Coherent charge transport in semiconductor quantum cascade structures

Abstract

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