Toroidal dipole-induced quasi-BICs in an all-dielectric metasurface for high-performance refractive index sensing

Quasi-bound states in the continuum (quasi-BICs) in all-dielectric metasurfaces provide a promising route to realizing ultra-high Q resonances. However, the excitation and tunability of toroidal dipole (TD)-induced quasi-BICs remain challenging. In this Letter, we propose an all-dielectric metasurface composed of silicon disk tetramers to excite TD-induced quasi-BICs. By introducing radius differences among the silicon disk tetramers, the in-plane C₂ symmetry breaking excites sharp quasi-BIC resonances, with the Q factor reaching as high as 34020. Through analysis on far-field multipole decomposition and near-field distributions, we demonstrate that TD along the z-axis dominate the quasi-BIC and explain the evolution from BIC to quasi-BIC. We demonstrate that the resonance position and strength of the quasi-BIC can be independently tailored through geometric asymmetry and incident polarization, offering a versatile strategy for mode control in all-dielectric metasurfaces. Furthermore, the proposed metasurface exhibits remarkable refractive index sensitivity of 477 nm/RIU and a high figure of merit (FOM) of 9540. The proposed metasurface not only presents an innovative strategy for tunable high-performance biosensing but also provides innovative approaches to nonlinear optics, photodetection, and integrated photonic devices.