Magnitude and nature of the quadratic electrooptic effect in potassium dihydrogen phosphate and ammonium dihydrogen phosphate crystals

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) m(2)V(-2) (as opposed to literature values of order 10 m(2) V-2), as follows: (KDP)g(xxxx) = -3.4 +/- 0.5, g(yyxx) = -0.2 +/- 0.4, and g(zzxx) = -0.7 +/- 0.4; (ADP)g(xxxx) -7.4 +/- 1.0, g(yyxx) = -1.7 +/- 0.9, and g(zzxx) = 1.4 +/- 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. (C) 2001 Optical Society of America.