Robustness of Bound States in the Continuum in Bilayer Structures against Symmetry Breaking

TL;DR

This paper studies the stability of bound states in the continuum (BICs) in bilayer structures, analyzing how symmetry-breaking perturbations affect their robustness and how to enhance their resilience for practical photonic applications.

Contribution

It provides a detailed analysis of the effects of symmetry-breaking on BICs in bilayer structures and proposes design strategies to improve their robustness against perturbations.

Findings

Material losses convert Fabry-Pérot BICs into quasi-BICs via second-order processes.

Symmetry-protected BICs remain non-radiative despite symmetry-breaking.

Robustness of BICs increases with larger interlayer distances.

Abstract

We investigate the robustness of bound states in the continuum (BICs) in a bilayer dielectric rod array against geometric and material perturbations. Our analysis focuses on both symmetry-protected and Fabry-P\'erot BICs, examining their transformation into quasi-BICs under three structural modifications: (i) in-plane displacement of one layer, which breaks the C$_2$ symmetry of the system; (ii) introduction of material losses that break time-reversal symmetry; and (iii) variation in the interlayer distance, which preserves structural symmetry. In particular, we demonstrate that material losses inevitably induce radiation in Fabry-P\'erot BICs via second-order perturbation processes, converting them into quasi-BICs, while symmetry-protected BICs remain non-radiative. We further show that, despite the inherent instability of BICs under symmetry-breaking effects, their resilience can be significantly enhanced through proper design. Both Fabry-P\'erot and symmetry-protected BICs exhibit exponentially weak sensitivity to C$_2$-breaking perturbations as the interlayer distance increases. Finally, we show that additional FP-BICs emerge under oblique incidence, originating from the interference of two high-Q quasi-BICs near the symmetry-protected ones. Our findings pave the way for the development of BIC-based photonic devices with improved robustness against fabrication imperfections, environmental variations, and material losses.