Optical Properties of Low Energy Ion Assisted Deposited TiO2 Films

Absorption in TiO2 films limits their efficiency and stability against high power laser radiation. The optical absorption in single layer TiO2 films is greatly influenced by deposition parameters such as the rate of deposition, oxygen partial pressure, substrate temperature and the starting material. It is also observed that low energy ion assisted deposition enhances the reactivity and results in reduction of absorption. In this investigation, single layers of TiO2 films have been prepared using ionized oxygen (produced using a Heitmann source) and TiO as the starting material. The optical properties (refractive index n, and extinction coefficient k) and the laser induced damage threshold as a function of deposition parameters has been studied. The discharge currents were varied between 0 and 400 mA with the rate of deposition being 9 and 18 nm/min. The oxygen pressure was maintained at 2×10−4torr. At low deposition rates, in situ optical monitoring of transmission has shown absorption free single layers for moderately low discharge currents (200 mA) whereas at higher deposition rates absorption free films were obtained at higher currents. The damage threshold of these films was found to be directly dependent on the absorption. Variation in damage threshold values is explained on the basis of film composition, structure and optical properties.