Antireflective film of porous silica.

We aimed to strengthen the antireflection coating of low refractive index, by using spinodal porous films based on phase separation of sodium borosilicate glass. Coating films were prepared by printing a glass paste onto a silica substrate, and melting at temperatures from 900°C to 1000°C, allowing phase separation at 600°C and finally etching to porosities of ∼60. Coating with reflections as low as 0.5% was achieved, which consisted of a homogeneous porous layer of several micrometers in thickness and a graded region of up to 400 nm in width at the interface with the substrate. As to the optical property, simulations based on three-layer and two-layer models were carried out, into which was incorporated a term expressing mutual coherence degree between reflections from the surface and interface with coherence length of the light source.