Sensing with Broken Symmetry: Revisiting Bound States in the Continuum

TL;DR

This paper investigates bound states in the continuum (BICs) in metasurfaces for optical sensing, revealing optimal asymmetry conditions for detection and providing guidelines to enhance sensing performance in the terahertz range.

Contribution

It offers a combined experimental and theoretical analysis of BIC-based sensing, identifying non-monotonic loss dependence and optimal asymmetry for improved detection limits.

Findings

LOD has a non-monotonic relation with asymmetry

Optimal sensing occurs at asymmetric conditions, not symmetric

Different optimal conditions for reflection and transmission schemes

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.