Physical characterization of semicontinuous silver and gold films on dielectric substrates

Semicontinuous metallic films possess unique optical and electrical transport properties resulting from strong localization and large enhancement of electric fields in the very near surface region. The field distributions and electrical transport properties of these films depend on the structural geometry and morphology of the samples and therefore also on the metal concentration, with distinctive properties near the percolation threshold where an insulator-to-metal transition occurs. This dissertation focuses on an experimental investigation of the linear and nonlinear optical, electrical transport and structural properties of semicontinuous silver-glass and gold-glass composites near the percolation threshold. The samples were synthesized by pulsed laser ablation and an effort was made to identify the percolation threshold in terms of structural properties using transmission electron microscopy as well as electrical resistivity and optical transmission through in situ measurements during sample synthesis. A study of the differences in silver and gold semicontinuous film morphology on silicon oxide versus carbon-formvar substrates was performed.; A detailed study was performed of visible and near infrared photoluminescence or "white light" generation by semicontinuous gold and silver films upon excitation by 780 nm 150 fs pulsed and CW laser irradiation.{09}The results of this study supported theoretical predictions that spectral continuum generation would be significantly enhanced at metal fill fractions close to the percolation threshold due to the high degree of electromagnetic field localization. It was shown that in both gold and silver semicontinuous films, visible photoluminescence occurs via a nonlinear mechanism and near infrared photoluminescence via a linear one, both dramatically enhanced by electromagnetic field localization. The mechanism of visible photoluminescence in silver is believed to be different from that in gold and white light generation capability in silver degraded quickly upon laser exposure and was very stable in gold.; The imaginary part of the cubic susceptibility of gold semicontinuous films was measured via the z-scan technique and an optical limiting or inverse saturable absorption effect was observed definitively in gold and suggestively in silver. A relatively low laser damage threshold observed for both the silver and gold films demonstrates the need for further materials engineering work.