Polarization-selective excitation of toroidal and magnetic dipoles for high-sensitivity sensing based on q-BICs

High-sensitivity refractive index sensors are crucial for a wide range of applications in biosensing and environmental monitoring. Among various approaches, all-dielectric metasurfaces have attracted significant attention due to their low optical losses and high quality-factors (Q-factors). However, achieving both high sensitivity and high Q-factors simultaneously in dual-polarization refractive index sensors remains a major challenge, limiting their potential for multi-channel and high-precision detection. To address this limitation, we propose an asymmetric dual-hollow silicon nanocylinder metasurface based on quasi-bound states in the continuum (q-BIC). The structure enables polarization-selective excitation of ultra-narrow Fano resonances, corresponding to the toroidal dipole (TD) mode under x-polarization and the magnetic dipole (MD) mode under y-polarization. Simulation results show that under y-polarization (MD mode excitation), the sensor achieves a sensitivity of 563.1 nm RIU−1, a Q-factor of 5879, and a figure of merit (FOM) of 1309.5 RIU−1; conversely, under x-polarization (TD mode excitation), the corresponding values are 376.6 nm RIU−1, 4701, and 1017.8 RIU−1, respectively. Compared to symmetric designs, the proposed structure exhibits significantly improved sensing performance under both polarization conditions. By leveraging the independent dual-channel resonances enabled by q-BIC, the sensor simultaneously optimizes both high-Q confinement and environmental sensitivity, thereby substantially improving detection capabilities. This work offers a novel metasurface design strategy and provides a robust foundation for the development of multi-analyte biosensing and microfluidic-integrated sensing platforms.