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Jacek Kasprzak

Condensation of exciton polaritons

Published on 23 October 2006


Thesis presented October 23, 2006

Abstract:
Because of their unique property of bringing pure quantum effects into the real world scale, phase transitions towards condensed phases - like Bose-Einstein condensation, superfluidity, and superconductivity - have always fascinated scientists. The BEC, appearing upon cooling a gas of bosons below a critical temperature, has been given a striking demonstration in dilute atomic gases of rubidium atoms at temperatures below 200nK. By confining photons in a semiconductor microcavity, and strongly coupling them to electronic excitations, one may create polaritons. These bosonic quasi-particles are 109 times lighter than rubidium atoms, thus theoretically allowing a BEC at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for a BEC of polaritons. Above a critical density, we observe massive occupation of the ground state, developing from a thermalized and saturated distribution of the polariton population at (16-20)K. We demonstrate as well the existence of a critical temperature for this transition. The spontaneous onset of a coherent state is manifested by the increase of temporal coherence, the build-up of long-range spatial coherence and the reduction of the thermal noise observed in second order coherence experiments. The marked linear polarization of the emission from the condensate is also measured. All of these findings indicate the spontaneous onset of a macroscopic quantum phase.

Keywords:
Bose-Einstein condensation, final state stimulation, long- range phase coherence, intensity correlations, exciton polaritons, semi- conductor microcavity, strong coupling, CdTe

On-line thesis.