Pure and Self-starting Harmonic Combs in THz Quantum Cascade Lasers: Theory and Experiments

Quantum Cascade Lasers (QCLs) are compact and powerful sources of frequency combs in the mid-IR and THz region [1] , [2] . They naturally tend to operate on the fundamental comb state i.e. the mode spacing is given by the round-trip time of the cavity. Lately it was observed that free running mid-IR QCLs operate, under optical feedback reduction, in a so-called harmonic comb state where the mode spacing is a multiple of the fundamental one [3] . The origin arises from a single mode instability close to threshold which evolves to the harmonic state and normally falls into the fundamental comb state. Harmonic combs in the THz were so far only observed in mode-locked THz QCLs [4] .