High sensitivity ( S ) and high quality factor ( Q ) 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 S and high Q near the bound states in the continuum (BIC). We theoretically achieved S > 800 nm/RIU and Q > 10 7 in refractive index sensing in the 1400–1600 nm telecom optical wavelength bands. We experimentally demonstrated an S of 94 nm/RIU and a Q of 1.2 × 10 4 , with a detection limit of 6 × 10 ?5 refractive index unit. These sensor designs can find applications in biochemical sensing, environmental monitoring, and healthcare.
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