Speaker
Description
Ultrafast Transmission Electron Microscopy (UTEM) with field-emitter sources [1] is one of the most advanced techniques to study materials dynamics on the nanoscale. Spatially coherent femtosecond electron pulses serve as nearly universal probes in ultrafast imaging, diffraction and spectroscopy. Despite the reduced complexity of electron-beam instrumentation at lower energies, Ultrafast Scanning Electron Microscopy (USEM) [2], has experienced somewhat fewer advancements in recent years, in part due to limitations in available contrast modes.
In this contribution, we present the development and application of a newly-designed multimodal USEM allowing for the simultaneous measurement of different electron probe signals. The instrument is based on pulsed photoemission from a Schottky field emitter driven by femtosecond laser pulses, enabling adjustable pulse duration and repetition rates. Electron transmission or reflection, as well as secondary electron signals can be recorded over a wide range of electron energies from 100 eV to 30 keV.
First proof-of-concept studies with the multimodal USEM demonstrate time-resolved four-dimensional scanning transmission electron microscopy (4D-STEM) on electron-transparent thin-film samples, including charge-density-wave materials. Samples are prepared in geometries that provide simultaneous access to transmission and secondary-electron signals under femtosecond pump–probe excitation. By raster-scanning a focused electron probe across the sample and recording a diffraction pattern at each position, we obtain four-dimensional datasets that correlate nanometer-scale real-space contrast with local reciprocal-space signatures. From these data, we reconstruct virtual bright-field and dark-field images, as well as maps of superlattice intensities, which visualize spatial variations in thickness, strain and ordered phases and their ultrafast evolution after optical excitation [3,4].
The multimodal detection scheme further enables time-resolved cathodoluminescence and electron energy-loss spectroscopy (EELS), including a momentum- and energy-resolved operation mode using a hemispherical energy analyzer, providing complementary access to changes in the electronic structure and excitations across the phase transition. Together, these capabilities establish multimodal ultrafast SEM as a flexible platform for nanoscale pump–probe studies, bridging real-space imaging, diffraction-based 4D STEM and spectroscopic contrast in a single instrument.
Acknowledgements and declaration of conflicts of interest
We thank Till Domröse for helpful discussions on the material system, Sophie Schaible for help in sample preparation, Dennis Naraschkewitz-Epp and Leon Brauns for technical support in the development of the instrument, and Armin Feist for discussions on instrument design. Funding by the European Research Council through an ERC-Advanced Grant (“ULEEM”, funding ID: 101055435) is gratefully acknowledged. M.S. and C.R. are co-founders of a start-up company (QSEM GmbH) marketing ultrafast scanning electron microscopes.
References
[1] A. Feist et al., Ultramicroscopy 176, 63-73 (2017).
[2] O. F. Mohammed et al., J. Am. Chem. Soc. 133, 7708–7711 (2011).
[3] S. Sun et al., Phys. Rev B, 92, 224303 (2015).
[4] S. Ji et al., Phys. Rev. B 101, 094303 (2020).