Thorium-doped strontium fluoride crystal: a unique candidate for a solid nuclear optical clock material.

Thorium-doped vacuum ultraviolet transparent crystals have attracted widespread attention to be used in the solid-state nuclear optical clock because of their advantages such as frequency stability, miniaturization, and spaceborne ability. However, the doping efficiency, doping loss, and doping uniformity of these existing crystals are still not ideal. Herein, a candidate with unique advantages in the cultivation of solid-state nuclear clock material, i.e., Th:SrF₂ crystal, is introduced for the first time, to the best of our knowledge. It not only has a segregation coefficient close to 1, which can achieve highly efficient and uniform doping of Th, but also ensures a high transmittance (∼69% at 149 nm) while achieving extremely high doping concentration (²³²Th > 6 × 10²⁰ cm^-3). In addition, the Th:SrF₂ crystal would not be irradiated-colored under strong α radiation, which is expected to ensure its transmission performance in the nuclear transition band is not severely affected by ²²⁹Th radiation damage. The discovery of this crystal would greatly promote the development of solid-state nuclear clock materials.