Highly coherent femtosecond electron pulses for ultrafast transmission electron microscopy

We describe the implementation and detailed characterization of a laser-triggered field-emitter electron source integrated into a modified transmission electron microscope. Specifically, localized linear photoemission from the front facet of a tip-shaped ZrO/W(100) Schottky emitter is employed, yielding electron pulses with a spectral bandwidth of 0.6 eV and pulse durations down to 200 fs (full-width-at-half-maximum). Furthermore, transverse electron beam properties are characterized for a range of TEM illumination conditions by caustic measurements of the focused beam in the sample plane of the electron microscope, demonstrating a beam emittance down to 1.8 nm mrad, photoelectron probe sizes below 1 nm and a degree of transverse coherence exceeding 10 %. The electron pulse properties achieved here enable ultrafast high-resolution phase-contrast imaging and Lorentz microscopy, electron holography and spatially-resolved electron spectroscopy, allowing for a comprehensive mapping of ultrafast processes in nanoscale systems.