Effective Rabi frequency in semiconductor lasers and the origin of self-starting harmonic frequency combs

Optical frequency combs have become a key research topic in optics and photonics. A peculiar comb state is the harmonic frequency comb (HFC), where optical lines are spaced by integer multiples of the cavity's free-spectral range. The spontaneous formation of HFCs has recently been observed in semiconductor lasers with fast gain recovery, such as Quantum Cascade Lasers (QCLs), although the underlying physical mechanism remains unclear. In this work, we provide a physical interpretation for the formation of HFCs in QCLs, based on a resonance phenomenon between an effective Rabi frequency and a mode of the laser cavity. This is corroborated by the results of the numerical integration of the effective semiconductor Maxwell-Bloch equations used to describe the multimode laser dynamics, as well as by the linear stability analysis of the continuous wave emission at threshold.