Femtosecond pulse distortion in GaAs quantum wells and its effect on pump-probe or four-wave-mixing experiments

We show that a low-intensity femtosecond pulse is severely distorted while propagating through a relatively thin (6300) GaAs multiple-quantum-well sample near the exciton resonance at low temperatures. This pulse distortion depends critically on the dephasing time T2, the total thickness l, detuning, and inhomogeneous broadening. In thinner, high-quality samples (l<3000, exciton linewidth <1 meV), the distortion is smaller and free-induction-decay-like. An interferometric measurement reveals the existence of well-defined nodes at which the envelope function changes its sign, demonstrating that the source of the pulse distortion is the reradiation of the induced dipoles. While the effects of this pulse distortion on pump-probe or four-wave-mixing experiments are relatively small for samples with l<3000, samples with l>6000 are strongly affected.