Quantum beats from extended electronic states in quantum wells

We study the polarization decay of excitons in GaAs/AlGaAs quantum wells using timeresolved degenerate-four-wave-mixing (DFWM) with subpicosecond time resolution. The coherent excitation of two energetically close exciton transitions enables the observation of quantum beats in the diffracted signal. We observe such quantum beats between (I) light-hole and heavy-hole excitons, (II) heavy-hole excitons split by spatially varying confinement, and (III) anticrossed levels in a coupled quantum well structure. We furthermore show that the DFWM lineshape reveals a novel polarization interaction.