Influence of hydroxyl content on second-order nonlinearity in thermally poled type-III fused silica

Effects of high (∼1000 ppm) and low (∼200 ppm) hydroxyl (OH) content on second-order optical nonlinearity (χ(2)) in thermally poled type-III fused silica (KU-1) are studied. Type-III fused silica with low OH content poled under 275 °C with 6 kV for various poling durations shows much stronger intensity of second-harmonic generation (SHG) signals compared with that with high OH content poled under the same condition. A near-anodic uniform surface and uniform bulk χ(2) profiles which are opposite in sign are calculated to be formed in the fused silica poled longer than the optimum poling duration. A two-charge-carrier model with charge injection and non-blocking electrodes is given to explain the creation of the χ(2) profile. More OH is believed to decrease the mobility of alkali ions during poling.