Anomalous Light Scattering by Topological ${\mathcal{PT}}$-symmetric Particle Arrays

TL;DR

This paper demonstrates that non-Hermitian $ ext{PT}$-symmetric particle arrays can exhibit anomalous sideways light scattering due to complex topological edge modes, with potential applications in covert communication.

Contribution

It introduces a novel scattering phenomenon in $ ext{PT}$-symmetric systems caused by complex topological edge modes, supported by analytical and numerical analysis.

Findings

Complex topological edge modes induce sideways scattering.

Negligible forward extinction cross section.

Potential for unnoticeable classical information leakage.

Abstract

Robust topological edge modes may evolve into complex-frequency modes when a physical system becomes non-Hermitian. We show that, while having negligible forward optical extinction cross section, a conjugate pair of such complex topological edge modes in a non-Hermitian $\mathcal{PT}$-symmetric system can give rise to an anomalous sideway scattering when they are simultaneously excited by a plane wave. We propose a realization of such scattering state in a linear array of subwavelength resonators coated with gain media. The prediction is based on an analytical two-band model and verified by rigorous numerical simulation using multiple-multipole scattering theory. The result suggests an extreme situation where leakage of classical information is unnoticeable to the transmitter and the receiver when such a $\mathcal{PT}$-symmetric unit is inserted into the communication channel.