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The Formation of the Second-Order Nonlinearity in Thermally Poled Fused Silica Glass

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Doctoral thesis

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This study examines the physics underlying the formation of a second-order nonlinearity through thermal poling in bulk fused silica glass. The technique of thermal poling involves applying a large voltage across the glass at an elevated temperature, and then allowing the glass to cool with voltage applied. Nominally, a nonlinearity is created from a high-field space-charge region that forms within 10 microns of the anode surface. This dissertation presents an investigation of the dynamics of the formation of the space-charge region. In situ experiments are reported that monitor second-harmonic generation and electrical conductivity as the nonlinearity is formed, and as the applied fields polarity is reversed as well as switched on and off. Complex dynamic behavior was observed. Several response time scales were noted. An incubation period of the second harmonic signal occurs until conductivity space-charge relaxation begins. A transient second harmonic signal from the cathode side was observed after field reversal. Two microscopic probe techniques are presented that indicate the extent of the nonlinearity in the glass. Poled fused silica samples were usually commercially available flame fused quartz optical flats with 1-5 ppm of various alkali ions and 100 ppm OH. Hydrofluoric acid etching was used to etch the cross-section of thin samples. A ridge indicating an affected region with a slow etch rate was identified that moved approximately logarithmically deeper into the sample with poling time. A groove indicating an effected region with a faster etch rate also was identified for long poling periods greater than or equal 1 hr. Secondary ion mass spectrometry SIMS was used to probe impurity distribution levels under the two electrode surface of poled samples. Regions depleted of Li and Na were observed. Li, Na, K and possibly H were identified as being mobile with different mobilities.

Subject Categories:

  • Ceramics, Refractories and Glass
  • Optics
  • Non-Radio Communications

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