  In this paper we present proof of principle experiments of an optical gating
concept for free electrons. We demonstrate a temporal resolution of 1.2+-0.3 fs
via energy and transverse momentum modulation as a function of time. The scheme
is based on the synchronous interaction between electrons and the near-field
mode of a dielectric nano-grating excited by a femtosecond laser pulse with an
optical period duration of 6.5 fs. The sub-optical cycle resolution
demonstrated here is promising for use in laser-driven streak cameras for
attosecond temporal characterization of bunched particle beams as well as
time-resolved experiments with free-electron beams. We expect that 10 as
temporal resolution will be achieved in the near future using such a scheme.
