High Q-factor multiple Fano resonances in all-dielectric metasurface based on quasi-bound states in the continuum

Bound states in the continuum (BICs) spectrum are localized states in the continuum domain with an infinite lifetime, zero radiation, and an infinite quality factor (Q-factor). The structural symmetry is typically broken to transform BICs into quasi-BICs with ultrahigh Q-factors, which are used in fields such as optical switches, nonlinear optics, and sensing. Herein, we present an all-dielectric metasurface fabricated by depositing Si on the SiO₂ substrate and etching an “8”-shape hole in the middle of each unit. By breaking the symmetry of the structure, the BIC is transformed into a quasi-BIC mode, which can realize triple Fano resonances with a high Q-factor. The effects of different structural parameters on the Fano resonances are investigated by varying the structural parameters. The physical mechanisms of the three Fano resonances are elucidated by combining the electromagnetic field and multipole decomposition of scattered power. Calculation results show that the maximum Q-factor reaches 4.7 × 10⁴, the maximum figure-of-merit (FOM) value reaches 675, and the modulation depths of the three resonances are approximately 100%. The metasurface exhibits a high Q-factor, FOM value, and modulation depth, thus rendering it suitable for application to high-performance biosensors, nonlinear optics, and optical switches.