Abstract-Robust Density Matrix Simulation of Terahertz Quantum Cascade Lasers

Benjamin A. Burnett, Andrew Pan, Chi On Chui, Benjamin S. Williams,

https://ieeexplore.ieee.org/document/8402230/

A common setback to electron transport models for quantum cascade laser active regions is the inability to freely simulate widely varying designs. One solution to this problem is to use a density matrix formalism with a generalized treatment of scattering, wherein the well-defined energy eigenbasis is used, and the relative simplicity of the density matrix can be taken advantage of for rapid simulations. We have developed such a model from first principles in the past, and now built a simulator for terahertz quantum cascade lasers that calculates a fully self-consistent solution to the coupled problem of bandstructure, lasing field strength, and space charge. This level of depth enables us to examine the model's performance across much of the design space and operating temperatures, for which we find generally good agreement. Areas for future improvement of the model are discussed, particularly the treatment of electron–electron scattering and continuum leakage. The model also enables us to make qualitative insights into the microscopic workings of the active regions, such as the nonequilibrium subband distributions and their response to the optical field, and the possibility for using two sequential optical transitions.

Abstract-Density matrix modeling of quantum cascade lasers without an artificially localized basis: A generalized scattering approach

Andrew Pan, Benjamin A. Burnett, Chi On Chui, and Benjamin S. Williams

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.085308

We derive a density matrix (DM) theory for quantum cascade lasers (QCLs) that describes the influence of scattering on coherences through a generalized scattering superoperator. The theory enables quantitative modeling of QCLs, including localization and tunneling effects, using the well-defined energy eigenstates rather than the ad hoc localized basis states required by most previous DM models. Our microscopic approach to scattering also eliminates the need for phenomenological transition or dephasing rates. We discuss the physical interpretation and numerical implementation of the theory, presenting sets of both energy-resolved and thermally averaged equations, which can be used for detailed or compact device modeling. We illustrate the theory's applications by simulating a high performance resonant-phonon terahertz (THz) QCL design, which cannot be easily or accurately modeled using conventional DM methods. We show that the theory's inclusion of coherences is crucial for describing localization and tunneling effects consistent with experiment.

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Abstract-Design strategy for terahertz quantum dot cascade lasers

Benjamin A. Burnett and Benjamin S. Williams

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-22-25471

The development of quantum dot cascade lasers has been proposed as a path to obtain terahertz semiconductor lasers that operate at room temperature. The expected benefit is due to the suppression of nonradiative electron-phonon scattering and reduced dephasing that accompanies discretization of the electronic energy spectrum. We present numerical modeling which predicts that simple scaling of conventional quantum well based designs to the quantum dot regime will likely fail due to electrical instability associated with high-field domain formation. A design strategy adapted for terahertz quantum dot cascade lasers is presented which avoids these problems. Counterintuitively, this involves the resonant depopulation of the laser’s upper state with the LO-phonon energy. The strategy is tested theoretically using a density matrix model of transport and gain, which predicts sufficient gain for lasing at stable operating points. Finally, the effect of quantum dot size inhomogeneity on the optical lineshape is explored, suggesting that the design concept is robust to a moderate amount of statistical variation.

© 2016 Optical Society of America

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Abstract-Origins of Terahertz Difference Frequency Susceptibility in Midinfrared Quantum Cascade Lasers

Benjamin A. Burnett and Benjamin S. Williams

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.5.034013

We present a density-matrix-based transport model applicable to quantum cascade lasers which computes both linear and nonlinear optical properties coherently and nonperturbatively. The model is applied to a dual-active-region midinfrared quantum cascade laser which generates terahertz radiation at the difference frequency between two midinfrared pumps. A new mechanism for terahertz generation is identified as self-detection, ascribed to the beating of current flow following the intensity, associated with stimulated emission. This mechanism peaks at optical rectification but exhibits a bandwidth reaching significantly into the terahertz range, which is primarily limited by the subpicosecond intersubband lifetimes. A metric is derived to assess the strength of self-detection in candidate active regions through experiment alone, and suggestions are made for improvement of the performance at frequencies below 2 THz.

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