Brillouin Scattering Study of Propagating Acoustoelectric Domains in n GaAs

A Brillouin-scattering study of amplified shear waves in propagating acoustoelectric domains in n GaAs is presented. On the basis of small-signal theory, a complete formalism is developed for the amplification of piezoelectrically active waves from the thermal background of lattice vibrations. Our experimental results show that this provides a good description of the acoustic flux when its intensity is less than about 10^-2 J/cm³. Here, the growth rate, intensity, frequency distribution, angular distribution, and spatial distribution of the amplified shear waves were all found consistent with small-signal theory. In the subsequent stages of growth, when the acoustic waves become very intense, many interesting deviations from small-signal theory were found, resulting from at least two nonlinear effects, parametric frequency conversion, and enhanced electron-phonon coupling. The acoustic spectrum is rapidly extended to low frequencies, with relatively narrow domains being initially produced at these frequencies. The acoustic energy density tends to saturate at about 1 J/cm³.