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Terahertz (THz) quantum cascade lasers (QCLs) are technologically important laser sources for the THz range but are complex to model. An efficient extended rate equation model is developed here by incorporating the resonant tunneling mechanism from the density matrix formalism, which permits to simulate THz QCLs with thick carrier injection barriers within the semi-classical formalism. A self-consistent solution is obtained by iteratively solving the Schrödinger–Poisson equation with this transport model. Carrier–light coupling is also included to simulate the current behavior arising from stimulated emission. As a quasi-ab initio model, intermediate parameters, such as pure dephasing time and optical linewidth, are dynamically calculated in the convergence process, and the only fitting parameters are the interface roughness correlation length and height. Good agreement has been achieved by comparing the simulation results of various designs with experiments, and other models such as density matrix Monte Carlo and non-equilibrium Green's function method that, unlike here, require important computational resources. The accuracy, compatibility, and computational efficiency of our model enable many application scenarios, such as design optimization and quantitative insights into THz QCLs. Finally, the source code of the model is also provided in the supplementary material of this article for readers to repeat the results presented here, investigate, and optimize new designs. [ABSTRACT FROM AUTHOR]

The phonon modes of materials contain critical information on the quality of the crystals. Phonon modes also offer a wide range of polarization-dependent resonances in infrared that can be tailored to applications that require large dielectric function contrast in different crystal directions. Here, we investigate the far-field characteristics of MOCVD-grown Ga2O3 thin films. With a combination of cross-polarization FTIR and AFM characterization techniques, we propose an easy and non-invasive route to distinguish κ and β phases of Ga2O3 and study the quality of these crystals. Using numerical methods and cross-polarization spectroscopy, the depolarization characteristics of β-Ga2O3 are examined and depolarization strength values as high as 0.495 and 0.76 are measured, respectively, for 400 and 800 nm-thick β-Ga2O3. The strong birefringence near optical phonon modes of an 800 nm β-Ga2O3 on a sapphire substrate is used to obtain several polarization states for the reflected light in the second atmospheric window 8–14 µm. We anticipate that our findings open a new path for material characterization and wave plate design for the mid-IR range and offer novel possibilities for the future of IR on-chip photonics, thanks to the compatibility of β-Ga2O3 with standard nanofabrication technology. [ABSTRACT FROM AUTHOR]