Self-Starting Harmonic Emission and Active Harmonic Modelocking in THz QCLs

In laser technology, harmonic modelocking is usually employed to attain higher repetition rates that is associated with lower noise systems. Despite the favourable dynamics of THz QCLs, harmonic active modelocking has not been experimentally demonstrated yet. In this work, we present the results of active modelocking experiments performed at the fundamental and second harmonic on a 6mm double metal THz QCL. We will also show that in certain cases, QCLs can spontaneously attain second harmonic emission without an active modulation, i.e. self-starting harmonic behaviour.