Self-starting harmonic comb emission in THz quantum cascade lasers

Harmonic comb states have proven to be ubiquitous in mid-IR quantum cascade lasers. 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 600GHz at 80K are reported. Such devices can be RF injected, and the free running coherence is assessed by means of a self-mixing technique performed at 50GHz. A theoretical model based on Maxwell-Bloch equations including an asymmetry in the gain profile is used to interpret the data.