## Abstract

Measurements of the magnitude and the sign of certain quadratic electro-optic coefficients of potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) were made with an actively stabilized Michelson interferometer. The results obtained for these coefficients are, in units of ${10}^{-20}{\mathrm{m}}^{2}{\mathrm{V}}^{-2}$ (as opposed to literature values of order ${10}^{-18}{\mathrm{m}}^{2}{\mathrm{V}}^{-2}),$ as follows: $(\mathrm{KDP}){g}_{\mathit{xxxx}}=-3.4\pm 0.5,\hspace{0.5em}{g}_{\mathit{yyxx}}=-0.2\pm 0.4,$ and ${g}_{\mathit{zzxx}}=-0.7\pm 0.4;\hspace{0.5em}(\mathrm{ADP}){g}_{\mathit{xxxx}}=-7.4\pm 1.0,\hspace{0.5em}{g}_{\mathit{yyxx}}=-1.7\pm 0.9,$ and ${g}_{\mathit{zzxx}}=-1.4\pm 0.9.$ The quadratic Faust–Henry coefficient describing the lattice and the electronic contributions to the quadratic electro-optic effect in KDP and ADP is estimated from our results. These show that the nonlinear susceptibility responsible for the quadratic electro-optic effect in these crystals is due mainly to nonlinear interactions of the low-frequency electric field with the crystal lattice.

© 2001 Optical Society of America

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