Diffraction of picosecond bulk longitudinal and shear waves in micron thick films; Application to their nondestructive evaluation

In this paper, acute focusing of the laser pump beam (≈0.5 μm) on the sample surface allows picosecond acoustic diffraction in thin metallic films. The resulting wavefronts propagate at a group velocity which differs from phase velocities in anisotropic films. Waveforms have been experimentally recorded in a gold layer (2.1 μm thick) for several distances between pump and probe on the sample surface. A specified signal processing based on a Synthetic Focalization Technique allows analyzing the space repartition of the acoustic wave vectors for both longitudinal and shear waves. Stiffness coefficients of the gold layer are then identified from wave arrival times.