Nanoroughness localization of excitons in GaAs multiple quantum wells studied by transient four-wave mixing

The interface roughness on a nanometer scale plays a decisive role in dephasing of excitons in GaAs multiple quantum wells. The excitonic four-wave mixing signal shows a free polarization decay and a corresponding homogeneously broadened line from areas with interface roughness on a scale larger than the exciton diameter. A photon echo and a corresponding inhomogeneously broadened line are observed from areas of interface roughness on a scale less than the exciton diameter. In the present study we observe both mechanisms simultaneously, and are able to clearly distinguish between the two mechanisms by spectrally resolving the transient four-wave-mixing signal.