Continuous-wave laser source at the 148  nm nuclear transition of Th-229

A continuous-wave laser source at 148.4 nm based on second-harmonic generation in randomly quasi-phase-matched strontium tetraborate, SrB 4 O 7 , is demonstrated. It provides 1.3 −0.6 +0.7 nW of VUV power in a single pass for an incident UV laser power of 325 mW. The laser system is developed for the resonant laser excitation of the 229 Th nucleus to its low-energy isomeric state. For a frequency-stabilized laser system, we expect to reach similar VUV power spectral densities as in previous pulsed laser excitation experiments of the nuclear transition in 229 Th-doped crystals.