Pockels Effect of KDP and ADP in the Ultraviolet Region

The Pockels effect, linear electro-optic effect, of KDP and ADP is studied in the ultraviolet region near the fundamental absorption edge of the materials. The half-wave retardation voltages of Z-cut crystals are measured, and are found to increase almost linearly with the wavelength in the region longer than 200 nm, but to deviate from the linear relation in the region close to the absorption edge. A new quantity–the electro-optic ρ-coefficient–is defined. The value of ρ 63 is different from the value of r 63 by a factor of the fourth power of the refractive index for the ordinary rays. The new electro-optic coefficient is more useful than the r -coefficient in discussing the atomic origin of the electro-optic effect, since the dispersion of ρ-coefficient depends only on the change in the dielectric tensor components by applied field, while the r -coefficient depends also on the refractive index.