Observation of non-Hermitian degeneracies in a chaotic exciton-polariton billiard

TL;DR

This paper demonstrates how non-Hermitian effects influence mode structures and spectral degeneracies in chaotic exciton-polariton billiards, revealing exceptional points and topological phenomena with implications for quantum transport and device functionalities.

Contribution

It introduces the first observation of non-Hermitian degeneracies, specifically exceptional points, in a chaotic exciton-polariton billiard system, highlighting their impact on quantum dynamics.

Findings

Observation of multiple non-Hermitian spectral degeneracies (exceptional points)

Detection of mode crossing, anti-crossing, and topological Berry phase effects

Evidence of mode switching and topological modal structures in the system

Abstract

Exciton-polaritons are hybrid light-matter quasiparticles formed by strongly interacting photons and excitons (electron-hole pairs) in semiconductor microcavities. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (i.e., non-Hermitian) quantum system: it requires constant pumping of energy and continuously decays releasing coherent radiation. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localisation, and dynamical properties. Using a spatially-structured optical pump, we create a chaotic exciton-polariton billiard. Eigenmodes of this billiard exhibit multiple non-Hermitian spectral degeneracies -- exceptional points. These are known to cause remarkable wave phenomena, such as unidirectional transport, anomalous lasing/absorption, and chiral modes. By varying parameters of the billiard, we observe crossing and anti-crossing of energy levels and reveal the nontrivial topological modal structure exclusive to non-Hermitian systems. We also observe the mode switching and topological Berry phase for a parameter loop encircling the exceptional point. Our findings pave the way for studies of non-Hermitian quantum dynamics of exciton-polaritons, which can lead to novel functionalities of polariton-based devices.