Laser acoustics with picosecond collimated shear strain beams in single crystals and polycrystalline materials

Two electromagnetic modes, ordinary and extraordinary, simultaneously propagating in a single crystal excite, via the optoacoustic effect, all three acoustic eigenmodes of the crystal, and, at a delayed time, detect these three modes via the acousto-optic effect. As a result of asynchronism in the propagation of the different acoustic modes, a volumetric, although a nonlocal excitation of shear sound is possible, even by spherical (isotropic) laser-induced stresses. The temporal profile of optical reflectivity, modified by sound, strongly depends on probe light polarization. It is shown that shear hypersound beams can be monitored by lasers, not only in single crystals in geometries with broken symmetry, but also in polycrystalline materials.