Generation of continuous-wave laser light at 148.4 nm using cavity-enhanced second-harmonic generation in BaMgF 4

The development of a nuclear optical clock based on 229 Th is expected to influence the next generation of time and frequency standards. At its heart is the need for a continuous-wave (CW) laser at 148.4 nm to drive the clock transition between the ground and excited isomeric states. In this work, we experimentally investigate the potential of BaMgF 4 crystals to create a CW solid-state laser at the vacuum ultraviolet (VUV) wavelength of 148.4 nm via cavity-enhanced second-harmonic generation. For this purpose, a BaMgF 4 crystal was grown, optically polished, and periodically poled. The crystal was inserted into a power-enhancement cavity, resonant at the fundamental wavelength of 296.8 nm, and the laser light generated at 148.4 nm was characterized. Within this proof-of-concept investigation, a VUV output power of typically (16±1)pW is obtained. This marks what is believed to be the first time that this type of crystal is used to generate VUV laser light. The experimental findings are compared to theoretical expectations and provide a clear path for future improvements, which could have implications not only for compact laser sources in the VUV but also for solid-state and single-ion-based 229 Th nuclear clocks.