Acoustic Beam Probing Using Optical Techniques

In Section I it is demonstrated that the amplitude of the light deflected or scattered by an advancing sinusoidal acoustic wave, as a function of the angle between the direction of light propagation and the acoustic wavefront, is proportional to the Fourier transform of the amplitude distribution of the acoustic wave in the plane of the wavefront. Studying the angular dependence of the optical-acoustic interaction accurately and directly determines the angular distribution or far-field diffraction pattern of the acoustic beam and incidentally determines the angular response of the acoustic transducer producing the beam. The angular resolution equals the angular spread in the probing light beam. Experiments illustrating and verifying the technique are described. In Section II the effect of volume acoustic loss is determined. It is shown that loss does not change the considerations of Section I apart from a slight reduction in angular resolution unless the decay distance is comparable to the acoustic wavelength. The loss parameter does introduce a maximum usable acoustic beam width for the interaction (coherence width). In addition, techniques for determining the acoustic loss are described. Particular attention is given to the near- and far-field energy distribution of the scattered light beam. It is shown that the far-field distribution is Lorentzian only under special circumstances. Consideration is given to probing beams with rectangular and Gaussian intensity distributions. Edge effects are taken into account, and it is shown that these can make important contributions to the line shape as well as lead to errors in the interpretation of phonon lifetimes from scattering experiments. Experiments confirming the results are described.