Generation of continuous-wave laser light at 148.4 nm using cavity-enhanced second harmonic generation in $BaMg{F}_4$

We experimentally investigate the potential of $BaMg{F}_4$ crystals to create a continuous-wave (CW) solid state laser at the vacuum ultraviolet (VUV) wavelength of 148.4 nm via cavity-enhanced second harmonic generation. This investigation is motivated by the development of a nuclear optical clock based on a transition between the ground and isomeric state in the ${}^{229}Th$ nucleus. For this purpose, a $BaMg{F}_4$ 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 generated laser light at 148.4 nm was characterized. Within this proof-of-concept investigation, a VUV output power of typically ($16\pm 1$) pW is obtained. This marks 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.