Sub-optical-cycle electron pulse trains from metal nanotips

The coherent modulation of swift electron beams with strong laser fields has enabled the generation of attosecond electron pulses, opening up new research avenues in ultrafast science. Here we study a comparatively simple alternative, the production of electron pulse trains directly at the source. In our theory work, we show that sub-optical-cycle electron bursts induced by tunneling photoemission from a metal nanotip can retain the temporal fingerprint of their emission dynamics in a typical low-energy point-projection microscope setup. We find that strong acceleration by a static field, a short propagation distance and a sufficiently large optical cycle duration mitigate temporal smearing due to matter-wave dispersion. Our approach enables studies of coherent interactions of slow electrons with matter on sub-femtosecond and nanometer scales, a regime which has hitherto remained inaccessible.