Nanoscale diffractive probing of strain dynamics in ultrafast transmission electron microscopy

Abstract

The control of optically driven high-frequency strain waves in nanostructured systems is an essential ingredient for the further development of nanophononics. However, broadly applicable experimental means to quantitatively map such structural distortion on their intrinsic ultrafast time and nanometer length scales are still lacking. Here, we introduce ultrafast convergent beam electron diffraction with a nanoscale probe beam for the quantitative retrieval of the time-dependent local deformation gradient tensor. We demonstrate its capabilities by investigating the ultrafast acoustic deformations close to the edge of a single-crystalline graphite membrane. Tracking the structural distortion with a 28-nm/700-fs spatio-temporal resolution, we observe an acoustic membrane breathing mode with spatially modulated amplitude, governed by the optical near field structure at the membrane edge. Furthermore, an in-plane polarized acoustic shock wave is launched at the membrane edge, which triggers secondary acoustic shear waves with a pronounced spatio-temporal dependency. The experimental findings are compared to numerical acoustic wave simulations in the continuous medium limit, highlighting the importance of microscopic dissipation mechanisms and ballistic transport channels.

High brightness ultrafast transmission electron microscope based on a laser-driven cold-field emission source: principle and applications

“…First studies of the dynamics of acoustic waves by ultrafast convergent-beam diffraction using a flat-photocathode have been performed in the cases of a silicon wedge [71] and graphite nanoslab [72]. Results in the same direction have recently been reported by the Göttingen group using Kossel-Möllensted patterns in graphite samples thanks to their Schottky based UTEM [73].…”

Ultrafast Transmission Electron Microscopy with High-Coherence Electron Pulses

“…In first experiments (Fig. 2), we demonstrated the broad applicability of ultrashort electron pulses in high-resolution imaging and diffraction, Lorentz microscopy, electron energy loss spectroscopy and scanning diffraction [10]. These capabilities allow us to, for example, map strain dynamics and probe nanoscale magnetic dynamics.…”

Structural dynamics probed by high-coherence electron pulses

“…5a). 93 The structure is illuminated with femtosecond laser pulses (50-µm focal spot size, 800-nm central wavelength, 50-fs pulse duration, 16-mJ/cm² optical fluence), and we diffractively probe the local structural distortion employing 700-fs electron pulses focused to a 28-nm diameter. An exemplary CBED pattern is shown in Fig.…”