Magneto-optics of excitons in quantum wells

Traynor, Nicholas James (1996) Magneto-optics of excitons in quantum wells. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Measurements of excitonic Zeeman splitting have been made in a range of InGaAs/GaAs and GaInP/AlGaInP quantum wells. The technique we have developed uses polarisation selection to separate the Zeeman split spectral line of the heavy hole exciton, and we are able to measure the magnitude of the splittings with μeV resolution, with direct determination of the sign. The data for InGaAs/GaAs has been compared with 8 band k.p. theory, together with previously published data on GaAs/AlGaAs quantum wells and the agreement is good, with the data reproducing the effects of valence band mixing predicted by theory. The comparison with theory has also enabled us to test a perturbation theory connection between the Luttinger κ and γ parameters.

The magnitude and sign of the electron g-factor in InGaAs/GaAs quantum wells has been estimated from Hanle depolarisation in a magnetic field applied perpendicular to the crystal growth axis. The results show that the g-factor is close to zero for well widths between 8 and 12 nm, and comparison with theory suggests that there is significant strain induced anisotropy in the magnitude of the g-factor.

We have measured the short range exchange energies for GaInP/GaAlInP quantum wells, which are in good agreement with calculations of exchange enhancement over bulk based on increased overlap of electron and hole wavefunctions in quantum wells. The measurements show the correct limiting behaviour for wide wells where the exchange enhancement is small due to minimal confinement and for narrow wells where the enhancement decreases sharply due to increased penetration of the electron and hole wavefunctions into the barriers.

We have used a novel technique based on time resolving pump induced changes in the polarisation of a probe beam reflected from the sample to investigate the spin and phase dynamics of excitons in GaAs/AlGaAs quantum wells under application of a magnetic field. We have observed quantum beating between the spin split heavy role exciton states, from which we have observed a dramatic slowing down of exciton spin relaxation upon application of a magnetic field, as reflected in the decay time of the beating.

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