Picosecond acoustic diffraction in anisotropic thin film (νm); Application to the measurement of stiffness coefficients

Abstract

Investigation of thin metallic film properties by means of picosecond ultrasonics has been under the scope of several studies. Generation of longitudinal and shear waves with a wave vector normal to the film free surface has been demonstrated. Such measurements can not provide complete information about properties of anisotropic films. Acute focusing of the laser pump beam ( 0.5 νm) on the sample surface has recently allowed us to provide evidence of picosecond acoustic diffraction in thin metallic films (1 νm) such as aluminum, gold, copper. Waveforms have been experimentally recorded in a gold layer (2 νm thick) for several distances between pump and probe on the sample surface. Due to acoustic diffraction, the acoustic wavefronts propagate at a group velocity which differs from phase velocity in the anisotropic film. However, a specified signal processing allows us analyzing the space repartition of the acoustic wave-vectors for both longitudinal and shear waves. Four stiffness coefficients of the anisotropic gold layer could thus be recovered accurately, demonstrating the feasibility of the measurement. © 2007 IOP Publishing Ltd.