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Tracing ultrafast modifications of the Franz-Keldysh absorption spectrum of biased AlxGa1−xAs heterostructure diodes, we directly analyze the dynamical build up of a nonequilibrium carrier avalanche due to impact ionization for electric fields F > 350 kV/cm. The timescale of the carrier multiplication is found to be in the order of 10 ps depending on the applied bias. Monte Carlo simulations in a simplified band structure agree well with the experiment. [ABSTRACT FROM AUTHOR]

After femtosecond photoinjection of carriers near the boundary of a parabolic shaped 250 nm wide potential well, electrons are found to coherently oscillate across the well instead of performing the intuitively expected unidirectional relaxation towards the bottom of the well. Surprisingly, the coherence of the periodic electron motion is maintained despite multiple phonon scattering events. This novel transport regime is predicted by Monte Carlo simulations and verified by analyzing the femtosecond transmission of the heterostructure. [ABSTRACT FROM AUTHOR]

Carrier capture into self-organized InGaAs/GaAs quantum dots is studied in a femtosecond transmission experiment. Resonantly generating carriers in the wetting layer, we analyze the population of both the band edge of the wetting layer and the quantum dot excited states. We find a carrier capture time of 3 ps that is independent of the carrier density providing that it remains small compared to the number of available electronic states. Moreover, we find that the capture process is predominantly spin-preserving in nature. These results suggest that phonon mediated scattering governs the quantum dot filling. [ABSTRACT FROM AUTHOR]