Optical and environmentally protective coatings for potassium dihydrogen phosphate (KDP) harmonic converter crystals

Potassium dihydrogen phosphate (KDP) crystals have been used as harmonic converters on the Nova laser at LLNL for over six years. All crystals were coated with a single layer, quarterwave AR coating of porous silica with a refractive index of 1.22. This was prepared by a sol-gel process and was applied from a colloidal suspension by spin coating at room temperature. A few crystals were also coated with a methyl silicone coating prior to the application of the AR coating for environmental protection. The initial optical performance of all crystals was very good but there has been some deterioration over the years because of environmental and laser damage degradation. The deterioration in the silicone samples was, however, much less than the others. We are now in the process of replacing all ten KDP arrays with new crystals and will apply the silicone undercoat to all samples. Recently we have been evaluating a new perfluorinated organic polymer coating which has a refractive index of 1.29. This material is soluble in fluorinated solvents and can be applied by dip coating from solution at room temperature. We hope that this can provide environmental protection when applied to KDP and also act as an AR coating at the same time. The optical performance is not as good as our porous silica because of the higher index; about 0.3% reflection per surface is obtained. 4 refs., 10 figs., 1 tab