Observation of time-resolved picosecond stimulated photon echoes and free polarization decay in GaAs/AlGaAs multiple quantum wells

We report measurements of exciton dynamics in GaAs multiple-quantum-well structures at low temperature using three-pulse picosecond four-wave mixing. The time-resolved measurements of the emission show both a delayed signal (stimulated photon echo) and a prompt signal (free polarization decay) with different decay times. The results suggest that, in addition to the inhomogeneously broadened resonance, a homogeneously broadened resonance contributes to the optical response of the exciton.