Stimulated-picosecond-photon-echo studies of localized exciton relaxation and dephasing in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells

We report measurements of the dynamics of localized excitons in GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple-quantum-well structures at low temperature based on stimulated-picosecond-photon-echo measurements. The results show that at low temperature excitons relax by phonon-assisted migration between localization sites and at higher temperatures are thermally activated to delocalized states. The measurements confirm recent theoretical predictions, but show the unexpected presence of additional dephasing.