Nanoscale diffractive probing of strain dynamics in ultrafast transmission electron microscopy

TL;DR

This paper introduces ultrafast convergent beam electron diffraction (U-CBED), a nanoscale technique for real-time mapping of strain dynamics in nanostructures, demonstrated on graphite membranes with high spatial and temporal resolution.

Contribution

The paper presents a novel U-CBED method enabling quantitative, ultrafast, nanoscale strain mapping, advancing the study of high-frequency strain waves in nanostructured systems.

Findings

Observation of a membrane breathing mode with spatial modulation.

Detection of in-plane polarized acoustic shock waves and secondary shear waves.

Comparison with simulations highlights microscopic dissipation and ballistic transport effects.

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 (U-CBED) with a nanoscale probe beam for the quantitative retrieval of the time-dependent local distortion 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.