Description
Thermoelastic properties at the nanoscale play a central role in condensed matter physics and nanostructured materials, yet remain challenging to probe experimentally. Extreme ultraviolet (EUV) transient grating (TG) technique has recently enabled access to these dynamics at selected wavevectors in the tens-of-nanometers range, providing insight into nanoscale thermal transport and acoustic phonons [1]. More recently, a novel time-resolved technique based on diffuse EUV scattering (DES) has emerged as a simple and effective method for studying phonons with nanoscale wavelengths [2]. In DES, an ultrafast optical pump excites lattice dynamics, while a coherent EUV probe measures the resulting diffuse scattering pattern. The scattered intensity forms a time-dependent fringe pattern on a two-dimensional detector, where each point corresponds to a specific wavevector q and oscillates at the frequency of the corresponding optically excited phonon. DES thus provides direct access to the dynamics of acoustic phonons spanning wavelengths from ~60 to 400 nm. Compared to conventional approaches, DES offers several key advantages, including continuous q-access within a single measurement, linear response to excitation fluence, and signal enhancement arising from surface roughness.
In this talk, we will introduce the DES technique and present its application in transmission geometry, which enables access not only to surface modes but also to bulk acoustic waves propagating in thin films. In addition to mapping the dispersion of high-wavevector guided modes, the observation of resonances associated with zero-wavevector longitudinal modes provides insight into the excitation mechanism, indicating a dominant role of thermoelastic generation followed by scattering from surface roughness.
Looking ahead, this approach can be naturally extended to the X-ray regime, opening the way to probing dynamics at even shorter length scales. DES also offers new opportunities to investigate magneto-elastic coupling, by enabling access to high-wavevector phonons and their interaction with magnetic degrees of freedom.
[1] Foglia et al., Photoacoustics 29, 100453 (2023)
[2] Capotondi et al., Phys. Rev. Lett. 135, 266101 (2025)
