Spectroscopy Technique Reveals Nanoscale Acoustic Waves
Surface acoustic waves (SAWs) propagate along a solid material’s surface like ripples on a pond. SAWs with wavelengths of hundreds of kilometers are generated by earthquakes, whereas SAWs with wavelengths at the micrometer scale can be excited by lasers. Flavio Capotondi at the Free Electron laser Radiation for Multidisciplinary Investigations (FERMI) facility at Elettra Sincrotrone Trieste in Italy and his colleagues have now demonstrated a simple laser-based technique for exciting and detecting SAWs with nanometer wavelengths [1]. They say that the work will allow the dynamics of materials’ crystal lattices to be probed over a wavelength range that has been inaccessible using other approaches.
Laser-generated SAWs typically have wavelengths roughly equal to the laser spot size—usually from tens to hundreds of micrometers. Shorter-wavelength SAWs can be excited using multiple interfering beams, but such schemes are technically challenging and are often limited to SAWs with a narrow spectral range. Capotondi and colleagues used a simple setup in which a metal or semiconductor film was irradiated with femtosecond pulses from a single optical “pump” laser. The same spot was then targeted with extreme-ultraviolet (EUV) femtosecond “probe” pulses from a free-electron laser. A CCD detector recorded the scattered EUV light.
The researchers were surprised to observe fringe patterns in the scattered EUV radiation that indicated the excitation by the pump beam of broad-spectrum SAWs, including wavelengths as short as 60 nm. They found that the irradiated film needed to have two properties for such short-wavelength SAWs to be generated. First, it needed to absorb energy from the pump beam. Second, its surface needed to have a certain degree of roughness. Capotondi says that the necessary roughness is present even on apparently smooth materials. If the technique can be demonstrated using tabletop EUV sources, then it could be beneficial to many areas of research.
–Rachel Berkowitz
Rachel Berkowitz is a Corresponding Editor for Physics Magazine based in Vancouver, Canada.
References
- F. Capotondi et al., “Time-domain extreme-ultraviolet diffuse scattering spectroscopy of nanoscale surface phonons,” Phys. Rev. Lett. 135, 266101 (2025).