We report the first direct laser excitation of the Th-229 nuclear transition in Th-doped CaF2 crystals using a tabletop tunable laser system. The Th:CaF2 crystals are grown at TU Wien with up to 5×10¹⁸ cm^-3 Th-229 concentration, and a VUV laser system developed at PTB, that provides a spectral photon flux of more than 2×10⁴ photons/(s Hz). A resonance fluorescence signal is observed in two crystals with different Th-229 dopant concentrations, while it is absent in a control experiment using Th-232. The nuclear resonance for the Th⁴⁺ ions in Th:CaF2 is measured at the wavelength 148.3821(5) nm, frequency 2020.409(7) THz, and the fluorescence lifetime in the crystal is 630(15) s. Because of the higher density of photon states in the dielectric optical medium, the measured spontaneous M1 decay rate is expected to be enhanced relative to the rate in vacuum by a factor n³ where n is the refractive index. Applying this correction, the measured radiative lifetime of 630(15) s corresponds to an isomer half-life in vacuum of 1740(50) s. These results pave the way towards a high-resolution Th-229 nuclear laser spectroscopy and realizing optical nuclear clocks.

Dr. Ke Zhang graduated with a bachelor's degree in physics from Shanxi University in July 2014, where she earned second prize of Science and Technology Challenge Cup of Shanxi university in 2013. She then pursued a PhD at the China Academy of Engineering Physics, earning multiple scholarships. From 2020 to 2022, she worked at GSI Helmholtz institute in Germany, where she solved two challenging problems in atomic and molecular spectroscopy: the first detection of the two lowest-lying atomic states in actinium[1] and the detection of fast oscillations of nuclear mass using molecular spectroscopy[2]. In 2022, she joined Ekkehard Peik's group for her second postdoctoral research position in PTB, where she was assigned the challenging scientific task of laser-exciting the Th-229 nucleus—a long-standing puzzle in Th-229 research since 1976. Together with her colleagues and collaborators, she succeeded in exciting the Th-229 nucleus on April 29, 2024[3].

[1] Phys. Rev. Lett. 125, 073001 (2020).

[2] Phys. Rev. Lett. 129, 031302 (2022).

[3] Phys. Rev. Lett. 132, 182501 (2024).