Stimulated Brillouin scattering and optical excitation of coherent shear waves

The theory of stimulated Brillouin scattering is applied to the experimental situation of crossed mode-locked laser pulses, centered at the same frequency, in isotropic and anisotropic materials. It is shown that generation of pure transverse (shear) ultrasonic waves should be possible with reasonable efficiency. In general, any acoustic phonon mode that the material can support— pure shear, quasishear, quasilongitudinal, and pure longitudinal—can be populated coherently. The results permit straightforward determination of the experimental configuration that most effectively couples laser energy to the desired mode. The acoustic frequency can be tuned conveniently from about 3 MHz to 30 GHz in most materials.