Aspects of cavity engineering in THz quantum cascade laser frequency combs

Terahertz quantum cascade laser (QCL) frequency combs are a disruptive technology for spectroscopy, imaging, and quantum technologies in this spectral range. For advanced development and tailoring of frequency comb properties, detailed modeling of the QCL device is necessary. Recent achievements in the field of THz QCLs have unveiled that custom modifications of the laser cavity by dispersion engineering or tapered waveguide sections significantly influence the laser's behavior. In this article, we present a numerical model based on the Maxwell-density matrix formalism that captures such cavity effects in detail, yielding a better understanding of the QCL dynamics and opening the possibility of designing cavities for custom laser applications. We show that waveguide engineering in terms of dispersion compensation and field enhancement can stabilize an unlocked multimode state into frequency comb operation and even shape its properties, such as the bandwidth or the mode spacing of harmonic frequency combs.