Terahertz (THz) quantum cascade lasers (QCL) may operate as harmonic frequency combs, exhibiting a mode-separation of multiple times the round trip frequency. This work aims to shed light on the coupled field-electron dynamics that lead to harmonic mode-locking in defect-engineered THz QCLs. Therefore, we use the Maxwell-Bloch equations describing a medium of two-level quantum systems interacting with the electric field in the laser cavity. We find that both the amplitude and phase of the electric field are coupled to the introduced defects, and the system quickly reaches a locked state. Despite the presence of the reflectors, spatial hole burning is necessary to enable multimode operation.
Mid-infrared optical solitons may be a powerful tool for applications in on-chip integrated photonics and spectroscopy, as they provide broadband, phase-locked frequency combs. Quantum cascade lasers (QCLs) embedded in a ring cavity have been found to enable self-starting optical soliton generation. In order to study these phenomena numerically, we use a model based on coupled Maxwell-density matrix equations. The introduction of backscattering in our model stabilizes self-assembled soliton field solutions, which is in very good agreement with experimental data. In this contribution, we present our model and discuss the mechanisms that lead to soliton operation in ring QCLs.
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