Optimum asymmetry for nanofabricated refractometric sensors at quasi-bound states in the continuum

A symmetry-protected bound state in the continuum (BIC) is one of the bases for high-resolution photonic refractometric sensors that rely on spectral shifts. However, a trade-off exists between the quality (Q) factors and the resonance amplitudes when the asymmetries of the unit cell are changed, making it difficult to intuitively determine the optimal nanostructural geometry. In this study, we present a theoretical and experimental approach for identifying the asymmetry parameters of dielectric metasurfaces that yield the lowest limit of detection (LOD). Silicon-based metasurfaces with asymmetric pair-rod arrays are fabricated experimentally, and the minimum LOD is obtained under a critical coupling condition with equal radiative and nonradiative Q factors. The results agree well with the theoretical model derived from the temporal coupled-mode theory. We reveal that the LOD and the optimum asymmetry are significantly influenced by nonradiative losses in the nanostructure, emphasizing the importance of loss reduction in dielectric metasurfaces at quasi-BICs for high-performance refractometric sensors.