Ultrafast Coupling of Optical Near Fields to Low-Energy Electrons Probed in a Point-Projection Microscope

We report the first observation of the coupling of strongopticalnear fields to wavepackets of free, 100 eV electrons with <50 fstemporal resolution in an ultrafast point-projection microscope. Opticalnear fields are created by excitation of a thin, nanometer-sized Yagi-Udaantenna, with 20 fs near-infrared laser pulses. Phase matching betweenelectrons and near fields is achieved due to strong spatial confinementof the antenna near field. Energy-resolved projection images of theantenna are recorded in an optical pump-electron probe scheme.We show that the phase modulation of the electron by transverse-fieldcomponents results in a transient electron deflection while longitudinalnear-field components broaden the kinetic energy distribution. Thislow-energy electron near-field coupling is used here to characterizethe chirp of the ultrafast electron wavepackets, acquired upon propagationfrom the electron emitter to the sample. Our results bring directmapping of different vectorial components of highly localized opticalnear fields into reach.