Narrow coherent beams of longitudinal acoustic waves are injected into a single crystal of PbMoO 4 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.

The transmission of a coherent beam of longitudinal acoustic phonons through a 12-layer Ag-Au superlattice is measured with high resolution at gigahertz frequencies. The phonons are generated with laser-induced thermomodulation of a Au transducer and, after passage through the superlattice, detected with Brillouin scattering in the PbMoO 4 single-crystalline substrate. The transmission is found to drop to about 1% in well-defined stop bands. Calculations based on wave equations with inclusion of acoustic damping quantitatively reproduce the stop bands as well as the oscillatory frequency dependence of the transmission at intermediate frequencies.

Coherent acoustic phonons emitted by a grating are observed in the gigahertz range, and found to show well-resolved interference maxima on either side of the forward direction. The phonons are generated by laser-induced periodic heating of a thin Au transducer, consisting of an array of narrow strips deposited onto a ͑001͒ face of a single crystal of PbMoO 4 . The phase and group velocities, which are noncollinear due to the elastic anisotropy, are distinguished with reliance on Brillouin scattering. The results are in accord with optical diffraction theory in the Fraunhofer limit, provided phonon defocusing is taken into account.

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