  Ultrashort electron beams with narrow energy spread, high charge, and low
jitter are essential for resolving phase transitions in metals, semiconductors,
and molecular crystals. These semirelativistic beams, produced by
phototriggered electron guns, are also injected into accelerators for x-ray
light sources. The achievable resolution of these time-resolved electron
diffraction or x-ray experiments has been hindered by surface field and timing
jitter limitations in conventional RF guns, which thus far are <200 MV/m and
>96 fs, respectively. A gun driven by optically-generated single-cycle THz
pulses provides a practical solution to enable not only GV/m surface fields but
also absolute timing stability, since the pulses are generated by the same
laser as the phototrigger. Here, we demonstrate an all-optical THz gun yielding
peak electron energies approaching 1 keV, accelerated by 300 MV/m THz fields in
a novel micron-scale waveguide structure. We also achieve quasimonoenergetic,
sub-keV bunches with 32 fC of charge, which can already be used for
time-resolved low-energy electron diffraction. Such ultracompact, easy to
implement guns driven by intrinsically synchronized THz pulses that are pumped
by an amplified arm of the already present photoinjector laser provide a new
tool with potential to transform accelerator based science.
