Transport and coherence properties of indirect excitions in coupled quantum wells

Doctoral thesis English OPEN
Mouchliadis, Leonidas
  • Subject: QC
    arxiv: Condensed Matter::Quantum Gases | Condensed Matter::Mesoscopic Systems and Quantum Hall Effect | Condensed Matter::Other

This dissertation consists of a theoretical investigation into the transport and coherence properties of indirect excitons in coupled quantum wells (QWs) at helium temperatures. The motion of excitons along the quantum well plane is described through a quantum diffusion equation and the possibility of excitonic cloud formation is studied both due to the natural potential fluctuations and externally applied confining potentials. The photoluminescence (PL) of decaying excitons is used as a probe for their properties such as concentration, effective temperature and optical lifetime. The exciton thermalisation from an initial high energy to the lattice temperature is achieved within their lifetime due to a very effective coupling between the exciton states and a continuum of phonon states, a direct consequence of the relaxation of momentum conservation along the growth direction of a QW. Moreover, the natural spatial separation between electrons and holes prevents their recombination, resulting in long lifetimes. The dynamics of the system of excitons in optically-induced traps is also studied and the numerical solution of the quantum diffusion equation provides an insight into the extremely fast loading times of the trap with a highly degenerate exciton gas. The hierarchy of timescales in such a trap allows for the creation of a cold and dense gas confined within the trap, opening a new route towards the long sought Bose-Einstein Condensation (BEC) in solid state. Finally the issue of exciton spatial coherence is studied and an analytic expression for the coherence function, i.e., the measure of the coherence in a system, is derived. A direct comparison with large coherence lengths recently observed in systems of quantum well excitons and microcavity polaritons is attempted and interesting conclusions are drawn regarding the build up of spontaneous coherence in these systems.
  • References (151)
    151 references, page 1 of 16

    5.1 Formation of Exciton Rings in Quantum W e l ls ................................................

    5.2 Optical Trapping of Indirect Excitons ..............................................................

    5.3 Spatial Coherence of E xcitons.............................................................................

    5.4 Future Work ......................................................................................................... A D erivation of the generalised Einstein relation

    A.0.1 The Einstein relation...............................................................................

    A.0.2 The generalised Einstein re la tio n ......................................................... B Calculation of th e capture coefficient

    B.0.3 The matrix element of the in te ra c tio n ................................................ 1.1 An idealised band structure of a direct-gap sem iconductor............................ 1.2 Energy band diagrams of a coupled quantum well s tr u c tu re ......................... 1.3 The photon and exciton dispersion relations..................................................... 1.4 A schematic diagram of the structure................................................................. 2.1 Spatial patterns of the indirect exciton PL intensity with increasing excitation

    power [1]................................................................................................................. 34

    2.2 Spatial profiles of the PL signal in the x -y and E -x plane [2] 35

    2.3 The effective potential profile as a function of the distance from the current

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