Instantaneous contribution in time-resolved four-wave mixing from GaAs quantum wells near zero time delay

We show the existence of an instantaneous contribution to the time-resolved four-wave mixing (FWM) from GaAs quantum wells near zero time delay (T=0), which is very sensitive to the excitation density and detuning. In spectrally resolved (SR) FWM, this instantaneous contribution corresponds to a broad background, which is stronger in samples with larger inhomogeneous broadening. This spectral background extends well below the heavy-hole exciton, implying that diffraction of laser spectral components with no corresponding density of states occurs near zero delay. Theoretical calculations show that the origin of this instantaneous signal is the free induction decay of off-resonance laser fields, which is dramatically enhanced by the strong pulse overlap and the resulting population grating near T=0. Away from T=0, the delayed, interaction-induced signal dominates. Three-beam SR-FWM also shows the existence of a similar contribution when the first two beams overlap and the third beam is diffracted off the population grating.