Ultrafast phase and amplitude dynamics of coherent transients in semiconductor quantum wells

We present investigations of the temporal evolution of phase and amplitude of coherent emission of GaAs/GaAlAs quantum well structures, excited by ultrashort laser pulses. We find that the coherent light emitted by excitons or by electron-hole pairs exhibits a complex phase behavior, which depends critically on the intensity and the central frequency of the exciting pulses. We observe nonlinear shifts of the instantaneous frequency within one ultrashort pulsed emission. We also discuss the theoretical interpretation the experiments. Although it is clear that non-local Coulomb correlation and non-Markovian dephasing processes are responsible for the observations, quantitative theories are not available presently. Therefore our experiments, point the need for theoretical refinements of nonlinear optical processes in semiconductors.