Optical generation and detection of surface acoustic waves on a sphere

Abstract

Surface acoustic waves (SAW’s) have been optically generated and detected on metallic spheres. The source was a focused yttrium aluminum garnet (YAG) laser operated in the Q-switched mode and the probe was a sensitive optical heterodyne interferometer. The curvature of the surface gives rise to dispersive effects. The variation of the SAW group velocity is deduced from the evolving waveforms of pulses detected after propagating a few turns around the sphere. The observation of SAW pulse waveforms at positions away from the pole (a point diametrically opposite to the source) reveals that the spectral components undergo a π-phase shift when passing through the pole.

Analysis of Laser Ultrasonic Measurements of Surface Waves on Elastic Spheres

“…The ratio of any two strengths among the longitudinal, shear, and Rayleigh waves depends on Poisson's ratio. It is known that the spherical Rayleigh wave is dispersive due to the surface curvature [9]. As seen in Fig.…”

“…A similar study [9] reported measurements of Rayleigh waves generated on a sphere by a thermoelastic laser source and illustrated the general properties of this wave (e.g. the focusing at the poles and the wave dispersion).…”

“…Instead of using direet integration to evaluate (5), eontour integration is more desirable and yields (9) where mn,i are the simple poles of E. As was shown in [13], the fundamental mode i=l and n>O forms the Rayleigh wave. For n=O, the sphere vibrates symmetrically with respeet to its center and hence these motions are denoted as the compressional modes.…”

Imaging of surface phonons on a sphere

“…A point-by-point study of laser-generated surface phonons on a sphere was carried out by Royer et al [2]. Because of the application to ball devices based on surface acoustic waves, there is much interest in characterizing how surface phonons propagate on the surfaces of elastically isotropic or anisotropic spheres [3].…”

Interferometric imaging of surface acoustic waves on a glass sphere

“…The profile of a circular wave front passing through a focus in 2D is known to undergo a shape change and a / 2 Gouy phase shift. 19,20 Our experimental resolution for ⌬ allowed us to study the former effect in detail but not the latter. The change in shape is governed by the complex value of the source function F 0 .…”