Quantum coherent dynamics of free carriers in semiconductors

Synchronized femtosecond pulses of different wavelength and bandwidth are used to investigate intraband and interband coherent effects in polar semiconductors. At low excitation densities the relaxation of nonthermal electron distributions by spontaneous emission of LO phonons in GaAs is monitored. Quantum kinetic behaviour of the dynamics beyond the semiclassical Boltzmann equation is demonstrated for the first time. Violation of the energy conservation as well as memory effects are shown to occur on an ultrashort time scale of 100 fs. For strong excitation we present the first observation of Rabi-oscillations of the occupation density in the absorption continuum of InP. Despite high carrier densities exceeding 10¹⁸cm^-3 dephasing times are long enough to allow for the appearance of two strongly damped Rabi-cycles at a peak intensity of the pump pulses of 17 GW/cm².