Structure of Nonlinear Optical Phenomena in Potassium Dihydrogen Phosphate

Previously available data on nonlinear optical phenomena in potassium dihydrogen phosphate (KDP) are interpreted together with new data on the linear electrooptic effect. It is shown that second harmonic generation is dominated by energy levels in the ultraviolet ("electronic" levels) whereas the dc and linear electrooptic effects may have contributions due to processes simultaneously dependent on ultraviolet and infrared ("ionic") levels. The contribution of this "electronic-ionic" process to the dc and linear electrooptic effects is less than 50% and may indeed be negligible. The close relationship between the dc and linear electrooptic effects is reexamined and shown to be in better agreement with experiment than previously reported. The limitations and implications of Kleinman's symmetry condition are discussed in the light of recent experimental data.