Photoacoustic Shear Waves in Gold Nanocrystals

Scientists from Université du Mains in Le Mans and from Université de Caen, both in France, and from Massachusetts Institute of Technology in Cambridge, Mass., USA, have generated and detected picosecond acoustic shear waves in gold films. The technique has potential in the study of supercooled liquids and glasses.

Probes at 800 or 400 nm detect dynamic reflectivity changes (ΔR) in arbitrary units, from the sputtered gold film nanostructure (inset, top right). Shear waves (S) are detected at 800 nm only, but longitudinal waves (L) are detected at both probe wavelengths. The inset Fourier transform evaluates their ringing (ν_S, ν_L) and Brillouin backscattering (ν_ß) frequencies. Courtesy of Thomas Pezeril of Université du Mains and Massachusetts Institute of Technology.

They used DC sputtering to deposit thin gold films with various thicknesses on silicon crystals. They split the 800-nm output from a Ti:sapphire laser into a pump beam and a variable delayed probe beam. Each pump pulse induces a transient thermal stress in the gold film that launches acoustic waves. They detected shear waves with a probe wavelength of 800 nm, but not with 400 nm, yet found longitudinal waves at both wavelengths. This reveals the different natures of the acoustic waves.

X-ray texture analysis showed that about 60 per cent of the crystallites were aligned differently from the main (111) component, giving a locally canted nanostructure that permits the generation and detection of the shear waves.

(Applied Physics Letters, 13 Feb. 2008, 061908)