# Ultrafast Relativistic Electron Nanoprobes

## TL;DR

This paper demonstrates the generation of ultrafast relativistic electron beams with picometer-scale emittance, enabling nanoscale material characterization with high temporal and spatial resolution, including diffraction and imaging of microstructures.

## Contribution

It introduces a novel ultrafast electron source with extremely low emittance and demonstrates its application in nanoscale material analysis.

## Key findings

- Generated relativistic electron beams with picometer-scale emittance.
- Achieved sub-micron resolution in mapping crystallographic domains.
- Enabled new ultrafast electron microscopy techniques.

## Abstract

One of the frontiers in electron scattering is to couple ultrafast temporal resolution with highly localized probes to investigate the role of microstructure on material properties. Here, taking advantage of the unprecedented average brightness of the APEX electron gun providing relativistic electron pulses at high repetition rates, we demonstrate for the first time the generation of ultrafast relativistic electron beams with picometer-scale emittance and their ability to probe nanoscale features on materials with complex microstructures. At the sample plane, the APEX beam is tightly focused by a custom in-vacuum lens system based on permanent magnet quadrupoles, and its evolution around the waist is tracked by a knife-edge technique, allowing accurate reconstruction of the beam shape and local density. We then use the focused beam to characterize a Ti-6 wt\% Al polycrystalline sample by correlating the diffraction and imaging modality, showcasing the capability to locate grain boundaries and map adjacent crystallographic domains with sub-micron precision. This work provides a new paradigm for ultrafast electron instrumentation, demonstrating the ability to generate relativistic beams with ultrasmall transverse phase space volumes enabling novel characterization techniques such as relativistic ultrafast electron nano-diffraction and ultrafast scanning transmission electron microscopy.