High-Q Toroidal Dipole Metasurfaces Driven By Bound States in the Continuum for Ultrasensitive Terahertz Sensing

A novel metallic toroidal dipole (TD) metasurface driven by Friedrich-Wintgen bound states in the continuum (FW-BIC) is theoretically proposed for terahertz (THz) sensing. By tuning the middle gap distance without breaking the symmetry of unit cell, the FW-BIC and quasi-BIC mode can be excited via resonance coupling between dipole modes. Based on the cyclic distribution of anti-aligned magnetic dipoles and the calculated scattering powers, TD resonance is demonstrated qualitatively and quantitatively; also FW-BIC is verified by far-field transmission spectrum and near-field enhancement spectrum. More importantly, it is the first time to exploit this quasi-BIC TD resonance for THz sensing to the best of our knowledge. For micron film sensing with frequency shift ( FS ) method, numerical results show the sensitivity, the Q -factor and the corresponding figure of merit (FoM) can simultaneously reach 775.7 GHz/RIU, 1016, and 284, respectively. Moreover, for nano film sensing where FS method is inapplicable, the amplitude difference method is utilized and the simulated results show it has superior sensing capability. Our proposed structure opens up an avenue to develop multifunctional and ultrasensitive THz sensors.