Sensing with Broken Symmetry: Revisiting Bound States in the Continuum

Abstract: Metasurface with bound states in the continuum (BICs) offer exceptional potential for optical sensing due to their inherently high quality (Q) factors. However, the detection of symmetry-protected BICs remains experimentally challenging due to their non-radiative nature. Introducing slight asymmetry makes these resonances observable, though it reduces the Q-factor. In real devices, intrinsic material losses further affect the resonance behavior and sensing performance. While it is often assumed that sensing is optimized at the critical coupling when radiative and non-radiative losses are balanced, the precise conditions for achieving the best limit of detection (LOD) and figure-of-merit (FOM) remain under active discussion. In this work, we experimentally and theoretically investigate BIC-based sensing in the terahertz (THz) range. We demonstrate that the LOD exhibits a non-monotonic dependence on asymmetry, reaching an unexpected optimum where radiative and non-radiative losses are not equal. Moreover, we show that this optimum differs between reflection and transmission sensing schemes. Our results provide practical guidelines for optimizing Q-factor, sensitivity, and signal amplitude together, and contribute to a deeper understanding of the fundamental limits of BIC-based sensing.