Experimental Evidence for Inhomogeneous-Pumping and Energy-Dependent Effects in Photon Bose-Einstein Condensation

Light thermalised at room temperature in an optically pumped, dye-filled microcavity resembles a model system of non-interacting Bose-Einstein condensation in the presence of dissipation. We have experimentally investigated some of the steady-state properties of this unusual state of light and found features which do not match the available theoretical descriptions. We have seen that the critical pump power for condensation depends on the pump beam geometry, being lower for smaller pump beams. Far below threshold, both intracavity photon number and thermalised photon cloud size depend on pump beam size, with optimal coupling when pump beam matches the thermalised cloud size. We also note that the critical pump power for condensation depends on the cavity cutoff wavelength and longitudinal mode number, which suggests that energy-dependent thermalisation and loss mechanisms are important.