OPTICAL 2ND-HARMONIC GENERATION IN FERROELECTRIC DICALCIUM LEAD PROPIONATE

All independent components of the second-order nonlinear optical (SHG) tensors and Miller's constants of Ca2Pb(C2H5CO2)6 (DLP) were determined in the temperature range from -60-degrees-C to 90-degrees-C. Both in paraelectric and ferroelectric phases, the components d11 and d14 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-degrees-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.