Local perfect chirality at reflection-zeros away from exceptional points in optical whispering gallery microcavity

TL;DR

This paper reveals the existence of local perfect chirality at reflection-zero points away from exceptional points in a perturbed optical microcavity, expanding understanding of chiral phenomena in non-Hermitian photonic systems.

Contribution

It demonstrates the occurrence of local perfect chirality at reflection-zero points away from exceptional points in a strongly perturbed microcavity system, using a first-principles scattering model.

Findings

Local perfect chirality occurs at reflection-zero points away from EPs.

The R-zero typically differs from the EP in structural parameters.

Only one eigenmode exhibits perfect chirality within the local azimuthal range.

Abstract

Recently, a local and imperfect chirality of the resonant eigenmode at the exceptional point (EP) has been reported in the optical whispering gallery microcavity system perturbed by two strong nanoscatterers [Phys. Rev. A 108, L041501 (2023)]. Here, we discover a local perfect chirality of the resonant eigenmode away from the EP in the parameter space of the strongly perturbed microcavity system. By considering the multiple scattering process of the azimuthally propagating modes (APMs) at the nanoscatterers with a first-principles-based model, the local perfect chirality is predicted to result from the unidirectional reflectionlessness, i.e., the reflection-zero (R-zero) of the APMs at the two nanoscatterers. Numerical results and model predictions consistently show that the structural parameters of the R-zero typically deviate from those of the EP, which means that the pair of split resonant eigenmodes at the R-zero have different complex resonance frequencies and electromagnetic fields. In general, only one of the pair of split eigenmodes exhibits a local perfect chirality within the local azimuthal range divided by the two nanoscatterers. With the decrease of the two nanoscatterers' sizes or their relative azimuthal angle, the R-zero tends to coincide with the EP.