Modeling the Influence of Charge Carrier Diffusion on Harmonic Comb Formation in Quantum Cascade Lasers

The quantum cascade laser (QCL), operating as an optical frequency comb, is highly attractive for applications in metrology and sensing. Recently, the generation of coherent optical frequency combs in QCLs with large intermodal spacing has been demonstrated. These harmonic combs promise superior quantum features and may expand the scope of photonic quantum technologies. This paper presents a numerical study of self-starting harmonic mode-locking in terahertz QCL devices in the presence of charge carrier diffusion. With our detailed multidomain simulation approach and the possibility of artificially turning off spatial hole burning, we observe a strong dependence of the laser state on the strength of the population grating.