Direct Laser Excitation of the Th-229 Nucleus in the CaF2 Crystal Environment Using a VUV Tunable Laser

We report on the resonant excitation of the low energy nuclear isomer state of the Th-229 using a tabletop tunable vacuum ultraviolet (VUV) laser system, and measured its long-lived fluorescence signal. To achieve this, we dope Th-229 nuclei as Th⁴⁺ ions into calcium fluoride (CaF₂) crystals [1]. The solid-state approach allows for a high Thorium concentration (up to 10¹⁹ cm⁻³) resulting in a high signal yield. The isomer signal was observed in two different crystals with different doping concentrations but was absent in a control experiment with a Th-232 doped crystal. These results are significant because they pave the way for Th-229 nuclear laser spectroscopy with a hertz-level resolution, similar to what has been achieved in the most advanced optical atomic clocks. This brings us one step closer to the development of the first nuclear clock.

[1] Beeks, K. et al. Sci Rep 13, 3897 (2023).