Enhanced sensing-field overlap and active emission for a high-performance BIC sensor

To effectively address the passivity and field distribution limitations inherent in conventional refractive index (RI) sensors, we present an active laser sensor that employs a single-layer guided-mode resonance (GMR) grating with slant ridges, which simultaneously functions as both an optical cavity and a gain layer. Specifically, by leveraging slant ridges, a quasi-bound state in the continuum is excited, which destructively interferes with the inherent GMR mode featuring a broadband spectrum, resulting in a transmissive resonance mode. By exploiting its unique field distribution concentrated within the gap regions and aligning its resonance wavelength with the gain medium, our design achieves active, high-performance sensing. This work presents a promising approach for RI sensing in compact systems, such as the end facet of an optical fiber, demonstrating a pathway for high-precision and integrated laser sensor applications.