Ultrasensitive metasurface sensor based on quasi‐bound states in the continuum

The quasi-bound state in the continuum (quasi-BIC) of dielectric metasurface provides a crucial platform for sensing, because its almost infinite <i>Q</i>-factor can greatly enhance the interactions between light waves and the analytes. In this work, we proposed an ultrasensitive all-dielectric metasurface sensor composed of periodic rectangular amorphous silicon pillars on a quartz substrate. By breaking symmetry of two pillars in unit cell, high <i>Q</i> quasi-BIC in the continuous near-infrared band can be excited. The magnetic toroidal dipole (MTD) is demonstrated to play a dominating role in the resonant modes by analyzing near-field distribution and multipole decomposition. The asymmetry degree has a significant impact on sensing performance of the proposed metasurface sensor, whose underlying physical mechanisms is analyzed by perturbation theory. The transmission spectrum and sensing performance of the fabricated metasurface sensor were measured. The experimental results show our designed metasurface sensor not only achieve a high sensitivity of 413/RIU, but also shows a high figure of merit (FOM) of 66 RIU^-1. This work provides excellent prospects for the excitation of strong MTD resonance quasi-BIC in sensing applications.