Mirror-coupled toroidal dipole bound states in the continuum for tunable narrowband perfect absorption

Metasurfaces have received a significant amount of attention in recent years as they offer novel ways for wave front control, subwavelength light focusing, and flat optics. Here, we employ sharp resonances in metasurfaces originated from the physics of bound states in the continuum (BICs) for both engineering and enhancing the absorption of light. Specifically, using temporal coupled-mode theory and numerical simulations, we demonstrate that an all-dielectric metasurface, of which the meta-atom consists of narrow-gap Si dimers with a monolayer graphene, can perfectly absorb the incident near-infrared light through critical coupling to the mirror-coupled toroidal dipole BIC. Due to unique loss engineering capabilities of the mode, multiple tuning knobs, including the narrow gap, meta-atom–mirror distance, and geometrical scaling factor, can be utilized for control over the coupling condition and the electric-field enhancement at the critical coupling condition. These results suggest that our platform could promise in on-chip optoelectronic applications.