Struttura della materia

Calometry of a Bose-Einstein-condensed photon gas

by Martin Weitz (Universität Bonn)

Aula Conversi (Dipartimento di Fisica - Ed. G.Marconi)

Aula Conversi

Dipartimento di Fisica - Ed. G.Marconi

Bose-Einstein condensation has been observed with cold atomic gases, exciton-polaritons, and more recently with photons in a dye-filled optical microcavity. I will here describe recent measurements of our Bonn group determining the heat capacity of a two-dimensional photon gas in the regime around the Bose-Einstein phase transition. Moreover, the entropy of the optical quantum gas has been determined. The photon Bose-Einstein condensate is generated in a wavelength-sized optical cavity, where the small mirror spacing imprints a low-frequency cutoff with a spectrum of photon energies restricted to well above the thermal energy. Thermal equilibrium is achieved by repeated absorption re-emission processes on the dye molecules. To determine calorimetric properties of the optical quantum gas, we analyze spectra of the dye microcavity emission at different levels of the phase space density, from which we first determine the internal energy per photon, and after differentiation with respect to the ratio of temperature and critical temperature the heat capacity can be determined. At the phase transition, the observed specific heat shows a cusp-like singularity, illustrating critical behavior, analogous to the \lambda-transition of liquid helium. From the optical spectra we have also determined the entropy per photon of the trapped photon gas.