The Effective Interaction Strength in a Bose-Einstein Condensate of Photons in a Dye-Filled Microcavity

TL;DR

This paper experimentally investigates photon Bose-Einstein condensation in a dye-filled microcavity, focusing on how the condensate size relates to photon interactions and determining an effective interaction strength parameter.

Contribution

It provides the first precise measurements of the effective interaction strength in photon BECs across various dye concentrations, revealing a value an order larger than prior estimates.

Findings

Condensate radius increases with photon number, indicating repulsive interactions.

Effective interaction strength parameter $ ilde{g}$ is approximately 10^{-2} across concentrations.

The measured interaction strength is an order larger than previously reported.

Abstract

We experimentally study Bose-Einstein condensation of photons (phBEC) in a dye-filled microcavity. Through multiple absorption and emission cycles the photons inside the microcavity thermalize to the rovibronic temperature of the dye solution. Raising the photon density of the thermalized photon gas beyond the critical photon density yields a macroscopic occupation of the ground state, i.e. phBEC. For increasing density, we observe an increase of the condensate radius which we attribute to effective repulsive interactions. For several dye concentrations we accurately determine the radius of the condensate as a function of the number of condensate photons, and derive an effective interaction strength parameter $\tilde{g}$. For all concentrations we find $\tilde{g} \sim 10^{-2}$, one order larger than previously reported.