Conditional robustness of propagating bound states in the continuum on biperiodic structures

TL;DR

This paper demonstrates that low-frequency propagating bound states in the continuum (BICs) on biperiodic structures with specific symmetries are robust against small symmetry-preserving structural perturbations, enhancing understanding and potential applications.

Contribution

It shows that certain propagating BICs on symmetric biperiodic structures are robust to symmetry-preserving perturbations, clarifying their stability and symmetry dependence.

Findings

Propagating BICs with one radiation channel are symmetry-robust.

BICs persist with slight frequency and wavevector changes under perturbations.

Symmetry plays a crucial role in BIC robustness on biperiodic structures.

Abstract

For a periodic structure sandwiched between two homogeneous media, a bound state in the continuum (BIC) is a guided Bloch mode with a frequency in the radiation continuum. Optical BICs have found many applications, mainly because they give rise to resonances with ultra-high quality factors. If the periodic structure has a relevant symmetry, a BIC may have a symmetry mismatch with incoming and outgoing propagating waves of the same frequency and compatible wavevectors, and is considered as protected by symmetry. Propagating BICs with nonzero Bloch wavevectors have been found on many highly symmetric periodic structures. They are not protected by symmetry in the usual sense (i.e., there is no symmetry mismatch), but some of them seem to depend on symmetry for their existence and robustness. In this paper, we show that the low-frequency propagating BICs (with only one radiation channel) on biperiodic structures with an inversion symmetry in the plane of periodicity and a reflection symmetry in the perpendicular direction are robust to symmetry-preserving structural perturbations. In other words, a propagating BIC continues its existence with a slightly different frequency and a slightly different Bloch wavevector, when the biperiodic structure is perturbed slightly preserving the inversion and reflection symmetries. Our study enhances theoretical understanding for BICs on periodic structures and provides useful guidelines for their applications.