From the experimental method section of the original paper: [1]

“A BX90-type diamond anvil cell equipped with 80 μm diameter diamond anvil culets was prepared. A 200 μm thick rhenium foil was indented down to 12 μm and a sample cavity of 40 μm in diameter was laser-drilled at the center of the indentation. Two agglomerates of submicron-sized gold particles, each of approximately 2 μm in size, were loaded into the sample chamber to serve as both YAG laser absorbers and pressure gauges. The cell was then loaded with pure nitrogen gas at ~1200 bars.”

[1]: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.12...