Bound states in the continuum (BICs) with ultrahigh quality ($Q$) factors hold great promise for enhancing wave–matter interactions. However, their practical realization is hindered by extreme fragility to structural imperfections. Here, we report an anomalously robust Fabry–Pérot (FP) BIC in a coupled triple-resonator system. Unlike conventional two-state systems, in which the formation of an FP BIC typically requires strict matching between two identical resonators, the proposed three-state system substantially relaxes this requirement and can still support an FP BIC even when the central resonator is distinct from the two side resonators. More importantly, this system intrinsically supports a pinned eigenvector of the form $\{-1,0,+1\}$, which enforces a vanishing field in the central resonator and thereby endows the BIC with strong robustness against local structural perturbations, a feature absent in two-state systems. Using a coupled waveguide–triple-resonator platform, we numerically and experimentally demonstrate the existence of both robust and sensitive BICs. This work deepens our understanding of BICs in finite systems and provides a viable way to realize ultrahigh-$Q$ resonances.

Close

PrevNext

View figure in article

• Acoustic metamaterials
• Acoustics
• Bound states in the continuum
• Non-Hermitian systems