High-Resolution Brillouin Scattering

A new technique for determining the velocity and attenuation of hypersound is established. Experiments illustrating the technique with water as the acoustic medium in the frequency range 500-1500 Mc/sec are described. The technique is based on superheterodyne detection of Brillouin-scattered light from a multi-frequency gas laser. The scattering sound is injected from a buffer rod into the medium under study, in contrast to the classic Brillouin-scattering technique, which uses scattering from ubiquitous thermal phonons, and detects by means of photon counting. Measurement of the amplitude and phase of the microwave photo-current (using a homodyne technique) as a function of the angle of the buffer rod determines the attenuation constant and phase velocity of the hypersound. The technique is especially useful at those hypersonic frequencies for which the attenuation is sufficiently great that the pulse-echo technique or other acoustic reflection techniques are difficult or impossible and yet small enough that the classic Brillouin-scattering technique has insufficient resolution. The experiments in water described here correspond to such a range, and bridge the gap between the available ultrasonic data and more recent measurements above 3 Gc/sec. In particular, the data indicate that for water an upper limit of ${10}^{-11\mathrm{}}$ sec can be set for the relaxation time associated with excess loss.