Laser-driven cold field emission source for ultrafast transmission electron microscopy.

Ultrafast transmission electron microscopy (UTEM) has emerged as a versatile technique for the time-resolved imaging of nanoscale dynamics on timescales down to few-hundred attoseconds but the temporal and spatial resolutions are still limited by the coherence properties of pulsed electron sources. Here, we report the development of a novel laser-driven linear cold field electron emitter integrated in a state-of-the-art UTEM system. Tuning the emitter's workfunction via an applied extraction field and illuminating the sharp tungsten emitter tip with UV light pulses generates ultrashort femtosecond electron pulses of 220 fs pulse duration at 200 keV electron energy, with energy widths as low as 360 meV. The photoelectron emitter demonstrates exceptional spatial coherence, achieving focal spot sizes down to 2 Å and a peak normalized brightness exceeding 6.7 ×10 A/msr. With an order-of-magnitude improvement compared to previously employed laser-driven Schottky field emitters, the present development opens up the field of ultrafast atomic-scale electron probing.