Dynamic modeling of quantum cascade lasers based on Maxwell-density matrix approaches

Quantum cascade lasers (QCLs) are based on quantum-engineered active regions, offering unprecedented possibilities in custom-tailoring the optical emission. Especially for the realization of dynamic regimes such as mode-locked short-pulse as well as fundamental and harmonic comb generation, also a targeted design of the optical waveguide is essential. Recent efforts additionally focus on the microwave properties of the waveguide, exploiting the fact that it simultaneously acts as a microwave transmission line. This enables hybrid microwave and photonic functionalities of the QCL, e.g., improved mode-locking due to co-propagation of the optical waveform and microwave modulation of the bias. A thorough investigation of the dynamic operating regimes and targeted device design requires a versatile and numerically efficient simulation approach. We show that Maxwell-density matrix models are highly suitable for this purpose. The optical and, for hybrid devices, microwave dynamics is then modeled by suitable propagation equations derived from Maxwell’s equations, and coupled to a multilevel density matrix description of the quantum active region. Combining this approach with carrier transport simulations leads to a self-consistent model without phenomenological parameters.