Bose condensation of upper-branch exciton-polaritons in a transferrable microcavity

TL;DR

This paper reports the first observation of Bose-Einstein condensation of upper-branch exciton-polaritons in a transferrable WS2 monolayer microcavity, demonstrating quantum coherence phenomena and specific conditions for condensation.

Contribution

It provides experimental evidence of upper-branch polariton Bose condensation in a transferrable monolayer microcavity and analyzes the conditions necessary for this phenomenon.

Findings

Nonlinear increase in upper polariton intensity near threshold

Decrease in linewidth and increase in temporal coherence

Condensation occurs within specific particle density range

Abstract

Exciton-polaritons are composite bosonic quasiparticles arising from the strong coupling of excitonic transitions and optical modes. Exciton-polaritons have triggered wide exploration in the past decades not only due to their rich quantum phenomena such as superfluidity, superconductivity and quantized vortices but also due to their potential applications for unconventional coherent light sources and all-optical control elements. Here, we report the observation of Bose-Einstein condensation of the upper polariton branch in a transferrable WS$_2$ monolayer microcavity. Near the condensation threshold, we observe a nonlinear increase in upper polariton intensity. This sharp increase in intensity is accompanied by a decrease of the linewidth and an increase of the upper polariton temporal coherence, all of which are hallmarks of Bose-Einstein condensation. By simulating the quantum Boltzmann equation, we show that the upper polariton condensation only occurs for a particular range of particle density. We can attribute the creation of Bose condensation of the upper polariton to the following requirements: 1) the upper polariton is more excitonic than the lower one; 2) there is relatively more pumping in the upper branch; and 3) the conversion time from the upper to the lower polariton branch is long compared to the lifetime of the upper polaritons.