An ultrasonic amplifier driven under dc conditions has been observed to oscillate in a stable high-frequency mode. The oscillation corresponded to a very high overtone of the fundamental acoustic resonance of the CdS plate, such as the 100th harmonic or greater. This oscillation is distinctly different from the periodic oscillations in current which occur in high-gain ultrasonic amplifiers at a period corresponding to an electron transit time, and is due to a buildup of acoustic flux. This new oscillation has very pure periodic waves which contain either one frequency or several harmonically related frequencies. Oscillations were detected by the rf current in the applied dc, or by ultrasonic transducers. There is no broad background of incoherent noise—all the available power goes into this one mode of oscillation. To oscillate, the CdS must have net round-trip ultrasonic gain, but not too much gain. Excessive gain allows a buildup of incoherent acoustic flux which destroys the high-frequency mode. The frequency of oscillation is near the frequency of the greatest ultrasonic gain. Although the exact mode on which the oscillator would resonate could not be predetermined, it could always be made to oscillate on the same mode under identical settings of voltage and resistivity.

• Received 11 March 1966

DOI:https://doi.org/10.1103/PhysRev.149.628