Femtosecond-pulse distortion in quantum wells

We show that a low-intensity femtosecond pulse is severely distorted while propagating through a relatively thin (<7000 Å) GaAs multiple-quantum-well sample and that this pulse distortion depends critically on the dephasing time ${\mathit{T}}_2$ and the total thickness l. An interferometric measurement reveals the existence of well-defined nodes at which the envelope function changes its sign. This pulse distortion significantly affects femtosecond experiments such as pump-probe or four-wave-mixing experiments.