Nonlinear electrooptic effects in KDP and DKDP crystals

An investigation is made of the relationship between the coefficients of the electrooptic effects and the tensors of the nonlinear electric susceptibilities. The form of the tensor $\mathbf K$, which describes the fourthorder nonlinear electrooptic effect, is found for crystals having $\bar{42}m$ symmetry. The results are given of polarization-optical measurements of the induced birefringence in single DKP and DKDP crystals for light polarized along the [100] axis and propagating along the [001] optic axis. The coefficients of the second-order electrooptic effect agree with the existing data but the coefficients of the fourth-order electrooptic effect differ from the published results by two orders of magnitude.