CW laser sources for exciting the thorium clock transition

The first low-energy nuclear excited state of thorium-229 has gained an increasing interest since direct laser excitations have been proven in various cases. This unique transition of the thorium isotope offers many applications, including a highly accurate nuclear clock, with short interrogation times due to high numbers of interrogated atoms in a solid-state environment, and a new testbed for physics beyond the standard model.

Still, one challenging key ingredient for a high-accuracy nuclear clock is a Hz-scale, narrow-linewidth laser source for the resonance wavelength of 148 nm. Different concepts for such a light source are under development like high-harmonic generation of a femtosecond laser frequency-comb, or four-wave mixing in gases or metallic vapors. We will show our results on an all-solid-state approach to achieve frequency doubling of laser radiation at 297 nm using the second-order nonlinear material strontium tetraborate (SBO). This material is characterized by a combination of high nonlinearity, a wide-bandgap reaching down to 125 nm, and the possibility for phase-matching. Phase matching in SBO is provided via spontaneous domain inversions that randomly occur during a Czochralski growth process.  We have generated up to 1.3 nW of vacuum-ultraviolet power in a single pass from 325 mW of 297 nm laser radiation. The resulting power spectral density is comparable to that of previous laser nuclear excitation experiments. We will discuss the prospects for the nuclear excitation of Th-229 with doped crystals and trapped ions with this source.