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
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