Absorptive resonant mirrors enabling high-Q perfect absorption for refractive index sensing

Prism-coupled layered structures, including surface plasmon resonance (SPR) sensors and resonant mirror (RM) sensors, have been widely used in refractive index sensing. However, the interference-based signal detection of RM is sophisticated, and the low quality factor of the SPR mode limits sensing performance. In this work, an absorptive RM with independently tunable radiative and absorptive losses is proposed and theoretically investigated. By controlling the buffer layer thickness and waveguide extinction coefficient separately, the radiative quality factor (Q_r) and absorptive quality factor (Q_a) can be adjusted without crosstalk. When Q_r and Q_a are matched at the critical coupling condition, perfect absorption with ultra-narrow bandwidth is achieved. With a simple layer stack and compatibility with existing prism-coupled schemes, the proposed absorptive RM may offer a promising platform for label-free sensing.