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

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.