  Femtosecond electron bunches with keV energies and eV energy spread are
needed by condensed matter physicists to resolve state transitions in carbon
nanotubes, molecular structures, organic salts, and charge density wave
materials. These semirelativistic electron sources are not only of interest for
ultrafast electron diffraction, but also for electron energy-loss spectroscopy
and as a seed for x-ray FELs. Thus far, the output energy spread (hence pulse
duration) of ultrafast electron guns has been limited by the achievable
electric field at the surface of the emitter, which is 10 MV/m for DC guns and
200 MV/m for RF guns. A single-cycle THz electron gun provides a unique
opportunity to not only achieve GV/m surface electric fields but also with
relatively low THz pulse energies, since a single-cycle transform-limited
waveform is the most efficient way to achieve intense electric fields. Here,
electron bunches of 50 fC from a flat copper photocathode are accelerated from
rest to tens of eV by a microjoule THz pulse with peak electric field of 72
MV/m at 1 kHz repetition rate. We show that scaling to the readily-available
GV/m THz field regime would translate to monoenergetic electron beams of ~100
keV.
