A Study of the Validity of the Weinreich Relationship for Cadmium Sulphide

Abstract

The validity of the Weinreich relationship was studied under both linear and non-linear conditions using an acoustic amplifier comprising a piece of single crystal, photoconducting, cadmium sulphide sandwiched between two coaxially plated, Y - cut quartz transducers of 15 MHz. fundamental frequency. Careful attention was paid to the choice of crystal, and also to amplifier fabrication. The experimental arrangement adopted incorporated facilities for varying the crystal conductivity from 10-9  (ohm cm)-1  to 10-4 (ohm cm)-1, applying a d.c. voltage from zero to 1 kV, and varying the duration, amplitude and frequency of the input acoustic signal. An essential feature of this arrangement was the simultaneous oscilloscope display of the acoustic echo train, applied d.c. voltage, and acoustoelectric voltage. Appropriate techniques were devised to enable an accurate determination of the acoustic attenuation coefficient, the input acoustic intensity, and the acoustoelectric voltage. An analysis of the Weinreich relationship consistent with these techniques was presented which allowed a quantitative comparison of theory and experiment to be made under both amplifying and attenuating conditions. The quantitative validity of the Weinreich relationship was established for linear acoustoelectric behaviour at 45 MHz. and 75 MHz. The results obtained at 15 MHz. indicated the Weinreich relationship to be qualitatively valid under attenuating conditions, but suggested a quantitative validity under amplifying conditions. The apparent disagreement between theory and experiment noted under certain non-linear conditions was attributed to the non-linear generation of harmonics, and subsequent investigation showed the Weinreich relationship to be valid for each harmonic generated. A means of establishing, quantitatively, the validity of the prominent carrier trapping theories was indicated, and it was concluded that these theories describe consistently the effects of carrier trapping on the acoustic attenuation and acoustoelectric voltage variation with applied d.c. voltage. Finally, some general observations relating to the amplification of broad band acoustic flux were given.