ULTRAFAST PHASE DYNAMICS OF COHERENT EMISSION FROM EXCITONS IN GAAS QUANTUM-WELLS

We investigate the temporal evolution of the instantaneous frequency o f ultrashort-pulse four-wave mixing in GaAs/GaxAl1-xAs quantum-well st ructures. We find that the coherent light emitted by excitons exhibits a complex phase behavior which depends critically on the density of e xcitons and the central frequency of the exciting pulses. Depending on the excitation conditions, we observe nonlinear shifts of the instant aneous frequency within one ultrashort pulsed emission or quantum beat s. In the latter case we determine that the beat pi-phase shift is ver y fast, only 40% above the fundamental quantum limit. We also present elaborate numerical simulations of the experiments based on a six-band generalization of the semiconductor Bloch equations. This formalism t akes into account Coulomb many-body interactions and Pauli exclusion. It reproduces the salient features of the experimental data. It fails to account for important details revealed by the very sensitive phase measurements. The discrepancies can be traced back to two approximatio ns in the theory: the statical treatment of screening, and the Lorentz ian description of dephasing. In both cases, this indicates the need f or theoretical refinements requiring a microscopic description of Coul omb scattering and dephasing processes in a non-Markovian theory.