Topologically protected defect states in open photonic systems with non-hermitian charge-conjugation and parity-time symmetry

TL;DR

This paper demonstrates that topologically protected defect states can exist in open, lossy photonic systems without a band gap, arising from exceptional points or charge-conjugation symmetry, expanding the understanding of topological states in non-Hermitian systems.

Contribution

It reveals the existence of topologically protected defect states in open systems with non-Hermitian symmetries, even when these systems are topologically trivial in the closed limit.

Findings

Defect states can exist within the continuum in open photonic systems.

Such states are generated via exceptional points or charge-conjugation symmetry.

Theoretical demonstration in a coupled-resonator optical waveguide.

Abstract

We show that topologically protected defect states can exist in open (leaky or lossy) systems even when these systems are topologically trivial in the closed limit. The states appear from within the continuum, thus in absence of a band gap, and are generated via exceptional points (a spectral transition that occurs in open wave and quantum systems with a generalized time-reversal symmetry), or via a degeneracy induced by charge-conjugation-symmetry (which is related to the pole transition of Majorana zero modes). We demonstrate these findings for a leaking passive coupled-resonator optical waveguide with asymmmetric internal scattering, where the required symmetries (non-hermitian versions of time-reversal symmetry, chirality and charge-conjugation) emerge dynamically.