Revealing the dark side of a bright exciton polariton condensate

TL;DR

This paper investigates the matter component of exciton polariton condensates using terahertz spectroscopy, revealing dark excitons and fundamental differences from photon lasing, which advances understanding of quantum condensates.

Contribution

It provides the first direct observation of dark excitons in polariton condensates and highlights the renormalization of intra-excitonic transitions upon condensation.

Findings

Dark excitons form a reservoir feeding the condensate

Intra-excitonic transitions are dramatically renormalized during condensation

Fundamental differences between polariton condensation and photon lasing are established

Abstract

Condensation of bosons causes spectacular phenomena such as superfluidity or superconductivity. Understanding the nature of the condensed particles is crucial for active control of such quantum phases. Fascinating possibilities emerge from condensates of light-matter coupled excitations, such as exciton polaritons, photons hybridized with hydrogen-like bound electron-hole pairs. So far, only the photon component has been resolved, while even the mere existence of excitons in the condensed regime has been challenged. Here we trace the matter component of polariton condensates by monitoring intra-excitonic terahertz transitions. We study how a reservoir of optically dark excitons forms and feeds the degenerate state. Unlike atomic gases, the atom-like transition in excitons is dramatically renormalized upon macroscopic ground state population. Our results establish fundamental differences between polariton condensation and photon lasing and open possibilities for coherent control of condensates.