Generation and propagation of coherent phonon beams
E. P. N. Damen, D. J. Dieleman, A. F. M. Arts, and H. W. de Wijn
Faculty of Physics and Astronomy, and Debye Institute, Utrecht
University, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands
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 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.