On Christmas Eve 2025, Phys. Rev. Lett. published a study led by Flavio Capotondi of FERMI-Elettra (Trieste, Italy), where we investigated a beautiful EUV diffused scattering phenomenon accidentally discovered by Flavio.
The experiment conducted at the FERMI free electron laser used a simple setup shown in Fig. 1(a): the sample is irradiated by a femtosecond optical pulse and diffuse scattering of a time-delayed EUV pulse is monitored by a CCD camera (with the direct reflection blocked). Normally, diffuse EUV scattering is featureless, as shown in Fig. 1(b). Upon the laser excitation, however, the scattering pattern yields a system of concentric rings evolving with the time delay between the optical pump and EUV probe, see Fig.1 (d-f).
It turns out that at each point on the detector the scattering intensity oscillates at the frequency of surface acoustic waves (SAW) at the corresponding scattering wave vector (see Fig. 2(a)), and a Fourier analysis of the data yields a SAW dispersion curve (Fig. 2(b)). Since the oscillations are detected at every point on the detector, SAWs causing this phenomenon must be propagating in all directions and cover a broad wave vector range corresponding to wavelengths 60 – 300 nm.
The excitation of SAWs with such nanoscale wavelength by 800 nm laser radiation focused into a 300 μm spot was totally unexpected: normally, laser excitation generates SAWs with wavelengths on the order of the spot size propagating away from the laser spot. Here, we had SAWs with thousands times smaller wavelengths propagating inside the laser spot.
A further investigation involving samples with different surface textures revealed that the excitation of nanoscale SAWs was facilitated by natural surface roughness present even on very smooth surfaces. The SAW pattern on the sample surface is random – and yet it produces regular fringe patterns in EUV scattering that contain information about the SAW velocity and attenuation.
Measurements conducted on multiple metal and semiconductor samples indicated that Capotondi scattering is a universal phenomenon occuring on any sample strongly absorbing at the optical pump wavelength. It offers a SAW spectroscopy tool covering a wavelength range inaccessible to traditional techniques such as surface Brillouin scattering.
For more details, read the paper: F. Capotondi, A. A. Maznev, F. Bencivenga et al., Phys. Rev. Lett. 135, 266101 (2025).
Fig. 1 (a) The experimental setup; (b) Static EUV diffuse scattering pattern from a multilayer Pt/Al sample; (c-f) differential diffuse scattering pattern (with the static background subtracted) following the optical excitation of the same sample for different pump-probe time delayes. Qx and Qy are the components of the scattering wave vector.
Fig. 2. (a) Time dependence of the differential diffuse scatterring intensity of at selected wave vector values; (b) the SAW disopersion curve obtained by a Fourier-analysis of the data. The red dashed curve shows the calculated dispersion relation. The sample is the same as in Fig. 1.