| Recent experiments in microcavity polaritons have shown many effects that can be associated with the phase transition known as Bose-Einstein condensation; these effects include a dramatic increase in both the population of the zero-momentum state and lowest-lying energy state, the formation of first- and second-order coherence in both space and time, and the spontaneous polarization of the polariton ensemble. However, these same results can also be a consequence of lasing. The primary focus of this dissertation is to examine these effects and determine to what degree the effects of lasing can be distinguished from those of Bose-Einstein condensation. Bose-Einstein condensation in a two-dimensional weakly-interacting gas, such as polaritons, is predicted to not occur without the aid of spatial confinement, i.e., a trap. Polaritons were subjected to various methods of confinement, including stress traps and exciton-reservoir traps, and the signatures of condensation in these traps are shown to be dramatically different than those of lasing in a system without confinement. It is also shown that, when driving the polariton condensate to very high density, the polaritons dissociate and the lasing transition succeeds Bose-Einstein condensation. The geometry of the trapping potential was also exploited to indicate that the symmetry of the condensate momentum-space distribution followed that of the ground state of the trap.;At reasonable densities, the lifetime of polaritons is of the same order as the polaritonpolariton interaction time, hence the previously shown effects are an incomplete Bose-Einstein condensation since thermodynamic equilibrium is not reached. A second part of this work has been to extend the lifetime of polaritons to achieve a more thermalized ensemble. We do this by increasing the Q factor of the microcavity through improving the re ectivity of the mirrors. These samples exhibit many interesting phenomenon since the polariton lifetime becomes long enough to traverse significant distances. Here, Bose-Einstein condensation occurs at a point spatially separated from the excitation source, ruling out the possibility of nonlinear amplification of the pump laser. Also, a super fluid-like transition is observed, giving rise to possible signatures of vortices.;Keywords: Polariton, Bose-Einstein Condensation, Lasing, Microcavity, Quantum Well, Exciton, Stress Trap, Electron-Hole Exchange, Valence-Band Mixing. |
