Generation and spectroscopic characterization of continuous-wave 148.4-nm light in strontium tetraborate
Continuous-wave (CW) radiation at 148.4~nm is required to drive the $^{229}$Th isomeric transition of a solid-state nuclear clock, and frequency doubling of 296.8-nm light is its most direct source. We generate CW 148.4-nm light by random quasi-phase matching in strontium tetraborate and characterize it with a compact CaF$_2$ prism spectrometer, which reaches picowatt sensitivity without filters or a grating and reads the wavelength from the rotation angle of a solar-blind photomultiplier. The observed harmonic profile has an angular FWHM of 0.36$^{\circ}$, corresponding to a convolution-limited spectral width of about 0.6 nm rather than the intrinsic linewidth. The directly estimated output is 1.9 nW at room temperature; scaling the measured temperature dependence gives an inferred 4.8 nW at the best observed temperature. The conversion varies by an order of magnitude across the crystal and is strongly structured in temperature: between 30 $^{\circ}$C and 50 $^{\circ}$C it shows several maxima only a few degrees apart, so a small excursion moves the output from a maximum to a minimum. These measurements provide valuable information on the realization and characterization of a high-quality 148.4-nm light source, and pave the way for the future development of high-performance nuclear optical clocks.