Robust high-Q quasi-BICs in double-layer high-contrast metagrating with temperature self-compensation for refractive index sensing

We propose a double-layer high-contrast metagrating structure with robust high-quality (Q) and temperature self-compensation for four-band refractive index sensing. The structure supports four-band symmetry-protected bound states in the continuum (SP-BICs) that transform into quasi-BICs as a result of structural symmetry breaking. However, the Q-factor of these quasi-BICs are limited by perturbation parameters, hampering practical fabrication. Interestingly, tuning the cavity length, we implement four-band Fabry–Pérot bound states in the continuum (FP-BICs) to transform the resonance mode back into high-Q quasi-BICs even at large perturbations. This approach is conducive to improving robustness and modulation freedom of Q-factors. In addition, we achieve temperature self-compensation by using the double-layer high-contrast metagrating consists of two materials with opposite thermo-optic (TO) dispersions. The simulation results indicate that the largest refractive index sensitivity is 470.9 nm/RIU, its figure of merit is 427818.2, and its Q-factor up to 9.3×105. The proposed double-layer high-contrast metagrating has potential application prospects for multiplex and high-performance sensing.