Time-resolved investigations of electronic transport dynamics in quantum cascade lasers based on diagonal lasing transition

In this study, the nature of electronic transport in quantum cascade lasers (QCLs) has been extensively investigated using an ultrafast time-resolved, degenerate, pump-probe optical technique. Our investigations enable a comprehensive understanding of the gain recovery dynamics in terms of a coupling of the electronic transport to the oscillating intracavity laser intensity. In QCLs that have a lasing transition diagonal in real space, studies of the near-threshold reveal that the transport of electrons changes bias region from phonon-limited relaxation (tens of picoseconds) below threshold to photon-driven transport via stimulated emission (a few picoseconds) above threshold. The gain recovery dynamics in the photon-driven regime is compared with conventional four-level lasers such as atomic, molecular, and semiconductor interband lasers. The depopulation dynamics out of the lower lasing state is explained using a tight-binding tunneling model and phonon-limited relaxation. For the superlattice relaxation, it is possible to explain the characteristic picosecond transport via dielectric relaxation; Monte Carlo simulations with a simple resistor model are developed, and the Esaki-Tsu model is applied. Subpicosecond dynamics due to carrier heating in the upper subband are isolated and appear to be at most about 10% of the gain compression compared with the contribution of stimulated emission. Finally, the polarization anisotropy in the active waveguide is experimentally shown to be negligible on our pump-probe data, supporting our interpretation of data in terms of gain recovery and transport.