Quasi‐Bound States in the Continuum on the Bloch‐Surface‐Wave Platform for Sensing With Simultaneous High Q‐Factor and High Sensitivity

Resonances with simultaneous high quality factor (Q‐factor) and sensitivity are essential for accurate detection of environmental changes and molecular interactions. However, current sensing designs often face a trade‐off between these two metrics. Here, we overcome this trade‐off by harnessing quasi‐bound states in the continuum (QBICs) on the Bloch surface wave (BSW) platform. We introduce a perturbation layer that induces the transition from a BSW‐based resonance to a QBIC. This design enables the simultaneous optimization of both Q‐factor and sensitivity without trade‐offs. The Q‐factor is primarily optimized through structural symmetry, whereas sensitivity is determined by structural parameters associated with the BSW properties. The optimal design achieves a simulated bulk refractive index sensitivity of 1155 nm/RIU and a corresponding figure of merit exceeding 10 5 . We experimentally demonstrate the QBIC with a Q‐factor of 11 000 and surface sensitivity comparable to that of plasmonic resonances, reaching a 9.85 nm resonance shift upon deposition of a 10‐nm‐thick SiO 2 layer. Further validation through immunoglobulin G (IgG) sensing reveals clear resonance shifts following each surface functionalization step, with no overlap with the original resonance peak. These results establish the design of QBIC on the BSW platform as a promising approach for high‐resolution, label‐free integrated photonic sensing systems.