Continuous-wave laser absorption spectroscopy of the thorium-229 nucleus

The low-energy nuclear transition in thorium-229 (Th-229) has been excited in thorium-doped crystals with laser light^1-3, opening the path towards a highly stable and robust solid-state optical nuclear clock⁴. The required laser radiation at 148-nm wavelength has so far been produced using pulsed laser systems, in which only a small fraction of the incident photons has been resonant with the narrow nuclear transition. Here we show that the nucleus can be excited with a continuous-wave (CW), narrow-bandwidth, solid-state laser source⁵ at sub-nanowatt power and that the nuclear resonance signal can be detected in absorption rather than fluorescence. This eliminates the slow nuclear fluorescence decay from the detection process, allowing for clock operation with fast signal acquisition. We characterize two different thorium centres in a calcium fluoride (CaF₂) crystal and measure the isomeric shift between them. One of the centres shows a very small static electric crystal field gradient <0.1 V Å^-2 compared with gradients in the range of 100 V Å^-2 observed previously^3,6. This indicates a centre with high symmetry of the ions surrounding the thorium nucleus, promising nuclear resonance lines that are nearly independent of the lattice spacing.