  Field-emission patterns from a clean tungsten tip apex induced by femtosecond
laser pulses have been investigated. Strongly asymmetric field-emission
intensity distributions are observed depending on three parameters: (1) the
polarization of the light, (2) the azimuthal and (3) the polar orientation of
the tip apex relative to the laser incidence direction. In effect, we have
realized an ultrafast pulsed field-emission source with site selectivity of a
few tens of nanometers. Simulations of local fields on the tip apex and of
electron emission patterns based on photo-excited nonequilibrium electron
distributions explain our observations quantitatively. Electron emission
processes are found to depend on laser power and tip voltage. At relatively low
laser power and high tip voltage, field-emission after two-photon
photo-excitation is the dominant process. At relatively low laser power and low
tip voltage, photoemission processes are dominant. As the laser power
increases, photoemission from the tip shank becomes noticeable.
