Topological polarization singularities induced by the non-Hermitian Dirac points

TL;DR

This paper introduces a new type of non-Hermitian Dirac point in photonic systems that, under differential loss, splits into exceptional points and induces polarization singularities, enabling control of unidirectional emission.

Contribution

It reveals a novel non-Hermitian Dirac point in the complex eigenfrequency plane that links band and polarization singularities, with implications for polarization control and unidirectional emission.

Findings

Dirac point splits into EPs with opposite chirality

Induces circularly polarized states (C points) with opposite handedness

Breaking mirror symmetry allows control and merging of C points

Abstract

A Dirac point in the Hermitian photonic system will split into a pair of exceptional points (EPs) or even spawn a ring of EPs if non-Hermiticity is involved. Here, we present a new type of non-Hermitian Dirac point which is situated in the complex plane of eigenfrequency. When there is differential loss, the Dirac point exhibits a dual behavior: it not only splits into a pair of EPs with opposite chirality in the band structure but also induces a pair of circularly polarized states (C points) with opposite handedness in the far-field radiation. Furthermore, breaking the corresponding mirror symmetries enables independent control of these Dirac-point induced C points, facilitating the merging of two C points and generation of unidirectional guided resonances. Our results demonstrate an explicit relation between the band singularities and polarization singularities, and provide a new mechanism to generate unidirectional emission, which can be useful in the band engineering and polarization manipulation.