  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.
