Quantum cascade lasers (QCLs) utilize quantized electron states as laser levels, which can be custom-tailored to the specific application by adequately designing the multi-quantum-well active region. With dephasing times between the upper and the lower laser level of about a picosecond, coherent light-matter interaction, along with other effects such as dispersion and spatial hole burning, governs the laser dynamics . Besides leading to multimode instabilities , the ultrafast dynamics in QCLs is increasingly exploited to implement innovative functionalities, such as the generation of frequency combs [2,3] and picosecond optical pulses  in the mid-infrared and terahertz regime.
For a targeted design of such structures and a deeper understanding of the complex QCL dynamics, a detailed theoretical model is required. We have developed a multi-domain simulation approach, which couples a Maxwell-Bloch type description of the light-matter interaction with advanced ensemble Monte Carlo (EMC) carrier transport simulations to eliminate empirical electron lifetimes . In the figure, simulation results for a QCL-based terahertz frequency comb source  are presented. The laser dynamics gives rise to a two-lobed power spectrum consisting of equidistant discrete comb lines [Fig. (a)]. Furthermore, a periodic temporal shifting of the power between the two spectral lobes is observed [Fig. (b)]. The two-lobed comb spectrum and the associated temporal switching dynamics have also been observed in experiment [3,5], validating our simulation model.
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