Conditions for anomalous acousto-optical diffraction by backward propagating acoustic waves

Experimental evidence is given of acoustic plate waves, whose group velocity is contradirected with respect to the phase velocity, through a determination of the acoustic wavelength dependence on frequency, in a limited range of frequencies. The dependence d Lambda /d Omega >0 between wavelength and frequency is experimentally verified, as the required condition for acousto-optical diffraction, where higher frequency components would scatter light into smaller diffraction angles.