Method for the preparation of porous silica antireflection coatings varying in refractive index from 122 to 144

Coatings of refractive index that vary between 1.22 and 1.44 were prepared from combinations of a colloidal silica suspension and a polysiloxane solution. The polysiloxane acted both as a binder and a filler for the colloidal silica particles. Increasing ratios gave coatings of lower porosity, hence higher refractive index, and better abrasion resistance. This variation in refractive index allowed highefficiency quarter-wave antireflection coatings to be prepared on substrates whose index varied from 1.45 to 2.1. The laser damage thresholds of all the coatings were >50 J/cm(2) at a wavelength of 1.06 microm with a 10-ns pulse length.