Generation and propagation of coherent phonon beams

Narrow coherent beams of longitudinal acoustic waves are injected into a single crystal of PbMoO₄ at gigahertz frequencies, and their properties are observed by means of Brillouin scattering. The waves are generated via the thermoelastic strain that results from periodic surface heating of a thin metallic transducer by interfering cw dye lasers. Frequency tuning is achieved simply by varying the optical difference frequency. A theoretical description based on heat diffusion and thermoelastic expansion agrees with the observed frequency dependence of the acoustic intensity, inclusive of acoustic resonances within the transducer, as well as its quadratic dependence on the laser power. The propagation of the acoustic beams is found to be governed by Fresnel diffraction provided due account is taken of phonon focusing. The beam furthermore is responsive to the phase profile over the laser-illuminated area, which allows us to manipulate the beam in various ways, such as modifying its divergence as if an acoustic lens were positioned just below the transducer or sweeping the beam sideward by a moving grating. Combined with Brillouin detection, distinguishing between phase and group velocities, this provides a direct measurement of phonon focusing. Finally, the decay of the acoustic beam with the distance is measured at various frequencies, to find confirmation of Herring's asymptotic theory for anharmonic phonon decay in anisotropic crystals.