Improvement of the environmental stability of sol-gel silica anti-reflection coatings

The environmental stability of sol-gel anti-reflection (AR) coatings on optical components is improved by an exposition to volumetric concentrated ammonia vapors. Coatings of 86 nm thickness are prepared by dip-coating in the silica sol synthesized by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in the ethanol solvents with a base catalyst. Due to the sol-gel process, the coatings have a large micro-porosity and consequently susceptible to contamination by adsorbing volatile organic compounds in the vacuum environment of high-powered lasers. The ammonia vapor (NH 3 ) followed by hexamethyldisilazene (HMDS) vapor respectively at room temperature is used to reduce the susceptibility. This work presents how the concentration of NH 3 vapor exposed to the coatings affects the environmental stability regarding to the optical performance, and shows that NH 3 vapor treatment with volumetric concentration of at least 10 vol% can make the coatings substantially durable, combined with excess HMDS vapor treatment both for 24 h. Besides, different off-line methods of cleaning to remove the deposition of the vapor contamination within the coatings, including infrared heating, ethanol dipping, and plasma cleaning, are investigated and compared. Graphical abstract