Effect of preannealing on the level of second-harmonic generation and defect sites achieved in poled low-water fused silica

The level of second-harmonic generation (SHG) in poled low-water fused silica was altered by preannealing in aqueous and dry-nitrogen atmospheres. The effect of water and heat was found to depend on the lot from which the material originated. The effect on the SHG of either a wet or a dry preanneal was sometimes found to be reversible by the opposite preannealing process. For both wet and dry preannealed samples the SHG was found to have both near-surface (<25-μm) and bulk (≈1.5-mm) components. Electron paramagnetic resonance (EPR) data revealed the presence of Si and Ge E′ defect sites and showed that the relative concentrations of these sites was altered by the preannealing processes. EPR and SHG data provided evidence for a second-order bond effect along with a third-order hole-filling effect as sources of the nonlinearity.