  We report on an experimental study of photon thermalization and condensation
in a semiconductor microresonator in the weak-coupling regime. We measure the
dispersion relation of light and the photon mass in a single-wavelength,
broad-area resonator. The observed luminescence spectrum is compatible with a
room-temperature, thermal-equilibrium distribution. A phase transition,
identified by a saturation of the population at high energies and a superlinear
increase of the occupation at low energy, takes place when the phase-space
density is of order unity. We explain our observations by Bose-Einstein
condensation of photons in equilibrium with a particle reservoir and discuss
the relation with laser emission.
