Progress toward direct VUV frequency comb spectroscopy of the ²²⁹Th nuclear clock transition

Due to its uniquely low-lying energy compared to all other known nuclear transitions, the ^229mTh isomeric nuclear transition has been proposed as a prime candidate for a nuclear clock. Such a system could provide insight into electronic-nuclear coupling or be used to probe temporal variation of fundamental constants. Previous nuclear physics experiments have populated the excited state via decay of the parent nucleus, thereby broadly constraining the transition energy, but coherent laser excitation of this transition has not been achieved. Using a vacuum ultraviolet (VUV) frequency comb, we aim to directly excite the nuclear transition and immediately reduce the known energy uncertainty by several orders of magnitude. We present our ongoing search for the nuclear transition in a solid-state thin film ThO₂ target using our VUV frequency comb. We discuss our in-house fabrication of the thin film target, our spectroscopy setup, and the results of our current scans over the 1-σ uncertainty range of the transition. We also present our plans for alternative thorium targets and detection systems that may offer better signal-to-noise ratio.