Observation of 450 GHz surface acoustic waves in suspended polycrystalline films by use of time-resolved resonant soft X-ray scattering*

We have used ultrafast optical pumping to generate nanoscale surface acoustic waves (SAWs) from 200 to 450 GHz, 10-50 nm wavelengths, in metallic films on SiN membranes. Our measurement demonstrates a novel soft X-ray elastic scattering mechanism that probes the coupling of the electronic and phononic degrees of freedom on a [Co₉₀Fe₁₀(0.6 nm)/Ni(0.2 nm)]x50 multilayer. The samples have an average grain size of 30 nm and a rms roughness of 1 nm. This nanostructured surface topography allows optical coupling to the in-plane SAW. In a transmission soft X-ray scattering geometry, with circular polarized X-rays tuned to the Ni L₃edge, we observe a prominent charge scatter ring with a radius of approximately 0.2 nm^-1. After optical pumping with fluences ranging from 24-27 mJ/cm², SAWs appeared as ripples on the charge scatter ring that oscillate at ps timescales. We use Brillouin light scattering (BLS) and modeling to identify the SAW as a dilation mode with an in-plane velocity of 5.6 km/s. Surprisingly, the SAW lifetime peaks sharply at q = 0.25 nm^-1(300 GHz), with a value greater than 60 ps, suggestive of a minimum in the SAW correlation length in the case of 2-d localization.