Simulation of the thermal and acoustic response of an elastically anisotropic solid to a nanosecond laser pulse in transient grating spectroscopy

Abstract: Transient grating spectroscopy (TGS) is a material characterization technique based on laser-induced thermoelastic excitation of thermal and acoustic gratings. On opaque samples, these gratings are dynamic surface displacements that reflect the sample's elastic and thermal properties, enabling both types of parameters to be determined from a single experiment. Here, we develop a detailed finite element model (FEM) simulation of the TGS experiment that fully captures the coupling between the thermal and mechanical fields, as well as the optical detection of surface displacement using a heterodyning approach. Using custom-designed two-dimensional elements, the model is particularly suitable for analyzing TGS measurements on anisotropic media, for which analytical theory is insufficient. The simulation captures not only the anisotropic relaxation of the thermoelastic field but also several acoustic features that arise at very short (ultra-transient) timescales and provide additional information about the elastic properties of the examined material. The model offers new opportunities for the in silico testing of various modifications of TGS experiments and their applications to a broad class of materials.