Bound States In The Continuum Induced Absorption Enhancement Of Graphene In Metasurface

Metasurface is a kind of artificially layered material with a thickness less than the wavelength and can be considered as the two-dimensional(planar)counterpart of metamaterials.As a frontier hotspot of micro and nano optics,it has broad application prospects in the fields of holographic imaging,nonlinear optics,novel beams and integrated optics.In recent years,a unique optical bound state in the optical waveguides of metasurface has attracted a lot of attention.Such bound states exist in the continuous spectrum of radiation waves while remaining perfectly localized,hence the name Bound States in the Continuum(BIC).The extremely restricted character of BIC and its exotic topological nature determine that it is difficult to be observed and controlled in the spectrum.However,it has been found that tiny perturbations of the resonant system can excite the BIC to collapse into a quasi-BIC with a finite Q-factor.For the quasi-BIC,the electromagnetic energy is highly localized,which undoubtedly provides favorable conditions for enhancing the intensity of light-matter interactions.Therefore,manipulation of the BIC becomes an effective way to enhance and modulate the light absorption of graphene.In this paper,based on silicon-based metasurfaces,two studies are proposed to enhance the light absorption of graphene via BIC,and interesting manipulations of the absorption characteristics are realized based on the special properties of BIC.An ultra-compact silicon-based grating metasurface(SGM)is designed,and an accidental BIC is found via varying the incident angle of the light wave.The quasi-BICs with high Qfactor are obtained by slightly modifying the incident angle of the light wave.After laying the monolayer graphene flat onto the upper surface of the metasurface,highly efficient light absorption of graphene is achieved at over-coupled resonance according to the coupled mode theory(CMT),and the location of the absorption peak can be dynamically tuned by varying the incident angle.Physical mechanisms for the enhanced light absorption of graphene can be interpreted as the resonant couplings between the toroidal dipole(TD)and electric quadrupole(EQ)according to the far-field multiple decompositions and near-field distributions of the structure.The absorption performances of the device are robust to the variation of structural parameters,and their absorption and reflection responses can be highly modulated by changing the Fermi level of graphene.This study achieves the enhancement of light absorption of graphene for the first time via accidental quasi-BIC,which broadens the application path of accidental BIC and provides certain reference for the development of photonic devices such as sensors and absorbers.A silicon-based two-dimensional photonic crystal slab(PCS)with square air slots etched in a single cell is designed.We find two Symmetry-protected BIC energy bands(TE1 and TE2)in the silicon-based 2D PCS,and deposit small square silicon blocks in the upper right side of the air slot to break the in-plane inversion symmetry of the PCS,breaking the TE1 mode to quasi-BIC.Two circularly polarized states with opposite spins(C point)are found by far-field polarization state analysis.The creation of the C points and the increase of the ellipticity of the polarization state promote the transmission spectra showing superb circular dichroism.After the monolayer graphene is added to the surface of PCS,perfect spin-selective absorption characteristics are exhibited at C points.And for the variation of the Fermi level of graphene,the high robustness of the absorption spectra corroborates the stability of the spinselective absorption ability determined by C points.The change of the position of the deposited small square can manipulate the movement of C points in momentum space,which in turn enables spin-selective absorption in multiple directions.For another BIC energy band TE2,the vortex singularity(V point)in its polarization vector field exhibits the singular phenomenon of splitting and then converging and then splitting.Through the detailed analysis of the energy bands and electromagnetic field patterns,we find that the coupling effect between the BIC and the guided mode resonance state causes such a phenomenon.And when the structural asymmetry increases to a certain degree,the interband coupling effect excites the splitting of the void in the momentum space far-field polarization field,thus deriving multiple C points and realizing a large range of high ellipticity in the far-field polarization field.Based on this phenomenon,we achieved the first wide-range spin-selective absorption in momentum space after tiling a monolayer graphene on the PCS.This study proposes a new mechanism to manipulate the light-matter interaction,broadening the application of C points supported by the quasi-BIC and exploring a completely new avenue of application for interband coupling effects.