Acoustoelectric Harmonic Generation in a Photoconductive Piezoelectric Semiconductor

Piezoelectric semiconductors can exhibit harmonic generation because of nonlinear interactions between the acoustic and electric fields in the solid. To observe this effect, longitudinal waves were excited in crystalline cadmium sulfide. Because CdS is highly photosensitive, its conductivity can be changed by several others of magnitude by varying the applied light level. The velocity and attenuation at 4.1 MHz and 8.2 MHz were measured and shown to be strong functions of conductivity in the range 0.001-0.010 C2-1m-1. Fitting the resulting data with linearized theory yielded values for the piezoelectric stress and elastic moduli that were consistent with literature values. Also, harmonic generation resulting from excitation at 4.1 MHz was measured. The amplitudes of the second to fifth harmonics exhibited oscillatory behavior as a function of conductivity, particularly in the aforementioned range. Finally, the second harmonic amplitude was also measured at multiple propagation distances, and the conductivity of its peak amplitude was shown to shift to lower conductivity at further distances.