Theoretical investigation of harmonic frequency comb formation dynamics in defect-engineered THz quantum cascade lasers

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.