Polarization-dependent picosecond excitonic nonlinearities and the complexities of disorder

Time-resolved emission of the hh1 exciton transient four-wave-mixing response in multiple quantum wells shows the unexpected result that the signal emission is either a photon echo or a free polarization decay, depending on the relative polarization of the excitation beams. The energy and temperature dependences of the dephasing rates and the transport and saturation characteristics show that the two excitations are significantly different. Explanations for the disappearance of the echo, which arises due to the presence of disorder, and the origin of the prompt signal are considered.