Phonon-focusing effect with laser-generated ultrasonic surface waves

Surface-phonon focusing is examined with laser generation of pulsed surface acoustic waves (SAW’s). Before presentation of the experimental results, the theory of the effect is summarized including both the ray approach and the stationary-phase analysis. To create a point source of SAW’s the focused beam of a Q-switched Nd:YAG (yttrium aluminum garnet) laser is employed. To visualize the SAW amplitude distribution we use dust patterns arising as a result of SAW-induced dust-particle removal from the surface under investigation. The SAW amplitude measurements with the probe beam deflection technique show that the above patterns reflect adequately the SAW amplitude angular dependence. The strong SAW focusing is observed on cubic crystals such as Si, Ge, GaAs, as well as on ${\mathrm{LiNbO}}_3$ crystal. The results obtained are discussed in comparison with the theory.