CONTINUUM CONTRIBUTION TO EXCITONIC 4-WAVE-MIXING DUE TO INTERACTION-INDUCED NONLINEARITIES - A NUMERICAL STUDY

We present a theoretical investigation of ultrafast transient four-wav e mixing (FWM) of GaAs quantum wells for coherent excitation of excito ns and a large number of continuum states. It is shown that in this ca se the line shape of the FWM signal is drastically altered due to an i nteraction-induced coupling of the exciton to all the excited continuu m states. The signal is dominantly emitted at the spectral position of the exciton and decays, as a function of delay, on a time scale set b y the duration of the laser pulse rather than by the intrinsic dephasi ng time. Nevertheless, the spectral width of the exciton line in the F WM spectrum and in the decay of the time-resolved FWM signal in real t ime are governed by the intrinsic excitonic dephasing rate. It is show n that for pulse durations of similar to 100 fs (for GaAs quantum well s) this behavior can be explained as the influence of the Coulomb exch ange interaction, while for even shorter pulses this behavior is domin antly caused by nonlinear polarization decay.