Laser-Ultrasonic Characterization of Strongly Anisotropic Materials by Transient Grating Spectroscopy

Abstract

Transient grating spectroscopy (TGS) is a laser-ultrasonic method allowing measurement of the surface acoustic wave (SAW) velocity in an examined material for a given direction of the wave vector. We explore the capability of TGS for determination of shear elastic coefficients ( $$c^\prime$$ and $$c_{44}$$ ) of strongly anisotropic cubic materials. TGS is tested on a set of single crystals with an anisotropy factor up to $$A=25$$ . Using a numerical simulation based on a Ritz-Rayleigh approach, we show that strong anisotropy may lead to significant coupling of SAWs with bulk shear waves, which complicates TGS measurements in specific directions. Based on the obtained TGS data, we discuss the possibility of also using the TGS technique for assessing the longitudinal elastic coefficient ( $$c_L$$ ). Despite the energy focusing and other effects originating from the strong anisotropy, the TGS method can be used to reliably determine the directional dependence of the SAW velocity in these materials, and the resulting experimental datasets are sufficient for inverse determination of both the soft shear elastic constant ( $$c^\prime$$ ) and the hard shear elastic constant ( $$c_{44}$$ ). The longitudinal coefficient can be determined with lower accuracy. TGS is a suitable experimental tool for contactless characterization of strongly anisotropic materials.