Measurement of quadratic electrogyration effect in KDP crystals for light propagating along the optical axis

In this paper, a method of measuring the β₃₁ coefficient of the quadratic electrogyration effect in potassium dihydrogen phosphate (KDP)-type crystals is proposed. It is shown that this very weak effect can be decoupled from other stronger effects occurring simultaneously in the real measuring system, even when small inaccuracies in crystal cutting and alignment are taken into account. Theoretical and numerical analyses are illustrated by experimental data obtained for the KDP crystal. High sensitivity, which is required for successful measurements at room and higher temperatures, has been achieved by using the polarimetric method for a light beam propagating along the optical axis of the crystal in combination with the lock-in technique and a new measurement procedure that compensates for the contribution of other effects such as electro-optic effects, natural birefringence and dichroism. It was found that the β₃₁ coefficient measured at a wavelength of 632.8 nm decreases linearly from 2.53 × 10⁻²¹ m² V⁻² at a temperature of 298 K to 2.08 × 10⁻²¹ m² V⁻² at 343 K.