Optical Refractive Index Sensing Based on High-Q Bound States in the Continuum in Free-Space Coupled Photonic Crystal Slabs.

High sensitivity () and high quality factor () are desirable to achieve low detection limit in label-free optical sensors. In this paper, we theoretically demonstrate that single-layer and coupled bi-layer photonic crystal slabs (PCS) possess simultaneously high and high near the bound states in the continuum (BIC). We theoretically achieved > 800 nm/RIU and > 10⁷ in refractive index sensing in the 1400-1600 nm telecom optical wavelength bands. We experimentally demonstrated an of 94 nm/RIU and a of 1.2 × 10⁴, with a detection limit of 6 × 10 refractive index unit. These sensor designs can find applications in biochemical sensing, environmental monitoring, and healthcare.