Coherent charge transport in semiconductor quantum cascade structures

Quantum cascade structures have found extensive application in electrically driven semiconductor lasers working in the mid- to far-infrared spectral range. Optical amplification in such unipolar devices is based on a population inversion between quasi-two-dimensional conduction subbands in coupled quantum wells. The population inversion in the active region is generated by electrons tunnelling from an injector region through a barrier into the upper laser subband and by ultrafast extraction of these electrons out of the lower laser subband through a barrier into the next injector region. Such transport processes on ultrafast timescales have been the subject of extensive experimental and theoretical work without, however, reaching a clear physical picture of the microscopic electron dynamics. In this review, we report a comprehensive experimental study of electron transport in electrically driven quantum cascade structures. Ultrafast quantum transport from the injector into the upper laser subband is investigated by mid-infrared pump-probe experiments directly monitoring the femtosecond saturation and subsequent recovery of electrically induced optical gain. For low current densities, low lattice temperatures and low pump pulse intensities, the charge transport is dominantly coherent, leading to pronounced gain oscillations due to the coherent motion of electron wavepackets. For higher current densities, lattice temperatures, or pump intensities, the gain recovery shows an additional incoherent component, which essentially follows the pump-induced heating and subsequent cooling of the carrier gas in the injector.