This manuscript describes harmonic generation in semiconductor superlattices, starting from a nonequilibrium Green's functions input to relaxation rate-type analytical approximations for the Boltzmann equation in which imperfections in the structure lead to asymmetric current flow and scattering processes under forward and reverse bias. The resulting current-voltage curves and the predicted consequences on harmonic generation, notably the development of even harmonics, are in good agreement with experiments. Significant output for frequencies close to 1 THz (7th harmonic) at room temperature, after excitation by a 141-GHz input signal, demonstrate the potential of superlattice devices for gigahertz to terahertz applications.

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We study the impact of interface roughness on the operation of mid-IR and THz quantum cascade lasers. Particular emphasis is given towards the differences between the Gaussian and exponential roughness distribution functions, for which we present results from simulation packages based on nonequilibrium Green's functions and density matrices. The Gaussian distribution suppresses scattering at high momentum transfer which enhances the lifetime of the upper laser level in mid-IR lasers. For THz lasers, a broader range of scattering transitions is of relevance, which is sensitive to the entire profile of the interface fluctuations. Furthermore we discuss the implementation of interface roughness within a two band model.

Quantum Cascade Lasers (QCLs) are generally designed to avoid negative differential conductivity (NDC) in the vicinity of the operation point in order to prevent instabilities. We demonstrate, that the threshold condition is possible under an inhomogeneous distribution of the electric field (domains) and leads to lasing at an operation point with a voltage bias normally attributed to the NDC region. For our example, a Terahertz QCL operating up to the current maximum temperature of 199 K, the theoretical findings agree well with the experimental observations. In particular, we experimentally observe self-sustained oscillations with GHz frequency before and after threshold. These are attributed to traveling domains by our simulations. Overcoming the design paradigm to avoid NDC may allow for the further optimization of QCLs with less dissipation due to stabilizing background current. arXiv:1806.01828v2 [cond-mat.mes-hall]

Abstract. In this work we include electron-electron interaction beyond Hartree-Fock level in our non-equilibrium Green's function approach by a crude form of GW through the Single Plasmon Pole Approximation. This is achieved by treating all conduction band electrons as a single effective band screening the Coulomb potential. We describe the corresponding self-energies in this scheme for a multi-subband system. In order to apply the formalism to heterostructures we discuss the screening and plasmon dispersion in both 2D and 3D systems. Results are shown for a four well quantum cascade laser with different doping concentration where comparisons to experimental findings can be made. IntroductionSince the quantum cascade laser (QCL) was introduced more than 20 years ago [1] it has continuously been improved and redesigned to operate from the mid-infrared all the way down to the Terahertz (THz) range. Compact laser sources at these wavelengths are valuable for spectroscopic applications [2] but a major problem is that THz QCLs do not operate at room temperature. This can be overcome via difference frequency generation which recently has been demonstrated at powers in the milli-watt range [3]. Direct THz-QCLs still generate a lot of interest in the research community due to the promise of higher wall-plug efficiency and the prospects of miniaturization of cryo-coolers.The main temperature degrading mechanism of the THz QCLs is currently not fully understood and is still debated by the community [4]. This is a challenge to theory and there is a great need of realistic modeling tools that are able to treat all important quantum effects on the same footing. A summary of different methods for simulating these structures is found in [5]. Monte Carlo simulations have shown that electron-electron interaction, beyond the meanfield or Hartree approximation can influence the dynamics of THz QCLs [6,7,8].In this work we include and study the effects of a simple electron-electron scattering via the Single Plasmon Pole Approximation (SPPA) [9,10]. In this approximation we capture both the static limit as well as dynamic effects. This gives an energy dependent (non-local in time) interaction beyond the Hartree-Fock approximation. This has been studied in a similar model with promising results [11], and with this work we want to adapt the idea into our model described in Ref. [12]. In other methods based on Non-Equilibrium Green's Functions (NEGF)

We present a systematic comparison of the results from our non-equilibrium Green's function formalism with a large number of AlGaAs-GaAs terahertz quantum cascade lasers previously published in the literature. Employing identical material and simulation parameters for all samples, we observe that discrepancies between measured and calculated peak currents are similar for samples from a given group. This suggests that the differences between experiment and theory are partly due to a lacking reproducibility for devices fabricated at different laboratories. Varying the interface roughness height for different devices, we find that the peak current under lasing operation hardly changes, so that differences in interface quality appear not to be the sole reason for the lacking reproducibility.

The gain spectrum of a terahertz quantum cascade laser is analysed by a non equilibrium Green's functions approach. Higher harmonics of the response function were retrievable, providing a way to approach nonlinear phenomena in quantum cascade lasers theoretically. Gain is simulated under operation conditions and results are presented both for linear response and strong laser fields. An iterative way of reconstructing the field strength inside the laser cavity at lasing conditions is described using a measured value of the level of the losses of the studied system. Comparison with recent experimental data from time-domain-spectroscopy indicates that the experimental situation is beyond linear response.Comment: 4 pages, 3 figures included in text, to appear in Applied Physics Letter

We use a numerical model based on non-equilibrium Green's functions to investigate the influence of interface roughness (IFR) scattering in terahertz quantum cascade lasers. We confirm that IFR is an important phenomenon that affects both current and gain. The simulations indicate that IFR causes a leakage current that transfers electrons from the upper to the lower laser state. In certain cases, this current can greatly reduce gain. In addition, individual interfaces and their impact on the renormalized single particle energies are studied and shown to give both blue- and red-shifts of the gain spectrum.