Quantum kinetic memory effects during electron-phonon interaction in semiconductors

We report on the first observation of wave mechanical features in free carrier relaxation. The emission of LO phonons by non-equilibrium electrons is studied via ultrasensitive transmission spectroscopy. Electrons are excited in a bulk GaAs sample by energetically narrow 120 fs pump pulses. The time evolution of the energy distribution is probed measuring the induced transmission changes with broadband 25 fs pulses. It is demonstrated that energy is not conserved in scattering events on ultrafast time scales. A phonon replica of the initial electron distribution starts energetically broadened. Within the phonon oscillation cycle of 115 fs, memory effects resulting from quantum interference become operative: The phonon satellite narrows towards the line width given by the photoexcited distribution. This process is repeated for every step in the LO cascade. These phenomena dominate hot carrier dynamics whenever scattering rates are comparable to or even faster than the frequency of the energy quanta exchanged. The influence of the electron-phonon coupling strength is investigated via non-degenerate four-wave-mixing experiments: LO phonon quantum beats are found in the decay of the coherent interband polarization in GaAs. In contrast, the oscillations are overdamped in InP as a result of the stronger Froehlich interaction in this more polar compound.