Abstract

We report the first observation of a permanent second-order nonlinearity in the near surface region of commercially available bulk fused SiO₂ induced by a combined temperature / static electric field poling process. The observed χ⁽²⁾ is three to four orders of magnitude larger than found in fiber experiments and approaches that of LiNbO₃. The preparation process for generating the χ⁽²⁾ nonlinearity involves heating to 250-325°C, in laboratory ambient, while applying a DC bias of 3-5 kV across the nominally 1.6-mm thick samples. After approximately 15 minutes the sample is allowed to cool with the bias still applied. Once cooled, a permanent nonlinearity is observed without any bias by second- harmonic generation (SHG) experiments. The nonlinearity can be erased by heating alone and reestablished by repeating the poling procedure. Poled samples kept at room temperature for almost a year without any special precautions show no noticeable degradation of the nonlinearity. The depth profile of the nonlinearity, probed with a chemical etching process, was found to be exponential with a characteristic decay length of a few μm's. The χ⁽²⁾ coefficient, measured by comparing the SHG power generated in LiNbO₃, was χ⁽²⁾₃₃ ≈ 1x10^-12 m/V. Additional measurements suggest that the nonlinearity is associated with a strong electric field frozen into the near surface region of the SiO₂ by electrons in deep traps associated with impurities and by charge transport of mobile ions such as Na⁺ under the applied field. Because of the ready manufacturability of silica and its established integration with semiconductor electronics and optoelectronics, this nonlinearity will have important applications.

© 1991 Optical Society of America