  In this paper we describe an ultrafast scanning electron microscope setup
developed for the research of inelastic scattering of electrons at optical
near-fields of periodic dielectric nanostructures. Electron emission from the
Schottky cathode is controlled by ultraviolet femtosecond laser pulses. The
electron pulse duration at the interaction site is characterized via
cross-correlation of the electrons with an infrared laser pulse that excites a
synchronous periodic near-field on the surface of a silicon nanostructure. The
lower limit of 410 fs is found in the regime of a single electron per pulse.
The role of pulse broadening due to Coulomb interaction in multielectron pulses
is investigated. The setup is used to demonstrate an increase of the
interaction distance between the electrons and the optical near-fields by
introducing a pulse-front-tilt to the infrared laser beam. Further we show the
dependence of the final electron spectra on the resonance condition between the
phase velocity of the optical near-field and the electron propagation velocity.
The resonance is controlled by adjusting the initial electron energy/velocity
and by introducing a linear chirp to the structure period allowing to increase
the final electron energy gain up to a demonstrated 3.8 keV.
