Structural dynamics probed by high-coherence electron pulses
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Introduction The physics of modern functional materials is governed by nanoscale heterogeneity and processes occurring on fast to ultrafast time scales. Observations with high spatial and temporal resolution allow for a comprehensive understanding of microscopic couplings and correlations, ultimately facilitating active control of properties, excitations, and transformations. Ultrafast metrology, including time-resolved optical and photoelectron spectroscopy,1 electron and x-ray diffraction,2–4 and spectroscopy, yields access to multiple degrees of freedom down to the femtosecond and attosecond range. Structural dynamics in strongly heterogeneous systems, such as nanostructures, surfaces, and monolayers, are of particular interest, but also pose great experimental challenges. In recent years, various techniques for ultrafast nanoscale probing were established, enabling, for example, time-resolved scanning near-field optical microscopy5–7 and scanning tunneling microscopy,8–10 electron point-projection microscopy,11–13 and lensless imaging using extreme ultraviolet radiation.14–16 Ultrashort electron pulses are ideally suited probes of materials structure and dynamics down to the atomic scale.17,18 Ultrafast diffraction in transmission,3,17,19–23 grazing incidence,24–26 and backscattering27 geometries is now established. Nanoscale real-space imaging, on the other hand, is possible using ultrafast transmission18,28,29 and scanning30,31 electron microscopy.
These techniques are rapidly expanding in scope, driven by strong developments of high-brightness pulsed electron sources.22,32–37 In this article, we discuss two complementary schemes to access ultrafast structural dynamics using high-coherence electron pulses from localized photoelectron sources. Specifically, ultrafast low-energy electron diffraction (ULEED) probes the orientation, symmetry, and long-range order of materials with ultimate surface sensitivity, while ultrafast transmission electron microscopy (UTEM) translates atomic-scale imaging and local diffraction to the ultrafast domain. We will introduce these methods and discuss selected examples of laser-induced structural dynamics probed by them.
Electron pulses from nanoscale sources Ultrafast electron diffraction and microscopy use a stroboscopic approach, following the principle of optical pump-probe techniques, as illustrated in Figure 1a–b. An ultrashort laser pulse (red) excites a transmission electron microscope specimen or surface, and the structural response is probed by a delayed electron pulse (green) in transmission or backreflection. By repetitive sampling at varying delays, a time-dependent “movie” of the induced dynamical process can be composed. The real-space and reciprocal space resolution of these techniques crucially depend on the quality of the employed
Armin Feist, University of Göttingen, Germany; [email protected] Gero Storeck, Department of Physics, University of Göttingen, Germany; [email protected] Sascha Schäfer, University Oldenburg, Germany; sascha
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