Ultrafast exciton dynamics in GaAs/AlGaAs semiconductor nanostructures

The initial scattering of excitations in semiconductor nanostructures, such as quantum wells and superlattices, occurs on a femtosecond time scale. The general features of the coherent nonlinear response have been investigated by spectrally and temporally resolved transient FWM experiments which have focused on the investigation excitons and biexcitons in III-V nanostructures.
The decay time of the FWM signal, in real and/or delay time, is a measure of the loss of coherence of the excited transitions. The effect of the excitation intensity on the exciton nonlinear response has been investigated on GaAs/AlGaAs quantum wells of different thickness and consequently different inhomogeneous broadening. A density dependent exciton dephasing time is found, that acts also as a source of FWM signal. The character of this extra non-linearity as a function of the inhomogenoeus broadening has been analyzed.
The temperature dependence of the exciton dephasing time has been measured in InGaAs/GaAs single quantum wells. A well width dependence of the exciton-acoustic phonon interaction is obtained, according to a reduced confinement of the excitonic wavefunction, strongly penetrating into the GaAs barrier, in InGaAs thin quantum wells.
An important source of optical nonlinearities in semiconductor nanostructures is the biexciton formation. The influence of biexcitons on the optical properties of semiconductor nanostructures is enhanced, compared to that of the bulk semiconductor, due to the confinement of the electronic state in these structures. We have investigated the formation, binding energy, and dephasing of biexcitons in InGaAs and GaAs quantum wells as well as in GaAs quantum wires.
A new technique to measure the coherence of optical excitations in solids, such as excitons, has been developed, which makes use of the random interference of scattered light.