Photon number distributions from a diode laser.

We use balanced homodyne detection to characterize light from a diode laser as it crosses the threshold. We measure the single-time second-order correlation function g(2), and also extract the photon number distribution. Just above the laser threshold, we find that the measured g(2) exceeds the prediction of the semiclassical single-mode laser model. From the reconstructed photon number distributions, we conclude that this excess is due to emission from nonlasing modes. For higher pumping current, the light noise increases due to a different mechanism, possibly mode competition or mode partition noise.

Power-law decay of the spatial correlation function in exciton-polariton condensates.

We create a large exciton-polariton condensate and employ a Michelson interferometer setup to characterize the short- and long-distance behavior of the first order spatial correlation function. Our experimental results show distinct features of both the two-dimensional and nonequilibrium characters of the condensate. We find that the gaussian short-distance decay is followed by a power-law decay at longer distances, as expected for a two-dimensional condensate. The exponent of the power law is measured in the range 0.9-1.2, larger than is possible in equilibrium. We compare the experimental results to a theoretical model to understand the features required to observe a power law and to clarify the influence of external noise on spatial coherence in nonequilibrium phase transitions. Our results indicate that Berezinskii-Kosterlitz-Thouless-like phase order survives in open-dissipative systems.

Gain-induced trapping of microcavity exciton polariton condensates.

We have performed real and momentum space spectroscopy of exciton polariton condensates in a GaAs-based microcavity under nonresonant excitation with an intensity-stabilized laser. An effective trapping mechanism is revealed, which is due to the stimulated scattering gain inside the finite excitation spot combined with the short lifetime. We observe several quantized modes while the lowest state shows Heisenberg-limited real and momentum space distributions. The experimental findings are qualitatively reproduced by an open dissipative Gross-Pitaevskii equation model.