Nelsen, Bryan
(2012)
Polariton Condensates in a Trap and Photon Lasing in Two-Dimensional Semiconductor Microcavities.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
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 polariton-polariton 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 reflectivity 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 superfluid-like transition is observed, giving rise to possible signatures of vortices.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
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| Date: |
29 June 2012 |
| Date Type: |
Publication |
| Defense Date: |
11 April 2012 |
| Approval Date: |
29 June 2012 |
| Submission Date: |
26 April 2012 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
135 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Dietrich School of Arts and Sciences > Physics |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Polariton, Bose-Einstein Condensation, Lasing, Microcavity, Quantum Well, Exciton, Stress Trap, Electron-Hole Exchange, Valence-Band Mixing |
| Date Deposited: |
29 Jun 2012 19:13 |
| Last Modified: |
15 Nov 2016 13:57 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/11906 |
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