  Harmonic comb state has proven to be emerging in quantum cascade lasers and
promoted by an interplay between parametric gain and spatial hole burning. We
report here on robust, pure, self-starting harmonic mode locking in
Copper-based double-metal THz quantum cascade lasers. Different harmonic orders
can be excited in the same laser cavity depending on the pumping condition and
stable harmonic combs spanning more than 600 GHz bandwidth at 80 K are
reported. Such devices can be RF injected and the free running coherence is
assessed by means of self-mixing technique performed at 50 GHz. A theoretical
model based on Maxwell-Bloch equations including an asymmetry in the gain
profile is used to interpret the data.
