Method for reducing the effect of environmental contamination of sol-gel optical coatings

Abstract

Antireflection (AR) coatings prepared form colloidal suspensions of silica have a large surface area because of their porosity. The surface is quite polar and readily absorbs vapor contamination to the detriment of the optical performance and the laser damage threshold. This effect is particularly bad in 'dirty' vacuum systems, such as target chambers. The polar surface is due to residual Si-OH and Si- ethoxyl groups formed as a result of the method of preparation of the coating suspension. We have now found that these groups can be removed by further treatment of the coating after preparation. This involves two steps, the first being exposure to ammonia and water vapor, which hydrolyses the ethoxyl groups to hydroxyl groups with the formation of more Si-OH groups. Some of these react further by self condensation to Si-O-Si linkages. The remaining Si- OH groups are removed in the second step by reaction with hexamethyl-disilazane, which converts them to trimethylsilyl groups. The latter are completely nonpolar and substantially eliminate vapor absorption. We have carried out a series of tests involving exposure of trace and untreated coatings to various types of vapor contamination and followed the degree of contamination by the reduction in optical transmission. In all cases, the treated coating showed a significant reduction in transmission loss. These tests also provide guidance as to which materials are acceptable for use in the NIF.