Raman Studies of Phase Transformations in Pulse Laser Irradiated Dielectric Films

摘要

Sputter deposited TiO sub 2 and ZrO sub 2 films on silica have been characterized. Multilayer stacks, useful for high energy laser mirror applications, have also been studied using probe wavelengths, which take advantage of the coating optical properties to enhance the Raman signal through interference effects. Spatially and time-resolved Raman spectra from these oxidic coatings have been acquired immediately following irradiation at 1.064, .532, or .355 mu m and at longer times. Transient measurements are designed to follow surface transformation/relaxation phenomena; measurements at longer times characterize the equilibrium damage state. Irradiated anatase coatings exhibit vibrational features that irreversibly shift to higher frequencies, suggesting an increase in localized stress. Catastrophic damage is manifested by the appearance of a higher density rutile phase. Rutile coatings examined after irradiation show only intensity changes, although marked changes in the polarized Raman scattering are apparent for grain-oriented coatings, suggesting that recrystallization has occurred. Amorphous coatings were found to rapidly crystallize under high-pulse energy irradiation. Results of these studies are compared with equilibrium Raman measurements of thermally-induced transformations in these materials. (ERA citation 11:016674)