Bound states in the continuum in toroidal dipole metasurface: Theoretical studies

Abstract

By exploiting toroidal dipole (TD) and bound states in the continuum, it is feasible to substantially mitigate radiation loss, thereby enhancing both the quality (Q) factor and local electric field intensity. Herein, we theoretically propose an asymmetric nanocavity-based rectangular metasurface with a simplified design. This indicates that the designed metasurface can effectively excite transverse and longitudinal magnetic quadrupoles (TMQ and LMQ), as well as TD, with high Q-factors within the near-infrared range. Within the investigated spectral region, the incorporation of cavities enables precise tuning of the spectral positions of TD resonances, further exciting TMQ and LMQ by breaking the in-plane symmetry of the metasurface. The maximum Q-factors of the three Fano resonances are calculated to be 2 × 106, 2.2 × 106, and 4.9 × 104, respectively, with a modulation depth approaching nearly 100%. Our proposed metasurface structure offers a robust theoretical basis for advancing optical devices, such as multichannel biomolecular sensing, narrow-band filtering, and optical communication.