Speaker
Description
Ultrafast nanoscale imaging utilizing femtosecond electron pulses offers a unique experimental access to structural, electronic and spin degrees-of-freedom. High-resolution, phase-based imaging methodologies require electron pulses with high spatial coherence and sufficient average electron current to outpace sample drift. To a large extent, electron beam properties are already set by the design of the photoelectron source with respect to electron phase-space filling and Coulomb correlations. In recent years, a number of specialized femtosecond electron sources were developed for a range of ultrafast electron diffraction and imaging modalities, each optimized for specific figures of merit such as pulse charge, average current, or peak beam brightness.
Here, I present our development of a laser-driven cold-field emitter for ultrafast transmission electron microscopy (uTEM) [1]. Based on linear photoemission from sharp single-crystalline tungsten tips at large extraction fields [2], we demonstrate photoelectron beam focussability down to 2 Å, electron spectral widths below 300 meV and 200-fs-scale electron pulse durations. The large relative transverse coherence above 50% renders this source especially promising for time-resolved phase-based imaging approaches, such as electron holography and ptychography as well as high-resolution Lorentz microscopy. Examples on applying such a source for photo-induced near-field electron microscopy are presented, focusing on the spectrally-resolved inelastic near-field scattering at MoS2 edges [3] and microdiscs, and the observation of simultaneous coherent and incoherent inelastic scattering channels.
Further recent instrumental developments will be discussed, including electron pulse properties generated with sub-10-fs optical driving and the combination of a laser-driven cold-field emitter with an aberration probe corrector. The latter minimizes the photoelectron’s spherical aberration coefficient to the micrometer-scale and enables a further reduction of electron focal spot-size or an increase of usable bunch charge due to a larger acceptance angle of the focusing optics.
References
[1] A. Schröder, A. Wendeln, J. T. Weber, M. Mukai, Y. Kohno, and S. Schäfer, Laser-driven cold field emission source for ultrafast transmission electron microscopy, Ultramicroscopy 275, 114158 (2025).
[2] D. Ehberger, J. Hammer, M. Eisele, M. Krüger, J. Noe, A. Högele, and P. Hommelhoff, Highly Coherent Electron Beam from a Laser-Triggered Tungsten Needle Tip, Phys. Rev. Lett. 114, 227601 (2015).
[3] N. Müller, S. el Kabil, G. Vosse, L. Hansen, C. Rathje, and S. Schäfer, Spectrally Resolved Free Electron-Light Coupling Strength in a Transition Metal Dichalcogenide, arXiv:2405.12017.