DFT study on the clamped linear electro-optic effect in KH2PO4

The clamped linear electro-optic (E-O) effect in KH₂PO₄ (KDP) is studied employing the electronic density-functional theory. It is found that proton atoms in the hydrogen bonds that connect the PO₄ units are much more flexible than the other atoms in KDP when an external electric field is applied. However, our results show that the main contribution to the clamped linear E-O effect in KDP crystals is from the relative displacements of the anions and cations in the phosphate group induced by a low frequency applied electric field, instead of the contribution of vibration of proton atoms. The ration Δn_i/Δl_k indicating the relationship between the atoms displacement and the refractive index are found to be well consistent with experimental results.