  Highly nonlinear optical processes, such as multiphoton photoemission,
require high intensities, typically achieved with ultrashort laser pulses and,
hence, were first observed with the advent of picosecond laser technology. An
alternative approach for reaching the required field intensities is offered by
localized optical resonances such as plasmons. Here, we demonstrate localized
multiphoton photoemission from plasmonic nanostructures under continuous-wave
illumination. We use synthesized plasmonic gold nanostars, which exhibit sharp
tips with structural features smaller than 5 nm, leading to
near-field-intensity enhancements exceeding 1000. This large enhancement
facilitates 3-photon photoemission driven by a simple continuous-wave laser
diode. We characterize the intensity and polarization dependencies of the
photoemission yield from both individual nanostars and ensembles. Numerical
simulations of the plasmonic enhancement, the near-field distributions, and the
photoemission intensities are in good agreement with experiment. Our results
open a new avenue for the design of nanoscale electron sources.
