Use of sol-gel hybrids for laser optical thin films

Abstract

AbstractThe CEA/DAM megajoule-class pulsed Nd:glass laser devoted to Inertial Confinement Fusion (ICF) research is requiring 7,000-m2 of coated area onto 10,000 optical components. Among these different optics, two specific examples of applied sol-gel chemistry will be described. First one is dealing with the 240 44-cm square cavity-end mirrors needing to be highly-reflective (HR)-coated onto deformable substrates. Such large dielectric mirrors are using interference quaterwave stacks of SiO2 and ZrO2-PVP (PolyVinylPyrrolidone) thin films, both starting from sol-gel colloidal suspensions (sols). The ZrO2-PVP high index layer is a nanohybrid material prepared from mixing nanosized-zirconia suspension with a transparent polymer solution (PVP). The oxide/polymer ratio of the hybrid system has been optimized regarding refractive index value and laser damage threshold. UV-curing of the nanohybrid has enabled optical coating stacking up to 20 layers, achieving 99% minimum reflection over the whole coated surface. FT-IR spectroscopy has been used to highlight particles/polymer chemical interactions and also polymer modifications under UV-irradiation.Second example is concerning development of a silica-based hybrid material to protect silvercoated light reflector used in laser pumping cavity. These metallic reflectors require a protective overlayer in order to preserve high-reflectivity front surfaces for long periods of operation under intense broadband flashlamp light and typical airborne contaminants. The so-called ormosil coating has been optimized in term of thickness and composition to enhance silver resistance to oxidation and tarnishing under UV-irradiation, to protect silver layer from clean-room cleaning procedure, to withstand 10,000 flashlamp glow-discharges exposure with the lowest possible change in the reflection value. To fulfil these requirements, the developed hybrid sol-gel material acts as an oxidation dense barrier, is chemical-resistant, is durable and remain transparent in the 400-1000nm wavelength range. Moreover, the sol-gel process allows industrial protective coating deposition onto large-sized and multi-shaped reflectors. These new protected reflectors will need to be replaced much less often than reflectors employed in current solid-state lasers, ensuring both higher performance and lower operating costs.