We report the first observation of the coupling of strong
optical
near fields to wavepackets of free, 100 eV electrons with <50 fs
temporal resolution in an ultrafast point-projection microscope. Optical
near fields are created by excitation of a thin, nanometer-sized Yagi-Uda
antenna, with 20 fs near-infrared laser pulses. Phase matching between
electrons and near fields is achieved due to strong spatial confinement
of the antenna near field. Energy-resolved projection images of the
antenna are recorded in an optical pump–electron probe scheme.
We show that the phase modulation of the electron by transverse-field
components results in a transient electron deflection while longitudinal
near-field components broaden the kinetic energy distribution. This
low-energy electron near-field coupling is used here to characterize
the chirp of the ultrafast electron wavepackets, acquired upon propagation
from the electron emitter to the sample. Our results bring direct
mapping of different vectorial components of highly localized optical
near fields into reach.