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

• A novel optical BIC classified as the mirror-coupled toroidal dipole mode is demonstrate in a dielectric metasurface on a lossless reflecting mirror. • Such the mode is characteristic by both the symmetry-protect BIC and accidental BIC, and thus its radiative lifetime can be controlled by two typical geometric parameters. • The unique loss engineering capability of the mode allows us to design metasurfaces critically-coupled to graphene, producing resonances with unity absorbance. • The platform holds for multiple tuning knobs to accurately control the coupling condition. 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. [ABSTRACT FROM AUTHOR]