Optical Second-Harmonic Generation in FerroelectricDicalcium Lead Propionate

All independent components of the second-order nonlinear optical (SHG) tensors and Miller's constants of Ca 2 Pb(C 2 H 5 CO 2 ) 6 (DLP) were determined in the temperature range from -60°C to 90°C. Both in paraelectric and ferroelectric phases, the components d 11 and d 14 which are forbidden by symmetry and Kleinmann's condition were also observed. Precise examination revealed that the phenomenon originated from the symmetry-lowering as a result of a strong internal electric field produced in the DLP crystal. By annealing the crystal at 250°C, the effect of symmetry-lowering could be diminished. The temperature dependence of the SHG tensors was analyzed by the bond-charge theory where Pb-O and Ca-O bonds were assumed to be the main contributions to the observed tensors. Opposite signs of the two-bond contributions cause the anomalous temperature dependence of the SHG tensors.