High spatial coherence in multiphoton-photoemitted electron beams

Nanometer-sharp metallic tips are known to be excellent electron
emitters. They are used in highest-resolution electron microscopes in
cold field emission mode to generate the most coherent electron beam in
continuous-wave operation. For time-resolved operation, sharp metal
needle tips have recently been triggered with femtosecond laser pulses.
We show here that electrons emitted with near-infrared femtosecond laser
pulses at laser oscillator repetition rates show the same spatial
coherence properties as electrons in cold field emission mode in cw
operation. From electron interference fringes, obtained with the help of
a carbon nanotube biprism beam splitter, we deduce a virtual source
size of less than (0.65 ± 0.06) nm for both operation modes, a factor of
ten smaller than the geometrical source size. These results bear
promise for ultrafast electron diffraction, ultrafast electron
microscopy, and other techniques relying on highly coherent and
ultrafast electron beams.