Porosity and composition effects in sol-gel derived interference filters

Abstract

The effects of porosity and composition on the optical thickness of sol-gel films is analyzed using the Clausius-Mossotti relationship. The optical thickness is predicted to decrease with shrinkage approximately linearly. For high density films the predictions agree with experimental data from SiO2 and SiO2-TiO2-Al2O3 coatings. At low densities, however, the optical thicknesses are much higher than predicted by the Clausius-Mossotti analysis. This difference is attributed to residual species in the unfired films. Using an empirical value for the ratio of the electronic polarizability αr to the molecular weight Wr of the residual species ( αr Wr = 1.65 × 10-25cm3 mol g-1) the difference between experiment and theory is accounted for quantitatively for both the single-component (SiO2) and multicomponent (SiO2-TiO2-Al2O3) coatings. Approaches to fabricating multicolor dichroic filters, which require large changes in optical thickness on heating, are presented. © 1995.