# Effect of Deuteration on the Temperature Dependence of the Quadratic Electro-Optic Effect in KDP Crystals

## TL;DR

This paper studies how deuteration affects electro-optic properties in KDP crystals, showing significant differences in temperature dependence.

## Contribution

The study reveals new insights into the quadratic electro-optic effect in KDP and DKDP crystals, challenging existing models.

## Key findings

- KDP and DKDP crystals show significant differences in electro-optic coefficients.
- Intrinsic coefficients vary more than previously believed.
- Current models of the linear effect are insufficient for describing the quadratic electro-optic effect.

## Abstract

The results of precise measurements of the temperature dependencies of quadratic electro-optic coefficients, namely g1111−g1122 and no3g1111−ne3g3311, in KH2PO4 (KDP) and KD2PO4 (DKDP) crystals at a wavelength of 632.8 nm are presented. We consider electro-optic coefficients describing changes in the optical impenetrability tensor resulting from an applied electric field, as well as intrinsic electro-optic coefficients defined in terms of induced polarization. The results show significant differences in the values of the analogous coefficients for the KDP and DKDP crystals and their temperature dependencies. Therefore, the quadratic electro-optic effect in KDP-type crystals cannot be easily described based solely on the contribution of PO4 tetrahedra, as assumed in current models of the linear effect. Moreover, the values of the intrinsic coefficients in the KDP and DKDP crystals differ even more than the corresponding usual electro-optic coefficients, which contradicts the conventional belief in their lower variability.