Coherent and incoherent aspects of the coupled exciton-phonon system

The dynamics of excitons interacting with a phonon bath following the generation by an ultrashort laser pulse can be subdivided into a coherent regime immediately after the excitation and the subsequent gradual transformation of coherent excitations into incoherent occupations. In the coherent regime the interaction with the phonon system modifies the dy namics of the highly phase sensitive interband transition amplitude, but does not immediately lead to a total loss of coherence. For a GaAs quantum-wire we compare a treatment of the coupled exciton phonon system on a quantum kinetic level with the results of Boltzmann dynamics demonstrating noticeable effects of the energy-time uncertainty on linear spectra. Furthermore, we discuss the influence of the Coulomb interaction during the exciton phonon scattering process and the resulting modifications of the spectra. The opposite limit of incoherent exciton dynamics is marked by redistribution processes between exciton occupation densities. Time and space resolved pump-probe experiments are compared with calculations based on the exciton Boltzmann equation in relaxation time approximation. These experiments are found to reflect the time evolution of the mean kinetic energy of the excitons. In particular for excitation above the band edge the diffusivities show a non-monotonic behavior reflecting the exciton formation process leading to large kinetic energies. To account for the gradual loss of coherence a theory is presented that deals consistently with the combined dynamics of coherent excitonic interband transitions and incoherent excitonic occupations. Of particular interest is the build-up of exciton densities with non-vanishing center-of-mass motion. Numerical results are presented for a quantum well system demonstrating that the loss of coherence is typically much faster than the thermalization, whereby both time scales crucially depend on the well width. Furthermore it is found that the build-up of incoherent exciton densities with finite center-of-mass motion is noticeably accelerated by a transfer process involving only coherent interband transitions.