Momentum microscopy of ultrafast electron emission from a strongly driven optical nanoantenna

RESUMO

Metallic nanostructures in combination with femtosecond lasers are a well-suited platform for the control of photoelectrons by strong and nano-localized driving fields, with high relevance for ultrafast, coherent electron emitters and petahertz electronics. Photoelectron spectroscopy resolves such photoelectron dynamics, but lacks nanoscale spatial resolution, making it only suited for single emitters or homogeneous arrays. We report the first strong-field experiment combining both photoemission electron microscopy and momentum microscopy, two complementary techniques providing spatial and momentum resolution within the same instrument. We apply this new methodology to a double-hole nanoantenna with sub-10 nm apex radii, demonstrating the potential of this approach for the control of photoelectrons in heterogeneous nanostructured samples using few-cycle light fields. Our measurements reveal distinct signatures of two classes of electron trajectories in the near-field. Quiver trajectories result in directed, angularly more focused emission, whereas subcycle trajectories give rise to a broader transverse momentum distribution. Surprisingly, this observation disagrees with previously reported emission characteristics from nanotip emitters, which we classify as a special case of a broader class of curved emitter surfaces driven by ultrashort light fields. This demonstrates both the impact of sophisticated electron detection methods and the potential of strong-field control of electrons in nanoscale geometries.