Brillouin-Scattering Analysis of Phonon Interactions in Acoustoelectric Domains in GaAs

Abstract

Acoustoelectrically amplified domains of ultrasonic flux in GaAs were used for analyses of various phonon interactions by Brillouin-scattering techniques. Both the growth of the flux from the thermal equilibrium phonon spectrum and particularly the attenuation (after the amplifying voltage pulse was shut off) were studied as a function of ultrasonic frequency in the broad range from 0.3 to 4.0 GHz at 300 °K. The data were analyzed to give the magnitude and frequency dependence of both the acoustoelectric gain and the lattice attenuation in the weak-flux regime. Comparison with small-signal gain theory in the $\mathrm{ql}\approx 1$ range, for piezoelectrially active [110] shear waves, gave good agreement. The lattice attenuation was determined not only for the amplified shear waves but also for the piezoelectrically inactive longitudinal waves, which were obtained by mode conversion upon reflection of the amplified shear-wave domain. The frequency dependence of the attenuation, proportional to $f^{1.8}$ and $f^{1.3}$, respectively, fell well below the expected $f^2$ behavior. In the strong-flux regime, striking anomalous attention was found, consisting of a too-rapid initial attenuation of low frequencies, and too-slow initial attenuation of higher frequencies. These results are interpreted to represent a dominant trend of up-conversion of intense ultrasonic flux from low to high frequencies by nonlinear frequency-mixing processes. Evidence is summarized favoring such frequency-mixing processes, over possible variation in the frequency dependence of the acoustoelectric gain in strong flux, as a dominant factor in the evolution of the strong-flux spectrum both in growth and attenuation.