Direct observation of the topological pruning in silica glass network; the key for realizing extreme transparency

Abstract The optical transparency of silica glass significantly improves when subjected to compression at its melting temperature. Using a rare hydrostatic iso-pressure apparatus capable of reaching 0.98 GPa at 1800 °C with Ar gas as the pressure medium, we obtained centimeter-sized glass samples, allowing us to measure various properties. Both the density and refractive index increased with pressure, while the refractive index dispersion decreased monotonically. However, Rayleigh scattering intensity, and small ring structures show a minimum around 0.8 GPa. High-energy X-ray scattering analysis indicates that the short-range structure, around 4 Å, governs the monotonic trends in the averaged physical properties, such as density and refractive index. In contrast, non-monotonic changes are observed with the disappearance of intermediate-range order at around 8 Å. This simplification of structural ordering is crucial for achieving extreme transparency in silica glass. The effect of suppression of the 8 Å order is well explained by the predicted topological pruning phenomenon, where large voids and small unstable ring structures vanish, leading to the minimal light scattering under high pressure. Our experimental findings also reveal that the optimal pressure for achieving this transparency is much lower than previously predicted, which makes the process more feasible for mass-production applications.