Ultrafast coherent electron transport in GaAs/AlGaAs quantum cascade structures

We report on the first direct time-resolved study of coherent electron transport in an electrically driven quantum cascade laser structure. The ultrafast quantum transport of electrons from the injector into the upper laser state is investigated by means of femtosecond mid-infrared pump-probe experiments directly monitoring the ultrafast saturation and subsequent recovery of electrically induced optical gain. For forward bias and spectral positions around the gain maximum we observe pronounced gain oscillations upon excitation giving direct evidence for a coherent wave packet motion from the injector into the upper laser state. (C) 2002 Elsevier Science B.V. All rights reserved.