PHONON FREQUENCY SPECTRA OF TRAVELING ACOUSTOELECTRIC DOMAINS IN CdS

Optical beam probing using Bragg diffraction from traveling acoustic domains in CdS has revealed a direct correlation between the frequency of maximum phonon intensity and the total integrated acoustic intensity. At the point of initiation of the acoustic domain the frequency of maximum phonon intensity was equal to that for maximum small signal gain, but as the domain propagated towards the anode the total acoustic intensity Φ_T increased and the frequency f_m of maximum phonon intensity decreased according to the relation ${\Phi }_T{\mathrm{\hspace{0.17em}\infty \hspace{0.17em}f}}_m^{\text{−4}}$ . After a sufficient distance of travel the acoustic intensity saturated and the peak phonon frequency became invariant. It is suggested that the drop in phonon frequency and its correlation with total phonon intensity are related to the trapping of free carriers by acoustic phonons.