Parametric Amplification and Frequency Shifts in the Acoustoelectric Effect

Mechanisms for the downshift in the frequency of maximum acoustic intensity f
mi
for high flux domains in piezoelectric semiconductors are reviewed. For the simple case where an externally introduced acoustic wave (pump) produces a single-frequency domain in photoconducting CdS, clear evidence is given that the downshift in f
mi
is due to parametric amplification of thermal acoustic noise. For a pump of 990 MHz, after some initial growth (v
d
=1.14 v
s
), the pump is found to be depleted. In the pump depletion region, signals in a 200 MHz band about the even subharmonic (445 MHz) are found to grow. At pump strains of about 10
⁻⁶
the signals propagated at angles to the pump equal to those that give phase matching according to the dispersion of linear theory. For higher pump strains, however, the collinear process is dominant. The signal domain is narrower than the pump domain, as expected, because the parametric growth is exponentially dependent on pump strain. The downshifting of f
mi
in the region where deviations from linear theory are still small is discussed in terms of a parametric interaction model, with the initial acoustic strain distribution considered as an incoherent pump.