Raman Studies of Inherent and Applied Stress in Thin Optical Films

Vibrational mode frequency shifts have been shown to correlate with strain in optical coatings the extent of which is dependent upon film deposition conditions. The presence of strain and strain inhomogeneity in thin films as well as crystallite phase and phase homogeneity affects the optical properties, environmental durability, and laser damage resistance. The influence of these variables on the response and stability of optical thin films can be effectively probed using Raman spectroscopy. Molecular vibrational spectra for several sol-gel and sputter-deposited TiO₂ films have been recorded at variable pressures approaching 100 kbar using a diamond anvil cell. Results demonstrate the presence of reversible pressure-induced phase transformations and suggest that introduction of voids in the film microstructure can influence the pressure at which such transformations are observed.