Ultrafast response of Harmonic Modelocked THz Lasers

Harmonic modelocking of lasers is usually employed to attain high repetition rates. Despite the favourable dynamics of THz quantum cascade lasers (QCLs), harmonic active modelocking has not been demonstrated. In this work, using ultrafast time resolved spectroscopy, we present the first demonstrations of active modelocking performed at the fundamental and second harmonic on metal-metal THz QCL. We will also show that in certain cases, QCLs can spontaneously attain self-starting harmonic behaviour up to the 15th harmonic through self-microwave generation, which highlights the electronic and photonic nature of these devices.