Brillouin-Scattering Study of Propagating Acoustoelectric Domains in Semiconducting CdS

Abstract

Brillouin scattering in an anisotropic medium is developed by taking account of the off-axis effect, and some aspects of amplified shear waves in propagating acoustoelectric domains in semiconducting CdS are presented. The small-signal theory for piezoelectrically active waves amplified from the thermal background of lattice vibrations is formalized by taking account of nonelectronic lattice loss. When the acoustic flux intensity was less than about ${10}^{\ensuremath{-}3}$ ${\mathrm{J}/\mathrm{c}\mathrm{m}}^{3}$, the growth rate and frequency dependence of the on-axis components of acoustic flux were all found to be consistent with the small-signal theory. As for the off-axis components, the angular distribution of the acoustic flux, angular dependence of net gain coefficient, and narrowing of the cone of the acoustic flux also gave a reasonably good agreement with the theory. However, they have been strongly subjected to the influence of the nonelectronic-lattice-loss term. Using a dual-pulse method, it was found that the frequency dependence of the attenuation of acoustic flux was proportional to ${f}^{1.5}$ and its angular dependence proportional to the square of the off-axis angle. The angular dependence of the nonelectronic lattice loss was tentatively explained with the help of boundary scattering. In the subsequent stages of growth, when the acoustic waves became very intense, many interesting nonlinear effects were found in contrast to the small-signal theory. The acoustic spectrum was rapidly extended to both low and high frequencies, compared with that as expected under the small-signal condition. The subharmonic was strongly amplified and its growth rate became three times larger than that of initially amplified flux. There was a reasonably large growth rate even at high frequencies.