Sol-gel prepared TiO2 – PMMA composite thin film for high laser induced damage threshold optical coating

Abstract

We report high-quality TiO2 and TiO2-PMMA composite thin films prepared by sol-gel process for application in optical coatings. The addition of PMMA to the TiO2 sol exhibits significant improvements in the optical quality and morphology of the TiO2 films. A comparison between TiO2 and TiO2-PMMA composite thin films is described with respect to various optical properties and their application in optical coatings for high-energy laser applications. The refractive index and bandgap of TiO2 are tailored by controlling the TiO2 sol: PMMA volume ratio. The refractive index varies in the range of 2.4 to 2.0 at 550 nm by controlling the TiO2: PMMA composition. Incorporation of PMMA leads to reduction in absorption from 1000 ppm to 20 ppm in the composite films, which is attributed to passivation of defect states. In addition, incorporation of PMMA into the TiO2 sol results in the filling of voids and a significant improvement in surface figure (from λ/6 to λ/10) of the composite films. These properties collectively enhance the laser induced damage threshold (LIDT) of 2.1 J/cm2 for TiO2 film to 17 J/cm2 of the TiO2-PMMA composite film. LIDT mechanisms in the films are discussed. Demonstration of antireflection coatings (AR) as well as high reflection coatings (HR) exhibiting wavelength tuning of the reflection peak using TiO2-PMMA composite thin films, is presented. An AR coating @1200 nm with 11.5 J/cm2 LIDT is reported.