Effects of introduction of sodium and water on second-order nonlinearity in poled synthetic silica glass

Second harmonic generation (SHG) from silica glasses doped with sodium or water was investigated to clarify the effects of the impurities on the second-order nonlinearity. Sodium ions were introduced into pure silica glasses from a polymer coating containing a trace amount of sodium with electrical poling. The doping of sodium drastically increased the SHG intensity from the poled silica glasses. The nonlinearity localized in the anode surface region is suggested to be ascribed to a frozen electric field formed by the drift of sodium ions. Introduction of water molecules into silica glasses was performed by hydrothermal treatment in purified water using an autoclave. The SHG intensity from the electrically poled glasses also increased with the increasing content of water molecules. In this case, the SHG was emitted from the entire region of the sample plate. The origin of the nonlinearity is tentatively assumed to be due to a frozen electric field created by charge separation with protonic conduction.