Topological properties of bound-states-in-the-continuum in geometries with broken anisotropy-symmetry

TL;DR

This paper explores how breaking symmetry in anisotropic waveguides fundamentally alters the existence and topological properties of bound states in the continuum, revealing new behaviors and state configurations.

Contribution

It uncovers the effects of azimuthal and polar symmetry-breaking on BICs, showing how these regimes change their existence loci and polarization states.

Findings

Azimuthal symmetry-breaking preserves BICs as lines but makes them interferometric.

Polar symmetry-breaking transforms BICs from lines to discrete points.

BICs' topological properties and polarization states are fundamentally altered by symmetry-breaking.

Abstract

Waveguiding structures made of anisotropic media support bound states in the continuum (BICs) that arise when the radiation channel of otherwise semi-leaky modes is suppressed. Hitherto, only structures with optical axes aligned in symmetric orientations inside the waveguide plane, where BICs appear as lines in the momentum-frequency dispersion diagram, have been considered. Here we address settings where such symmetry is broken and unveil a number of fundamental new features. Weak and strong symmetry-breaking regimes are identified, corresponding to azimuthal and polar optical axes orientation asymmetries, respectively. The azimuthal symmetry-breaking is found to still preserve the existence loci of BICs in the momentum-frequency dispersion diagram as lines. However, all possible BICs become interferometric, while the polarization separable states that occur in symmetric settings cease to exist. The polar symmetry-breaking has stronger effects and transforms the BICs' existence loci from lines to points, which correspond to full-vector states that exist at discrete values of the optical axis orientation for a given wavelength. Such transformation results in fundamental changes in the topological properties of the radiated field around the BICs.