Ultrafast coherent electron transport in quantum cascade laser structures

Summary form only given. Until now, a large amount of work has been done on quantum cascade lasers (QCL). However, direct experimental information on the microscopic mechanisms is still poor. A very important point is the transport of electrons from the injector through the injection barrier (IB) into the upper laser level. Here, two contradictory pictures exist. On the one hand it is assumed that this transport is coherent via resonant tunneling. Due to the high electron density in the injector it is, on the other hand, considered to be incoherent via dissipative scattering processes. So far, a direct experimental proof is missing. Here, we present the first direct time-resolved experimental study of transport from the injector into the active region. We have performed pump-probe measurements on three different GaAs/AlGaAs QCL structures under current injection.