Transient Grating Spectroscopy for Complete Elastic Anisotropy: Beyond the Measurement of Surface Acoustic Waves

Transient grating spectroscopy (TGS) with heterodyne detection is an opto-acoustic method for contactless measurement of surface acoustic wave velocities. The present objective is to explore the possibility of extracting bulk wave velocities from the TGS spectra. The measurement has been carried out on CuAlNi shape memory alloy. By measuring in 180 directions on a sample surface, an angular map of inverse acoustic velocities (slownesses) has been constructed from the TGS frequency spectra. A Ritz–Rayleigh approach was used to calculate the slowness surfaces to verify the results. A correspondence has been found between the calculated and experimentally obtained slowness maps, indicating that the TGS spectra contain information about both the surface acoustic waves and the bulk waves propagating along the surface.