Phase-space views into dye-microcavity thermalised and condensed photons

TL;DR

This study visualizes and analyzes the momentum and position-resolved spectra of dye-microcavity photons, revealing their dispersion, weak interactions, and condensation behavior, with implications for understanding photon Bose-Einstein condensation.

Contribution

It provides the first phase-space view of thermalised and condensed dye-microcavity photons, including dispersion relations and interaction strength estimates.

Findings

Photon effective mass matches that of a free photon.

Photon-photon interactions are weak, with an upper bound on interaction strength.

Condensation occurs first in the lowest energy mode, with multimode condensation at high pump powers.

Abstract

We have observed momentum- and position-resolved spectra and images of the photoluminescence from thermalised and condensed dye-microcavity photons. The spectra yield the dispersion relation and the potential energy landscape for the photons. From this dispersion relation, we find that the effective mass is that of a free photon not a polariton. We place an upper bound on the dimensionless two-dimensional interaction strength of $\tilde{g}\lesssim 10^{-3}$, which is compatible with existing estimates. Both photon-photon and photon-molecule interactions are weak. The temperature is found to be independent of momentum, but dependent on pump spot size, indicating that the system is ergodic but not perfectly at thermal equilibrium. Condensation always happens first in the mode with lowest potential and lowest kinetic energy, although at very high pump powers multimode condensation occurs into other modes.