Giant absorption of monolayer graphene empowered by degenerate quasi-bound states in the continuum

Light absorption of monolayer graphene in the visible (Vis) to near-infrared (NIR) region can be enhanced via many types of resonances; however, maximal absorption of 0.5 is the fundamental restriction for the absorption of graphene in two-port systems. Herein, we demonstrate that the absorption limitation in the two-port systems can be broken through degenerate quasi-bound states in the continuum (QBICs) of a silicon (Si) metasurface, which supports a symmetry-protected BIC (S-BIC) and a merging BIC (M-BIC) at the Γ point. The combination of absorption from the two QBICs results in significant absorption of 0.65 at θ = 16° and 22° for TE and TM polarizations, respectively. The coupled-mode theory (CMT) for the degenerate QBICs is proposed, and the predicted results are in good agreement with the finite element method (FEM). By varying the incident angle and Fermi level of graphene, the absorption from the degenerate QBICs can be switched between the “On” and “Off” states for a series of wavelengths. We believe this discovery implies a new paradigm toward efficient light absorption in advanced optoelectronic applications.