Influence of Eigenstate Basis Sets on Self-Starting Harmonic Comb Simulations in THz QCLs

Quantum cascade lasers (QCLs) in self-starting harmonic comb operation are highly attractive for applications in optical and quantum communication. Recently, harmonic mode-locking in QCLs has been reported in several experimental studies, which demands for a robust theoretical description. Here, we provide an extensive numerical study of a terahertz QCL active gain medium capable of self-starting harmonic mode-locking. Our theoretical characterization is divided into stationary carrier transport simulations based on the ensemble Monte Carlo method and dynamical simulations of the light-matter interaction based on multi-level Maxwell-Bloch equations. We investigate the influence of the chosen eigenstates basis on the gain spectrum and present self-consistent simulation results of stable harmonic comb operation with a mode spacing of four times the free spectral range.