Femtosecond ultrabright electron
sources with spatially structured
emission are an enabling technology for free-electron lasers, compact
coherent X-ray sources, electron diffractive imaging, and attosecond
science. In this work, we report the design, modeling, fabrication,
and experimental characterization of a novel ultrafast optical field
emission cathode comprised of a large (>100 000 tips), dense
(4.6 million tips·cm<sup>–2</sup>), and highly uniform
(<1 nm tip radius deviation) array of nanosharp high-aspect-ratio
silicon columns. Such field emitters offer an attractive alternative
to UV photocathodes while providing a direct means of structuring
the emitted electron beam. Detailed measurements and simulations show
pC electron bunches can be generated in the multiphoton and tunneling
regime within a single optical cycle, enabling significant advances
in electron diffractive imaging and coherent X-ray sources on a subfemtosecond
time scale, not possible before. At high charge emission yields, a
slow rollover in charge is explained as a combination of the onset
of tunneling emission and the formation of a virtual cathode