Plasmon-Enhanced Photoemitters as Bright Ultrashort Electron Pulse Generators

Short and coherent electron beams play an increasingly important role as probes of materials dynamics at atomic length (~0.1 nm) and time (~100 fs) scales and are being explored as patterning tools in lithography applications. Here, we demonstrate nanostructured plasmonic photocathodes that enhance and exploit multiphoton photoemission to generate femtosecond electron beams with engineered spatiotemporal characteristics. We present studies of two cathode geometries: nanogroove arrays for high current beams and plasmonic lenses for single or multiple nanoscale beams originating from a flat surface. Photocathodes were fabricated using e-beam lithography and then characterized in a newly commissioned 20 kV DC photogun testbed at Lawrence Berkeley National Laboratory. We first present photoemission characteristics of nanogroove arrays, including measurement of the 4-photon photocurrent yield and asymmetric emittance. We then illustrate characteristics of gold plasmonic lens emitters excited by near-infrared light using electromagnetic simulations, which predict sub-100 nm emitted electron beam size and sub-10 fs emitted pulse durations. Prototypes fabricated using e-beam lithography have sub-nm surface roughness and smooth plasmonic resonance modes characterized by cathodoluminescence spectromicroscopy. Finally, we present preliminary photoemission measurements from plasmonic lens cathodes. These results demonstrate a concept for designing nanoscale and ultrashort electron beams while paving the way for broader investigation of plasmon-enhanced photoemitters for scientific and industrial applications.