Dynamic Gratings for Contactless Material Diagnostics

This paper demonstrates the potentialities of the dynamic gratings (spatial modulation spectroscopy) method using for nondestructive material diagnostics, separation of different nonlinearity mechanisms, measurements of thermo-optical and kinetic characteristics. Recording of short-lived (hundreds of microseconds) and long-lived (several seconds) gratings in photorefractive crystals of bismuth silicate have been studied. It has been established that a mechanism of the short-lived gratings is local whereas that of the long-lived gratings is nonlocal (diffusion). In the process of the surface transient gratings recording in samples of ZnGeP2 single crystal the amplitude and phase components of a diffracted signal have been separated. It has been shown that attenuation of a signal from the amplitude grating is governed by the local-type relaxation processes (recombination of free carriers). In-plane thermal diffusivity has been measured; and it has been found that there is no anisotropy of thermal diffusivity for different orientations of the thermal grating vector with respect to the crystal axis. The acoustic wave formation in the sub-surface area of a crystal and in the air has been studied. The dependence of the ultrasound propagation speed in the sub-surface area of a sample on the mutual orientations of a crystal and the transient grating vector has been revealed. The contactless method for temperature estimation of the sample sub-surface layer by the acoustic-wave frequency measuring has been proposed.