Fano resonance enhanced multiphoton photoemission from single plasmonic nanostructure excited by femtosecond laser

High-flux photoelectron pulses with femto-nano spatiotemporal properties enhanced with plasmonic metallic nanostructures can provide possibilities for potential photoelectron-based devices. In this work, a large multiphoton photoemission enhancement in a single plasmonic Au-heptamer nanostructure, which supports the Fano resonance, is investigated using photoemission electron microscopy (PEEM). By exploiting the high spatial PEEM resolution and performing calculations, it is found that the photoemission electrons are mainly emitted from the junction area between the lower surface of the nanostructure and the indium tin oxide film on the substrate. It is also observed that photoemission is strongly affected by the Coulomb field. Specifically, the strong photoemission locates in the gap region with the Coulomb attraction rather than Coulomb repulsion, even though the latter exhibits a higher charge density. The measured photoelectron enhancement achieved by employing a Fano-resonant Au heptamer is supported by calculations, combining the photoemission effects of both near-field intensity in the lower surface and nanostructure absorption.