Dual-functional all-dielectric metasurface enabled by high-Q quasi-BICs for refractive index sensing and third-harmonic generation

We present a dual-functional all-dielectric platform based on cross-shaped silicon resonators that simultaneously supports high-sensitivity sensing and high-order nonlinear generation. By introducing controlled transverse symmetry breaking while preserving C 2 rotational symmetry, symmetry-protected bound states in the continuum (SP-BICs) are transformed into high-Q quasi-BICs. Multipole decomposition analysis confirms that the excitation of these quasi-BICs is dominated by toroidal dipole (TD) resonance, and during the transition from SP-BICs to quasi-BICs, the TD shifts from being dominated by the x component to being dominated by the y component, thereby clarifying the physical origin of the BIC mechanism in such dielectric metasurfaces. The proposed structure enables linear and precise spectral tuning of resonant wavelengths while also achieving the regulation of the Q values, demonstrating excellent spectral controllability. As a refractive index sensor, the metasurface achieves an exceptional sensitivity (S) of 588.89 nm/RIU and a figure of merit (FOM) up to 67844. Furthermore, efficient third-harmonic generation (THG) is demonstrated, highlighting its strong potential for nonlinear frequency conversion. This work provides what we believe to be a novel approach for developing tunable, high-sensitivity BIC-based sensors and shows promising prospects for integrated nonlinear photonic applications.