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The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. Here, using time-resolved THz techniques, we show the first of demonstration harmonic injection and mode- locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. This generates multiple THz pulses per round trip in both active and self-starting harmonic regimes. This behavior is supported by time-resolved Maxwell-Bloch simulations of induced gain and loss in the system. This work exploits the inherent ultrafast dynamics of QCLs and opens up new avenues in THz pulse generation.
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