Brillouin-Scattering Studies of Acoustoelectric Gain and Lattice Attenuation in Semiconducting CdS

Brillouin-scattering measurements were used to study acoustoelectric interactions in semi-conducting CdS. In the GHz frequency range, a double-pulse technique described earlier was employed to suppress domain formation and thereby reduce down-conversion processes. In this manner considerable extension of the small-signal regime towards higher flux levels could be achieved. At lower frequencies, nonlinear effects were invoked to produce a propagating low-frequency domain whose attenuation and amplification were subsequently studied under small-signal conditions. Analysis of the time evolution of the spatial and frequency distribution of the amplified flux, measured under different conditions, provided accurate data on the frequency dependence of the acoustoelectric gain and the lattice attenuation over the broad range of 0.15-4 GHz. Comparison of the measured gain parameters with the small-signal theory of acoustoelectric amplification gave very good agreement throughout the frequency range studied. The lattice attenuation was found to follow the expected $f^2$ Akhiezer law at high frequencies and to vary much more slowly, if at all, in the low-frequency range (0.2-0.6 GHz). In this latter range the lattice attenuation is very low (∼1 ${\mathrm{cm}}^{-1\mathrm{}}$) and is most probably controlled by crystal imperfections. Although the studies in this paper were mostly confined to the small-signal regime, several interesting features have emerged concerning nonlinear effects. In particular, the down-converted low-frequency flux in a propagating domain was observed to lag slightly behind the high-frequency flux from which it originated. No explanation can be offered at present for this rather surprising phenomenon.