  Bose-Einstein condensation, the macroscopic accumulation of bosonic particles
in the energetic ground state below a critical temperature, has been
demonstrated in several physical systems. The perhaps best known example of a
bosonic gas, blackbody radiation, however exhibits no Bose-Einstein
condensation at low temperatures. Instead of collectively occupying the lowest
energy mode, the photons disappear in the cavity walls when the temperature is
lowered - corresponding to a vanishing chemical potential. Here we report on
evidence for a thermalised two-dimensional photon gas with freely adjustable
chemical potential. Our experiment is based on a dye filled optical
microresonator, acting as a 'white-wall' box for photons. Thermalisation is
achieved in a photon number-conserving way by photon scattering off the
dye-molecules, and the cavity mirrors both provide an effective photon mass and
a confining potential - key prerequisites for the Bose-Einstein condensation of
photons. As a striking example for the unusual system properties, we
demonstrate a yet unobserved light concentration effect into the centre of the
confining potential, an effect with prospects for increasing the efficiency of
diffuse solar light collection.
