Ultrafast coherent dynamics in semiconductor nano-structures

• Main Author: Davis, J. A.
• Published: University of Cambridge 2005
• Subjects: 537.622
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598402

This thesis reports on the work done in studying coherent dynamics of excitons in quantum wells and exciton spins in quantum dots. Four-wave-mixing experiments were performed in a magnetic field on GaAs/AlGaAs single quantum wells known to contain spatially large monolayer islands. The properties of the dynamics of the signals are discussed, and compared to previous values. Beating was observed between the monolayer islands, and the variation of the phase of these beats was examined in an attempt to determine the mechanism for coupling between them. The majority of the data suggests the monolayer islands are not coherently coupled, however, the behaviour under some conditions brought this into question and required further modelling. Models for a three-level system and two on-interacting two-level systems including the effects of a local field are presented. Neither model was able to reproduce the experimental data on its own, however, a combination of the two was successful. On the basis of the modelling, it was shown that the contribution to the signal from coherently coupled transitions decreases as a function of magnetic field. This gives some insight into the possible coupling mechanisms, and these are discussed. Pump-probe experiments are performed on InGaAs/GaAs quantum dots in an attempt to obtain details of the coherent exciton-spin dynamics in such systems. Decoherence of the exciton spin is observed in sufficiently high magnetic fields, and the dependence of the dephasing time on field strength and temperature are studied. To my knowledge, these are the first reported experimental results showing decoherence of exciton spin. Comparisons of the observed behaviour with existing predictions are made in an attempt to determine the mechanism for loss of exciton spin coherence in quantum dots. None of the predicted mechanisms were able to fit my experimental data adequately, suggesting the presence of some other more efficient mechanism.