TRANSIENT CARRIER AND FIELD-DYNAMICS IN QUANTUM-WELL PARALLEL TRANSPORT - FROM THE BALLISTIC TO THE QUASI-EQUILIBRIUM REGIME

We have performed a series of experiments, applying femtosecond optical techniques to probe transient electron transport and electric field dynamics in high field parallel transport, in a horizontal p-i-n quantum-well structure. We have attempted to measure physical quantities associated with transport transients, such as electron distribution functions, electron velocity, and electron acceleration, by means of femtosecond absorption saturation, excitonic electroabsorption, and THz electromagnetic radiation, at electric fields of up to 16 kV/cm. Important transport phenomena such as ballistic acceleration on a time scale of 150 fs, and the onset of a saturated velocity on a time scale of 1 ps, have been observed in our experiments. Furthermore, we have measured the time-dependent electric field. We find that both space-charge screening and THz radiation contribute to collapse of the applied electric field at high carrier densities. Under such circumstances, transport and electric field dynamics are strongly coupled. We demonstrate an energy overshoot due to the field collapse coupled with strong electron-LO phonon scattering on a time scale of 200 fs. Self-consistent transport and field considerations are required in theoretical modeling.