Thermalization of photons

Jan Klaers1, Frank Vewinger1 & Martin Weitz1

Bose–Einstein condensation1, the macroscopic accumulation of bosonic particles in the energetic ground state below a critical temperature, has been demonstrated in several physical systems2, 3, 4, 5, 6, 7, 8. The perhaps best known example of a bosonic gas, blackbody radiation9, however exhibits no Bose–Einstein condensation at low temperatures10. 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 thermalized two-dimensional photon gas with a freely adjustable chemical potential. Our experiment is based on a dye-filled optical microresonator, acting as a ‘white wall’ box for photons. Thermalization is achieved in a photon-number-conserving way by photon scattering off the dye molecules, and the cavity mirrors provide both an effective photon mass and a confining potential—key prerequisites for the Bose–Einstein condensation of photons. As a striking example of 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 collection11.

**Groupmeeting by Xingxing Xing, August 4th, 2010**