Session W10: Invited Session: Novel Bose-Einstein Condensates: Photons, Excitons, Magnons, Rydberg Atoms, and Polar Molecules

Bose-Einstein condensation of photons

Bose-Einstein condensation, the macroscopic ground state accumulation of particles with integer spin (bosons) at low temperature and high density, has been observed in several physical systems, including cold atomic gases and solid state physics quasiparticles. However, the most omnipresent Bose gas, blackbody radiation (radiation in thermal equilibrium with the cavity walls) does not show this phase transition. The photon number is not conserved when the temperature of the photon gas is varied (vanishing chemical potential), and at low temperatures photons disappear in the cavity walls instead of occupying the cavity ground state. Here I will describe an experiment observing a Bose-Einstein condensation of photons in a dye-filled optical microcavity [1]. The cavity mirrors provide both a confining potential and a non-vanishing effective photon mass, making the system formally equivalent to a two-dimensional gas of trapped, massive bosons. By multiple scattering of the dye molecules, the photons thermalize to the temperature of the dye solution. In my talk, I will begin with a general introduction and give an account of current work and future plans of the Bonn photon gas experiment. \\[4pt] [1] J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, Nature \textbf{468}, 545 (2010).   

Superfluid Phase Transition of Long-Lifetime Polaritons

Exciton-polaritons are quanta of electronic excitation which can have their properties tailored in semiconductor structures to have extremely light mass, about four orders of magnitude less than a free electron. One can think of them as photons dressed with an effective mass and an atom-like interaction. Because of their very light mass, exciton-polaritons show Bose quantum effects even at moderate densities and temperatures from tens of Kelvin up to room temperature. In the past five years, multiple experiments have shown effects of polaritons analogous to Bose condensation of cold atoms, such as a bimodal momentum distribution, quantized vortices, a Bogoliubov excitation spectrum, spatial condensation in a trap, and Josephson junction oscillations. In these experiments, though, the lifetime of the polaritons has been just a little longer than their thermalization time, which means that nonequilibrium effects play an important role; in particular, the transition to superfluidity has been smeared out rather than a sharp transition. In this talk I report new results with polaritons that have very long lifetime compared to their thermalization time. We see a discontinuous jump in the properties of the polariton gas indicative of a true phase transition, and we see ballistic transport over hundreds of microns. We also now have a way to use a laser to create a potential barrier for the polaritons.