On the robustness of bound states in the continuum in waveguides with lateral leakage channels

TL;DR

This paper investigates the robustness of bound states in the continuum (BICs) in symmetric optical waveguides with lateral leakage channels, showing they are resilient to certain structural perturbations, which enhances understanding for practical applications.

Contribution

It provides analytical and numerical evidence that symmetric BICs in waveguides are robust against symmetry-preserving perturbations, advancing theoretical knowledge of BIC stability.

Findings

BICs are robust under symmetry-preserving perturbations.

Symmetric waveguides support stable BICs.

Analytical and numerical methods confirm BIC robustness.

Abstract

Bound states in the continuum (BICs) are trapped or guided modes with frequencies in radiation continua. They are associated with high-quality-factor resonances that give rise to strong local field enhancement and rapid variations in scattering spectra, and have found many valuable applications. A guided mode of an optical waveguide can also be a BIC, if there is a lateral structure supporting compatible waves propagating in the lateral direction, i.e., there is a channel for lateral leakage. A BIC is typically destroyed (becomes a resonant or a leaky mode) if the structure is slightly perturbed, but some BICs are robust with respect to a large family of perturbations. In this paper, we show (analytically and numerically) that a typical BIC in optical waveguides with a left-right mirror symmetry and a single lateral leakage channel is robust with respect to any structural perturbation that preserves the left-right mirror symmetry. Our study improves the theoretical understanding on BICs and can be useful when applications of BICs in optical waveguides are explored.