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

Abstract

High sensitivity (<i>S</i>) and high quality factor (<i>Q</i>) 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 <i>S</i> and high <i>Q</i> near the bound states in the continuum (BIC). We theoretically achieved <i>S</i> > 800 nm/RIU and <i>Q</i> > 10⁷ in refractive index sensing in the 1400-1600 nm telecom optical wavelength bands. We experimentally demonstrated an <i>S</i> of 94 nm/RIU and a <i>Q</i> of 1.2 × 10⁴, with a detection limit of 6 × 10^-5 refractive index unit. These sensor designs can find applications in biochemical sensing, environmental monitoring, and healthcare.