  The generation of stable trains of ultra-short (fs-ps), terahertz
(THz)-frequency radiation pulses, with large instantaneous intensities, is an
underpinning requirement for the investigation of light-matter interactions,
for metrology and for ultra-high-speed communications. In solid-state
electrically-pumped lasers, the primary route for generating short pulses is
through passive mode-locking. However, this has not yet been achieved in the
THz range, defining one of the longest standing goals over the last two
decades. In fact, the realization of passive mode-locking has long been assumed
to be inherently hindered by the fast recovery times associated with the
intersubband gain of THz lasers. Here, we demonstrate a self-starting
miniaturized ultra-short pulse THz laser, exploiting an original device
architecture that includes the surface patterning of multilayer-graphene
saturable absorbers distributed along the entire cavity of a double-metal
semiconductor 2.30-3.55 THz wire laser. Self-starting pulsed emission with
4.0-ps-long pulses in a compact, all-electronic, all-passive and inexpensive
configuration is demonstrated.
