Exciton spin dynamics in quantum wells

Grevatt, Treena (1996) Exciton spin dynamics in quantum wells. University of Southampton, Doctoral Thesis.

Abstract

We have used time-resolved optical techniques to investigate the spin and phase relaxation and quantum beat phenomena of heavy-hole excitons in GaAs/Al_xGa_1-xAs multiple quantum wells.

The spin relaxation of optically oriented excitons has been measured using reflective polarisation sensitive probing of the pump induced population changes in the sample. We have observed weak well width dependence of the spin relaxation time at low temperatures and much greater sensitivity to well width at room temperature where the spin relaxation rate has been found to follow the electron subband confinement energy as E_1e^2.1. The decays of the pump induced reflectivity difference are monoexponential at low temperature but exhibit two component structure at room temperature comprising a rapid initial decay followed by a slowly decaying tail. These two decays have been attributed to hole and electron spin relaxation. At low temperature the spin relaxation mechanism is one of simultaneous electron-hole spin flip within the exciton as a consequence of the exchange interaction which is greatly enhanced in quantum well systems. At room temperature the electron spins decay by the D'Yakanov-Perel mechanism. We have investigated the temperature dependence of the spin decays in the range 10-160K and observed the onset of the two component, free carrier decay, at a temperature roughly corresponding to the exciton ionisation energy.

A new technique of coherent exciton spectroscopy using transient reflective probing of optically induced linear birefringence has been developed and applied to measurements of exciton dephasing as a function of excitation energy and intensity and sample temperature. The mechanisms for the induced optical nonlinearities have been investigated and for spin relaxation we have found that phase space filling induces a small blue shift of the exciton spin state, but in the case of coherence studies we observe a saturation of the exciton oscillator strength and a pump induced rapid variation of the homogeneous linewidth across the inhomogeneous line.

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