Photoluminescence-Driven Quasi-BICs in Photonic Nanomembranes for Active Refractometric Sensing

Quasi-bound states in the continuum (quasi-BICs) offer a powerful platform for refractometric sensing by combining narrow resonances with strong electromagnetic confinement, promising high spectral resolution and large figures of merit (FOM). In practice, however, these advantages are often compromised by stringent angular alignment in passive transmission/reflection interrogation. Here, we demonstrate an active sensing platform that couples the defect-related photoluminescence (PL) of a freestanding silicon nitride nanomembrane to quasi-BIC modes. The substrate-free architecture suppresses radiation leakage and sustains tightly confined high-Q resonances, while emission-based readout bypasses stringent angular alignment. The resulting PL resonances exhibit line widths of 0.17 nm (Q ≈ 4000) in air and 0.26 nm in water. In aqueous sensing, the platform achieves a FOM of 250, substantially exceeding representative values reported for passive quasi-BIC sensors in the visible regime. This synergy translates the high-Q advantage of quasi-BICs into a practical active platform for high-performance refractometric and surface-binding sensing.