High-Frequency Monochromatic Acoustic Waves Generated by Laser-Induced Thermomodulation

Monochromatic acoustic waves are produced in a single crystal of PbMoO₄ by periodic heating of a deposited Au film over an area about 40 μm in diameter by two interfering single-frequency dye lasers. Detection of these waves at optical difference frequencies up to 4 GHz is accomplished by Brillouin scattering. The generated power depends on the film thickness by acoustic interference within the film. Besides weak phonon defocusing effects, the divergence of the generated acoustic wave is limited by Fraunhofer diffraction.