This thesis is a theoretical investigation into the properties of indirect excitons in coupled quantum wells at low lattice temperatures. The relaxational thermodynamics, optical decay and diffusion of the statistically-degenerate excitons are modelled theoretically and numerically. The excitons' thermalization from an initial energy, EJkB = 20-300 K, to the lattice temperature, Th 1 K, is investigated. The exciton optical creation and decay mechanisms are then included, as well as the change in the exciton effective temperature due to these mechanisms. While the optical creation heats the excitons, their optical decay produces an effect called 'recombination heating and cooling', and whether it produces a net cooling or a net heating of the exciton system depends on the exciton effective temperature, T. The system of excitons is also studied in two dimensions by using a quantum diffusion equation. The excitons are created by a laser pump with a cylindrically-symmetric spatial intensity profile. The created excitons move outwards from the excitation spot by drift and diffusion, and cool down while doing so. They become more optically- active as they cool, creating a ring of photoluminescence around the excitation spot. This ring was also seen in experiments of this kind. Theoretical results are fitted to experimental results, and the diffusion coefficient for exciton concentrations in the range of 0 < n2D < 2.5 x 1010cm-2 varies from 0.06 to 25cm2/s when Tb = 1.5 K, and the disorder amplitude in the sample is U 0.9 eV. Finally, a novel kind of laser trap used in experiments to spatially confine the excitons is modelled theoretically. While the experiments were carried out at Th = 1.5 K giving an occupation number of the ground state of 8, theoretical simulations show that for a lattice temperature of Tb = 0.4 K the occupation number of the ground state is 500. The trap is also modelled as a homogeneous trap, and simulations show that when Tb is decreased further the fraction of excitons in the ground state increases dramatically.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:583801 |
| Date | January 2006 |
| Creators | Smallwood, Lois Elenid |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/54566/ |
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