Evanescent-wave acceleration of ultrashort electron pulses

High-power femtosecond laser pulses have been used to excite surface plasmons in 500 Å silver and gold films. Nonlinear excitation results in the emission of electron bunches through multiphoton excitation at low power and laser-induced field emission at high power. The energies of photoelectrons are found to extend as high as 0.4 keV. Calculations show that these high energies are due to ponderomotive acceleration in an evanescent field extending from the metal film out into the vacuum. The theoretical calculations suggest that femtosecond electron pulses with relativistic energies can be generated using longer wavelengths or by developing the surface morphology.