  We demonstrate an experimental technique for both transverse and longitudinal
characterization of bunched femtosecond free electron beams. The operation
principle is based on monitoring of the current of electrons that obtained an
energy gain during the interaction with the synchronized optical near-field
wave excited by femtosecond laser pulses. The synchronous
accelerating/decelerating fields confined to the surface of a silicon
nanostructure are characterized using a highly focused sub-relativistic
electron beam. Here the transverse spatial resolution of 450 nm and femtosecond
temporal resolution achievable by this technique are demonstrated.
