  We investigate thermodynamic properties of a two-dimensional photon gas
confined by a dye-filled optical microcavity. A thermally equilibrated state of
the photon gas is achieved by radiative coupling to a heat bath that is
realized with dye molecules embedded in a polymer at room temperature. The
chemical potential of the gas is freely adjustable. The optical microcavity
consisting of two curved mirrors induces both a non-vanishing effective photon
mass and a harmonic trapping potential for the photons. While previous
experiments of our group have used liquid dye solutions, the measurements
described here are based on dye molecules incorporated into a polymer host
matrix. We describe studies of fluorescence properties of dye-doped polymers,
and discuss the applicability of Kennard-Stepanov theory in this system. We
observe a thermalized two-dimensional photon gas in the solid state based
microresonator system. In the future, dye-based solid state systems hold
promise for the realization of single-mode light sources in thermal equilibrium
based on Bose-Einstein condensation of photons, as well as for solar energy
concentrators.
