Relationship between refractive index and density of synthetic silica glasses

The effect of impurity and density on refractive index was studied for silica glasses synthesized with direct, sol-gel, and vapor phase axial deposition (VAD) processes. Linear relationships between the refractive index and density n=Cρ+D were found in a wide wavelength region from ultraviolet to infrared. The correlation coefficient C decreased from 0.081±0.007cm3∕g at 1.707μm to 0.061±0.008cm3∕g at 0.238μm. The relationship was different from those of the VAD or direct glasses formed by high-pressure densification, that is, the coefficient increased from 0.200±0.011cm3∕g at 1.707μm to 0.229±0.012cm3∕g at 0.238μm for VAD glass. Chlorine and hydroxyl impurities affected the refractive index; however, these impurities did not cause the linear relationship. After adjusting the index according to the chlorine related effect and the effect of the sample density, the difference between the sample’s refractive indices Δ(n2−1) showed a similar dispersion relation irrespective of the synthetic process. An analysis using a two-term harmonic oscillator model suggested that the origin of the refractive index dispersion is the oxygen 2p-silicon 3s interband transition, as is the dispersion relation of silica glass. The difference in the refractive indices in these synthetic silica glasses is deduced to be due to a stoichiometric excess or deficiency of oxygen atoms of forms ≡Si–OHHO–Si≡, ≡Si–Si≡, ≡Si––SI, etc., and the density arising from these forms in these synthetic processes.