Selective excitation of single-mode acoustic waves by phase velocity scanning of a laser beam

A novel method for selective generation of single-mode acoustic waves in multimode media has been developed using a laser beam scanned at the phase velocity of a specified mode. In dispersive media, the acoustic frequency can be varied by changing the scanning velocity. The number of carriers in the generated wave packet is proportional to the difference between the phase and the group velocities. These features were experimentally verified in the fundamental symmetric and asymmetric Lamb waves on an aluminum plate generated by a long-pulse Nd:YAG laser. Applications to anisotropy and thickness measurements are discussed.