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arXiv · 2610.32419

Electron Wavefronts Capture Nanoscale Movies of Hypersound at Surfaces

Johannes Otto·Leon Brauns·Benjamin Schroeder·Claus Ropers
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Abstract

Acoustic waves transport mechanical energy and elastic strain through solids, with material composition and shape governing their propagation. In the gigahertz hypersound regime, acoustic wavelengths reach tens to hundreds of nanometres, comparable to nanoscale device dimensions. Harnessing these waves underpins high-frequency signal processing, sensing, electromagnetic coupling and quantum control. Acoustic metrology probes elastic modes and internal interfaces. Yet directly imaging how nanoscale acoustic waves interact with local structure remains challenging, requiring picometre displacement sensitivity combined with high spatiotemporal resolution. Here we demonstrate full-field imaging of laser-induced hypersound at surfaces using reflected electron wavefronts. We introduce ultrafast low-energy electron microscopy (ULEEM), exploiting the intrinsic surface sensitivity of few-electronvolt electrons. Coherent electron pulses down to femtosecond durations reflect from the topmost atomic layers, imprinting the evolving surface profile onto their wavefronts. Defocus converts phase variations into image contrast. In a thin van der Waals material, we image the thermoelastic generation and propagation of picometre-amplitude acoustic waves. Measured dispersion relations, interpreted through elastodynamic modelling, distinguish substrate-supported and locally detached regions, revealing how interfacial contact shapes propagation. Delayed wave emission reveals scattering at buried mechanical defects. Our results establish electron-wavefront imaging of nanoscale surface dynamics, enabling the exploration of ultrafast energy transport, structural transformations and coupled mechanical, electronic and magnetic phenomena.

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Johannes Otto, Leon Brauns, Benjamin Schroeder, Claus Ropers. 2026-10-23. Electron Wavefronts Capture Nanoscale Movies of Hypersound at Surfaces. https://arxiv.org/abs/2610.32419

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