Nonhermitian defect states from lifetime differences

TL;DR

This paper demonstrates that nonhermitian defect states can form in open optical systems due to lifetime differences alone, without requiring asymmetric backscattering, expanding understanding of topological defect state formation.

Contribution

It shows that lifetime differences in lossy coupled resonators can create defect states without asymmetric backscattering, supported by both theoretical and numerical analysis.

Findings

Defect states exist due to lifetime differences in lossy resonators.

Spectrally isolated defect states can be realized in simple photonic devices.

Full-wave simulations confirm the theoretical predictions.

Abstract

Nonhermitian systems provide new avenues to create topological defect states. An unresolved general question is how much the formation of these states depends on asymmetric backscattering, be it nonreciprocal as in the nonhermitian skin effect or reciprocal as encountered between the internal states of asymmetric microresonators. Here, we demonstrate in a concrete, practically accessible setting of a lossy coupled-resonator optical waveguide that nonhermitian defect states can exist in open optical systems due to lifetime differences, without the need for asymmetric backscattering within or between the individual resonators. We apply our findings to a finite system of coupled circular resonators perturbed by nanoparticles, following the concept of creating an interface by inverting the position of the nanoparticles in half of the chain. We compare a coupled-mode tight-binding approximation to full-wave numerical simulations, showing that spectrally isolated defect states can indeed be implemented in this simple nonhermitian photonic device.