Phonon Optics with Fraunhofer-Diffracted Phonon Beams.

A narrow Fraunhofer-diffracted beam of monochromatic phonons is injected into a PbMoO₄ single crystal by periodic heating of a gold transducer. This thermomodulation is accomplished by illuminating the transducer by two interfering focused dye-laser beams over an area 40 μm in diameter. The generated phonons, whose frequency equals the frequency difference of the lasers, are detected by the use of Brillouin-scattering spectroscopy, which enables us to view the phonon beam selectively for site and momentum. When the two laser beams are incident at a mutual angle φ, the thermomodulation induces a sideward moving grating, with the result that the phonon beam departs at an angle determined by the grating constant. By varying φ, the phonon beam can thus be swept through the crystal over 15^circ to either side, as was verified by experiments. Here, of course, phonon focusing effects play a role. Furthermore, experiments on the reflectivity of the crystalline surfaces are conducted at 2 K, where the mean free path exceeds the size of the crystal and the reflection coefficient may be derived from a comparison of the Stokes and anti-Stokes Brillouin spectra. It is found that the reflectivity can be tailored by depositing thin metallic films.