Transient carrier and field dynamics in quantum-well parallel transport: From the ballistic to the quasi-equilibrium regime

Femtosecond optical techniques have been used to probe transient electron transport and electric field dynamics in high field parallel transport, in a horizontal p-i-n quantum-well structure. Physical quantities associated with transport transients, such as electron distribution functions, electron velocity, and electron acceleration, have been measured by means of femtosecond absorption saturation, excitonic electroabsorption, and terahertz 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 the experiments. The time-dependent electric field has also been measured. It is found that both space-charge screening and terahertz radiation contribute to the collapse of the applied electric field at high carrier densities. Under such circumstances, transport and electric field dynamics are strongly coupled. An energy overshoot due to the field collapse coupled with strong electron-LO phonon scattering on a time scale of 200 fs is demonstrated