Femtosecond few- to single-electron point-projection microscopy for nanoscale dynamic imaging

Femtosecond electron microscopy produces real-space images of matter in a series ofultrafast snapshots. Pulses of electrons self-disperse under space-charge broadening,so without compression, the ideal operation mode is a single electron per pulse. Here,we demonstrate femtosecond single-electron point projection microscopy (fs-ePPM)in a laser-pump fs-e-probe configuration. The electrons have an energy of only150 eV and take tens of picoseconds to propagate to the object under study.Nonetheless, we achieve a temporal resolution with a standard deviation of 114 fs(equivalent to a full-width at half-maximum of 269 +/- 40 fs) combined with a spatialresolution of 100 nm, applied to a localized region of charge at the apex of a nanoscalemetal tip induced by 30 fs 800 nm laser pulses at 50 kHz. These observations demonstratereal-space imaging of reversible processes, such as tracking charge distributions,is feasible whilst maintaining femtosecond resolution. Our findings could find applicationas a characterization method, which, depending on geometry, could resolve tensof femtoseconds and tens of nanometres. Dynamically imaging electric and magneticfields and charge distributions on sub-micron length scales opens new avenues ofultrafast dynamics. Furthermore, through the use of active compression, such pulsesare an ideal seed for few-femtosecond to attosecond imaging applications which willaccess sub-optical cycle processes in nanoplasmonics