# Plasmonic lenses for tunable ultrafast electron emitters at the nanoscale

## TL;DR

This paper introduces plasmonic bullseye lenses designed to generate tunable, ultrafast, nanoscale electron pulses, advancing the capabilities of electron sources for high-resolution scientific applications.

## Contribution

It presents the design, fabrication, and optical characterization of plasmonic lenses that enable control over ultrafast electron emission at the nanoscale.

## Key findings

- Simulations show pulse duration can be tuned to sub-10 fs.
- Fabricated lenses exhibit suitable plasmonic behavior for ultrafast photoemission.
- Design enables control over electron pulse properties through geometric parameters.

## Abstract

Simultaneous spatio-temporal confinement of energetic electron pulses to femtosecond and nanometer scales is a topic of great interest in the scientific community, given the potential impact of such development on a wide spectrum of scientific and industrial applications. For example, in ultrafast electron scattering, nanoscale probes would enable accurate maps of structural dynamics in materials with nanoscale heterogeneity, thereby understanding the role of boundaries and defects on macroscopic properties. On the other hand, advances in this field are mostly limited by the electron source brightness and size. We present the design, fabrication, and optical characterization of bullseye plasmonic lenses for next-generation ultrafast electron sources. Using electromagnetic simulations, we examine how the interplay between light-plasmon coupling, plasmon propagation, dispersion, and resonance governs the properties of the photoemitted electron pulse. We also illustrate how the pulse duration and strength can be tuned by geometric design, and predict sub-10 fs pulses with nanoscale diameter can be achieved. We then fabricated lenses in gold films and characterized their plasmonic properties with cathodoluminescence spectromicroscopy, demonstrating suitable plasmonic behavior for ultrafast, nanoscale photoemission.