Exciton–LO-Phonon Quantum Kinetics: Evidence of Memory Effects in Bulk GaAs

Oscillations of the transient four-wave-mixing signal with a period of about $100\mathrm{fs}$ are observed in bulk GaAs using 14 fs pulses tuned to the exciton resonance at low temperatures. The measurements are explained in terms of the non-Markovian quantum kinetics for electron-hole pairs due to LO-phonon scattering. It is shown that the observed oscillations are evidence for memory effects. The experiments provide a first test of the central ideas of quantum kinetics, in which the effects of quantum coherence and of dissipation are intrinsically connected.