Probing Transient Localized Electromagnetic Fields Using Low- Energy Point-Projection Electron Microscopy

Low kinetic energy electrons are of interest for probing nanoscale dynamic processes using ultrafast electron microscopy techniques. Their low velocities reduce radiation doses and enhance the interaction with confined electromagnetic fields and, thus, may enable ultrafast spectroscopy of single nanostructures. Recent improvements in the spatial and temporal resolution of ultrafast, low-energy electron microscopy have been achieved by combining nanotip photoemitters and point-projection imaging schemes. Here, we use such an ultrafast point-projection electron microscope (UPEM) to analyze the interaction of low-energy electrons with transient electric fields created by photoemission from a nanogap antenna. By analyzing their kinetic energy distribution, we separate angular deflection due to radial field components from electron energy gain and loss due to their axial acceleration. Our measurements open up a route toward the spatial and temporal characterization of vectorial near-fields by low energy electron streaking spectroscopy.