Topological unidirectional guided resonances emerged from interband coupling

TL;DR

This paper introduces a topological mechanism for creating unidirectional guided resonances in photonic crystal slabs through interband coupling and symmetry breaking, enabling new light control methods.

Contribution

It demonstrates a novel topological approach to realize UGRs by tuning interband coupling and polarization singularities in photonic crystal slabs.

Findings

UGRs can be systematically generated via interband coupling.

Circular-polarized states emerge from trivial polarization fields.

Ubiquity of UGRs in photonic crystal slabs is confirmed.

Abstract

Unidirectional guided resonances (UGRs) are optical modes in photonic crystal (PhC) slabs that radiate towards one side without the need for mirrors on the other, represented from a topological perspective by the merged points of paired, single-sided, half-integer topological charges. In this work, we report a mechanism to realize UGRs by tuning the interband coupling effect originating from up-down symmetry breaking. We theoretically demonstrate that a type of polarization singularity, the circular-polarized states (CPs), emerge from trivial polarization fields owing to the hybridization of two unperturbed states. By tuning structural parameters, two half-charges carried by CPs evolve in momentum space and merge to create UGRs. Our findings show that UGRs are ubiquitous in PhC slabs, and can systematically be found from our method, thus paving the way to new possibilities of light manipulation.