All-optical injection and detection of ballistic charge currents in germanium

All optical techniques are used to inject and to study the relaxation dynamics of ballistic charge currents in clean germanium at room temperature without the application of external contacts or the use of externally applied fields. Ballistic currents are injected by the quantum interference between the transition amplitudes for direct one and two photon absorption of a pair of phase-locked and harmonically related ultrafast laser pulses. The transport of carriers following ballistic injection is temporally and spatially resolved using optical differential transmission techniques that are sensitive to the relative optical phase of the two injection pulses. The electron-hole dynamics are determined by the initial ballistic injection velocity, momentum relaxation, and space charge field effects. The injection process in Ge is similar to that in direct band gap materials but the indirect nature of Ge complicates the monitoring of the carrier dynamics, allowing the holes to play a more prominent role than in direct gap materials. The latter opens the possibility of following the hole (as opposed to the electron) dynamics. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3500547]