Spectroscopic limits of diamond anvils to 520 GPa and projected bandgap closure

Diamond anvils serve as optical windows in static ultrahigh-pressure experiments, now reaching the terapascal regime. However, they exhibit poorly understood changes in their optical properties under multimegabar pressure. Here, we present broadband absorption measurements (ultraviolet to infrared wavelengths) up to 520 GPa, revealing a pronounced loss of transparency with pressure. Diamond Raman scattering is used to infer the stress profile along the anvils' axis under the assumption of tetragonal distortion, and crucially at the sample interface. This enables a quantitative analysis of absorption spectra, showing an indirect bandgap narrowing towards the infrared, with metallization projected near 560 GPa sample pressure within our stress model. A universal optical behavior is observed across different anvil geometries, which is consistent with the universality of the Raman edge pressure scale, here refined. These findings help define the spectroscopic operational limits of diamond anvil cells under extreme pressure with important implications for recent claims of hydrogen metallization.