We report the first direct measurements of the refractive index of silica glass up to 145 GPa that allowed quantifying its density, bulk modulus, Lorenz-Lorentz polarizability, and band gap. These properties show two major anomalies at ∼10 and ∼40 GPa. The anomaly at ∼10 GPa signals the onset of the increase in Si coordination, and the anomaly at ∼40 GPa corresponds to a nearly complete vanishing of fourfold Si. More generally, we show that the compressibility and density of noncrystalline solids can be accurately measured in simple optical experiments up to at least 110 GPa.

Refractive index provides fundamental insights into the electronic structure of materials. At high pressure, however, the determination of refractive index and its wavelength dispersion is challenging, which limits our understanding of how physical properties of even simple materials, such as MgO, evolve with pressure. Here, we report on the measurement of room-temperature refractive index of MgO up to ∼140 GPa. The refractive index of MgO at 600 nm decreases by ∼2.4% from ∼1.737 at 1 atm to ∼1.696 (±0.017) at ∼140 GPa. Despite the index at 600 nm is essentially pressure independent, the absolute wavelength dispersion of the refractive index at 550–870 nm decreases by ∼28% from ∼0.015 at 1 atm to ∼0.011 (±8.04 × 10−4) at ∼103 GPa. Single-effective-oscillator analysis of our refractive index data suggests that the bandgap of MgO increases by ∼1.1 eV from 7.4 eV at 1 atm to ∼8.5 (±0.6) eV at ∼103 GPa.

Gated detection of supercontinuum pulses enables optical probing of solid and molten silicates at extreme pressure-temperature conditions. -