretical support for the design of absorber and modulator based on the quasi-BIC.（2）A tetrameric symmetric metasurface composed of silicon（Si）cuboids is designed,the metasurface supports two resonance modes,magnetic dipole（MD）resonance and toroidal dipole（TD）resonance,both of which the resonances are all symmetrically protected BICs.Since the cuboid tetramer of the metasurface maintains C₄ and mirror symmetry,both resonances are polarization-independent.After integrating graphene with the metasurface,a dual-band polarization-independent absorber can be obtained in the communication band.The results provide theoretical support for the design of dual-mode polarization-independent resonators,which facilitates the design of high-performance high-Q factor absorber applications.