Space Charge in Thermally Poled Fused Silica

摘要

Various groups continue to make progress improving in-line and all-fiber electro-optic switches and wavelength converters based on thermal poling of fused silica. However, there is still much to be understood of the thermal poling processes that lead to the establishment of the second-order nonlinearity (Χ~(2)) that makes these devices possible. The predictable control of the the Χ~(2) distribution is a difficult endeavor. The location, strength, and profile of the Χ~(2) distribution based on poling conditions has not yet been fully described, although several groups have contributed to our basic understanding using Maker fringes, etching techniques, laser-induced pulse probes, and other measurement techniques. We performed several Secondary Ion Mass Spectrometry (SIMS) analyses to complement earlier etching and electrical current/second harmonic generation (SHG) measurements in an attempt to understand the dynamic behavior of the Χ~(2) distribution. These measurements revealed an alkali depleted space charge region whose depth into the glass evolves logarithmically with poling time. Our data demonstrate that Na~+, Li~+, and K~+ are mobilized by thermal poling in type II fused silica (flame fused natural quartz, 1-10 ppm alkali impurity levels). We also suspect H~+ or H_3O~+ plays a role, due to the abundance of H and H_2O in glass surfaces and air. We observed that the poling environment plays an important role in the charge profile and depth of the space charge region. Impurities from the electrodes, handling, the atmosphere, as well as impurities already adsorbed onto the glass surface, act as reservoirs for an ion exchange process in which alkali ions are pulled into the space charge region. We have observed that the presence of impurities near the glass surface can reduce overall mobility and slow formation of the space charge region. Pruneri et al. have also noted that the poling atmosphere makes a difference in the Χ~(2) distribution, observing that given similar poling times, the Χ~(2) distribution extends further into the glass for vacuum-poled samples than for samples poled in air.