  It is believed that passive mode locking is virtually impossible in quantum
cascade lasers (QCLs) because of too fast carrier relaxation time. Here, we
revisit this possibility and theoretically show that stable mode locking and
pulse durations in the few cycle regime at terahertz (THz) frequencies are
possible in suitably engineered bound-to-continuum QCLs. We achieve this by
utilizing a multi-section cavity geometry with alternating gain and absorber
sections. The critical ingredients are the very strong coupling of the absorber
to both field and environment as well as a fast absorber carrier recovery
dynamics. Under these conditions, even if the gain relaxation time is several
times faster than the cavity round trip time, generation of few-cycle pulses is
feasible. We investigate three different approaches for ultrashort pulse
generation via THz quantum cascade lasers, namely passive, hybrid and colliding
pulse mode locking.
