R. T. Phillips

Exciton g-Factors, Diamagnetic Shifts, and Exchange Splittings in Quantum Dots in GaAs Quantum Wells

Excitons localised in the random interfacial potential that occurs naturally in GaAs quantum wells (QWs) have been studied. The states are those of random quantum dots (QDs), and give narrow, homogeneously broadened photoluminescence emission lines; this makes them ideal for the study of small effects such as splittings and shifts induced by a magnetic field and exchange interaction. We show that the random nature of the interfacial potential leads to a distribution of both g-factor and diamagnetic shift, which we determine for individual QD excitons. Although there is little systematic correlation of g-factor with position in the inhomogeneous distribution, there are systematic correlations between the diamagnetic shift and the exchange splitting. These results are consistent with highly localised states showing symmetry lower than that of the QW, and an enhanced exchange interaction. We analyse the results in terms of a spin Hamiltonian which treats the heavy-hole excitons as an effective spin-1/2 system.

Carrier Relaxation in (GaIn)As Quantum Dots

Time-resolved photoluminescence experiments are performed to study carrier relaxation in strain-induced (GaIn)As quantum dot structures. We observe a sub-picosecond onset of the photoluminescence from the lowest quantum-dot electron–hole transition after optical excitation of higher-energetic barrier states. This shows that carrier capture into the quantum-dot ground state can be extremely fast at high electron–hole densities. The recombination lifetime of the lowest quantum-dot transition and the interlevel relaxation time are extracted from the decay of the photoluminescence transients.

The role of spin in interacting excitonic gases

The spin-dependent exciton-exciton interactions in GaAs quantum well (QW) systems have been studied, under resonant and non-resonant excitation conditions at liquid helium temperatures. In the former case, a pure exciton gas is created, whereas in the latter the excitons are surrounded by a charged electron-hole plasma. In a double QW structure, the distance between electrons and holes comprising the excitons could be externally tuned by applying an electric field perpendicular to the well planes. The interactions were studied in terms of their dependence on excitation intensity, excess energy, spin polarization and electron-hole separation using time- and polarization-resolved photoluminescence up-conversion spectroscopy.

Coherent Control of Resonant Rayleigh Scattering

Phase-locked pulse pairs are used for resonant excitation of excitons confined within GaAs/AlGaAs quantum wells. The time-resolved secondary emission displays strong dependence on the relative phases of the two pulses for inter-pulse delays of the order of 1ps or less. Constructive interference is combined with an increase in the rise time of the emission following the second pulse. For longer inter-pulse delays the different phases of inhomogeneous excitons complicate the effect of the second pulse, with the apparent effect changing with time after the second pulse. To model this, the optical Bloch equations for a line of three-level systems are integrated over time and the resulting polarisations summed over a range of angles. The importance of the inhomogeneity in descriptions of coherent control of secondary emission is shown.