Highly sensitive polarization-tunable Fano resonant metasurface excited by BICs for refractive index detection

Utilising the asymmetry of the micro-nano structure to excite Fano resonance in dielectric metasurfaces presents low loss and high field enhancement factors, and has a broad range of applications in optical sensing. Nevertheless, the asymmetry of the structure often leads to polarization-sensitive characteristics, significantly reducing the quality factor and sensitivity of the metasurface, thereby hindering its further application. In this study, the metasurface composed of dielectric materials is designed to be completely insensitive to x and y polarization. The metasurface is composed of semicircular disks, which are asymmetrically and periodically placed on a quartz substrate. By controlling the radius of the two semicircular disks, the symmetry is broken, and the bound states in the continuum (BIC) transition to quasi-bound states in the continuum (q-BIC), resulting in an ultra-narrow Fano resonance peak. Simulation results show that the magnetic dipole (MD) dominates the resonance. The designed metasurface structure was fabricated and then integrated into a microfluidic system for spectral refractive index sensing experiments. In the near-infrared band, our tests demonstrate refractive index sensing performance of 405 nm/RIU and a Q factor of up to 319, with significant symmetry of the polarization angle, which agrees well with theoretical simulations. Furthermore, the spectral response of the designed structure can be flexibly controlled by adjusting the polarization angle of the incident light. We believe that this study provides a novel integrated platform for applications such as biosensing and environmental detection, and also has certain reference value for the design of polarization optical switches and metasurface absorbers for encrypted communication.