Photon echoes and free-polarization decay in GaAs/AlAs multiple quantum wells: Polarization and time dependence

We have studied picosecond two-pulse degenerate four-wave mixing in GaAs/AlAs quantum wells under different polarization configurations of the two pump beams and the analyzed optical polarization. We observe two components, a prompt free polarization decay (FPD) and a delayed photon echo (PE), which show different polarization rules and dephasing times. The observed polarization rules are consistently explained in a theoretical model. Within this model, the two-component behavior is a direct consequence of short-range disorder. The PE dephasing rate is associated with the homogeneous broadening of the excitons while the FPD corresponds to the total dephasing rate, consisting of the homogeneous and inhomogeneous (short-range and long-range) broadenings.