Generation and propagation of coherent phonon beams - art. no. 174303

Narrow coherent beams of longitudinal acoustic waves are injected into a si ngle crystal of PbMoO4 at gigahertz frequencies. and their properties are o bserved by means of Brillouin scattering. The waves are generated via the t hermoclastic strain that results from periodic surface heating of a thin me tallic transducer by interfering cw dye lasers. Frequency tuning is achieve d simply by varying the optical difference frequency. A theoretical descrip tion based on heat diffusion and thermoelastic expansion agrees with the ob served frequency dependence of the acoustic intensity, inclusive of acousti c resonances within the transducer, as well as its quadratic dependence on the laser power. The propagation of the acoustic beams is found to be gover ned by Fresnel diffraction provided due account is taken of phonon focusing . The beam furthermore is responsive to the phase profile over the laser-il luminated area, which allows us to manipulate the beam in various ways, suc h as modifying its divergence as if an acoustic lens were positioned just b elow the transducer or sweeping the beam sideward by a moving grating. Comb ined with Brillouin detection, distinguishing between phase and group veloc ities, this provides a direct measurement of phonon focusing. Finally, the decay of the acoustic beam with the distance is measured at various frequen cies, to find confirmation of Herring's asymptotic theory for anharmonic ph onon decay in anisotropic crystals.