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

Abstract

A continuous-wave laser source at 148.4 nm based on second-harmonic generation in randomly quasi-phase matched strontium tetraborate, SrB4O7, 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 229Th 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 229Th-doped crystals.

Figures (5)

• Figure 1: Experimental setup for SHG at 148.4 nm. UV light at 296.8 nm is focused into the RQPM SBO crystal that generates VUV light. The temperature stabilized crystal is mounted on a 5-axis translation stage. Two dichroic mirrors, a VUV grating and a solar blind CsI PMT are used for the detection of the VUV power. The VUV radiation can be directed either to the spectrometer or to the spectroscopy beam line by a mirror mounted on a vacuum compatible translation stage.
• Figure 2: The recorded PMT signal showing fundamental and second-harmonic spectra. The SHG signal at 148 nm vanishes under atmospheric conditions due to high absorption of the VUV light. The peaks between 170 and 340 nm are residuals of the UV radiation.
• Figure 3: VUV signal measured with the SBO crystal in vacuum and under high-purity N$_2$ buffer gas. The laser radiation was initially blocked and then applied to the crystal. The crystal in vacuum shows strong local temperature variations that change the RQPM conditions and therefore the resulting VUV signal.
• Figure 4: SHG power as a function of crystal temperature for varying fundamental power under vacuum (a) and N$_2$ buffer gas environment (b). The dashed lines are fits with sinc$^2$-functions. In the absence of nitrogen the optimal temperature for SHG decreases with increased fundamental power due to heating caused by absorption in the crystal.
• Figure 5: (a) Typical dependence of PMT VUV current and counts on the fundamental power with quadratic fits. For the photon counting dependence the fit ignores experimental points at UV powers $\ge$225 mW where a saturation effect of the photon counting method is observed. (b) SHG power as function of the fundamental power.