Bose-Einstein condensation, the macroscopic accumulation of bosonic particlesin the energetic ground state below a critical temperature, has beendemonstrated in several physical systems. The perhaps best known example of abosonic gas, blackbody radiation, however exhibits no Bose-Einsteincondensation at low temperatures. Instead of collectively occupying the lowestenergy mode, the photons disappear in the cavity walls when the temperature islowered - corresponding to a vanishing chemical potential. Here we report onevidence for a thermalised two-dimensional photon gas with freely adjustablechemical potential. Our experiment is based on a dye filled opticalmicroresonator, acting as a 'white-wall' box for photons. Thermalisation isachieved in a photon number-conserving way by photon scattering off thedye-molecules, and the cavity mirrors both provide an effective photon mass anda confining potential - key prerequisites for the Bose-Einstein condensation ofphotons. As a striking example for the unusual system properties, wedemonstrate a yet unobserved light concentration effect into the centre of theconfining potential, an effect with prospects for increasing the efficiency ofdiffuse solar light collection.
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