Tailoring the light absorption of monolayer graphene via accidental quasi-bound states in the continuum

Bound states in the continuum (BICs) have drawn fundamental and technological interests due to their distinct features such as infinite quality factor and extremely localized fields. Recently, it has been shown that the light absorption of graphene can be effectively enhanced by using symmetry-protected quasi-BICs; however, the important role of the counterparts of accidental quasi-BICs for light absorption enhancement of ultrathin films has not been studied, to our knowledge. Herein, light absorption enhancement of graphene is demonstrated through the excitation of accidental quasi-BICs based on a simple silicon grating metasurface (SGM). Highly efficient light absorption of monolayer graphene can be achieved at over-coupled resonance, and the locations of the absorption peaks and their peak values can be dynamically tuned by varying the incident angle. The enhanced light absorption of graphene is originated mainly from the hybrid toroidal dipole and electric quadrupole mode according to the far-field multiple decompositions and near-field distributions of the unit cell of the structure. In addition, the absorption responses of the SGM with graphene are robust to the variation of structural parameters, and their optical performances can be highly modulated as the Fermi level of graphene is altered.