Time resolved excitonic coherent polarization dynamics in GaAs multiple quantum wells

We investigate pump induced modulation in the temporal evolution of the excitonic coherent polarization generated by a probe pulse in GaAs multiple quantum wells. For this, we perform fs time resolved pump-probe reflectivity measurements using frequency upconversion at both positive and negative pump-probe delays. A contribution arising from interference of excitonic linear and nonlinear polarization, which is usually not considered in describing pump-probe experiments, is required to understand the results.