Humidity effect on the decay of second-order nonlinearity in thermally poled fused silica

Type I (GE124) and Type II (KV) fused silica were thermally poled in a vacuum and in air under identical poling conditions. Second-order nonlinear (SON) strength and nonlinear depth were found all to be the same. Samples were then stored in high and low humidity to study their SON stability. The SON of poled GE124 was stable over time despite different poling atmospheres and humidity in storage. The SON of both the air-poled and vacuum-poled KV samples decayed over time in both low and high humidity, with the exception that the air-poled KV sample stored in low humidity remained stable. High humidity accelerated the decay process of the KV samples. A porous surface model was used to interpret the decay mechanism. The decay curves implied multiple carriers or a multiple-porosity model for the decay mechanism.