Harmonic mode-locking of THz quantum cascade lasers (Conference Presentation)

Despite their ultrafast dynamics, modelocking of THz QCLs has been shown recently as a robust method to generate short THz pulses with pulse trains as short as ~4ps at a central frequency of ~2.5THz. This opens up applications of QCLs to synthesizer controlled frequency combs as well as operation of QCLs in more exotic modelocked regimes. In this work we demonstrate for the first time harmonic active modelocking of QCLs where pulses are generated at a harmonic of the round trip frequency. This is achieved by active modelocking at the second harmonic of the QCL fundamental round trip frequency, generating a short pulse train with two pulses per round-trip. This realisation potentially allows for higher average power from modelocked QCLs, modulated at multiple harmonics of the fundamental frequency, and is ideally adapted to the QCLs ultrafast dynamics with modulation beyond 60 GHz attainable.