On generation of picosecond inhomogeneous shear strain fronts by laser-induced gratings

Abstract

The processes leading to excitation of inhomogeneous, plane compression/dilatation bulk acoustic strain pulses and shear bulk acoustic strain fronts following the creation of a transient laser interference pattern at a mechanically free surface of an elastically isotropic medium are described. It is shown that the characteristic frequencies of these acoustic disturbances can be much higher than those corresponding to the spatial period of the laser-induced grating. As well, it is shown that the inherent dispersive nature of these bulk acoustic eigenmodes provides plausible explanation for the reported observations of acoustic echo transformation in thin film, ultrafast laser ultrasonics experiments.