Multipolar Origin and Active Control of High-Q Quasi-BIC Fano Resonances in Dielectric Metasurfaces for Sensing Applications

We designed an ingenious all-dielectric metasurface, employing cuboid structures patterned with bow-tie-shaped nanoholes, exhibiting multiple Fano resonances induced by quasi-bound states in the continuum (quasi-BICs) through structural asymmetry. Among them, several resonant modes demonstrated high quality factors in the range of 10^3-10^4, along with near-unity modulation depth and strong spectral contrast. The optical responses were analyzed utilizing the finite-difference time-domain (FDTD) method, with Fano profiles fitted to theoretical models and the BIC governed modes validated via the squared inverse ratio law. Furthermore, multipolar decomposition and electromagnetic spatial field profile revealed the origins of the resonance, while LC circuit modeling provided additional physical insight into the Fano profiles. The proposed metasurface also exhibited strong polarization dependence, indicating its potential for active optical switching. Finally, refractive index sensing performance, including the detection of Vibrio cholerae, reached a sensitivity of 342 nm/RIU and a figure of merit of 217.14 RIU^-1. Advancing the control of high-Q quasi-BIC Fano resonances, this study highlights Fano resonators' potential for refractive index sensing and active switching.