Besides the Pockels effect, crystals of the dihydrogen phosphate type (symmetry class 4¯2 <i>m</i>) also exhibit an indirect electro-optic effect or piezo-optic effect. This effect is examined both experimentally and theoretically, and expressions for the spatial distribution, amplitude, and frequency response of modulation are derived for the case of a (<i>zxt</i>)0°-cut square plate. The frequency response is found to depend upon the light-beam diameter. The amplitude of the piezo-optic resonances is found to be nearly independent of frequency.
JOSEPH F. STEPHANY, "Piezo-Optic Resonances in Crystals of the Dihydrogen Phosphate Type," J. Opt. Soc. Am. 55, 136-141 (1965)