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Predictive quantum theory of current and optical emission in quantum cascade lasers

We have implemented a self-consistent non-equilibrium Green's functions approach for vertical charge transport and optical gain in terahertz quantum cascade lasers (THz-QCL) in the stationary limit. We present theoretical results of the current-voltage characteristics and the temperature dependence of the optical gain for GaAs/Al.15Ga.85As THz-QCLs and compare our results with experimental data. We find excellent quantitative agreement for the current-voltage characteristics and the peak gain energy and identify non-radiative transitions between the laser levels as primary factor that limits the maximum operation temperature. Furthermore, we find significant coherent leakage of electrons in the upper laser level that increases the threshold current density. We propose a broadening of the collector well to efficiently suppress this coherent leakage and to reduce the threshold current.