Bright VUV frequency combs for a thorium nuclear clock

The identification of a thorium-229 (^229mTh) isomer transition at 148.382 nm in the vacuum ultraviolet (VUV) has raised the prospect for implementing a nuclear-referenced optical clock. Such a nuclear clock promises ultrahigh precision in a compact and robust system that avoids the complexities of ion or atom trapping, since ^229mTh can be embedded in a crystal host. However, a viable clock system requires a bright and ultrastable comb-referenced VUV laser to interrogate the transition.

We are developing such a source. Using phase-matched four-wave mixing in a gas-filled, low-loss anti-resonant hollow core fiber, we have demonstrated 4ω + 4ω – ω = 7ω upconversion of light at 1.03 µm from a femtosecond Yb-based laser to 147 nm, with an estimated 2% overall efficiency, 80 mW average VUV power, MHz repetition rate and excellent beam quality. To scale to >10 MHz repetition rates, we are also developing a compact Yb-fiber laser at 1039 nm, seeded by a stabilized frequency comb laser. This high-brightness source with <100 Hz linewidth and ~1 µW per comb mode around 148.4 nm will allow for precision spectroscopy of this transition and for implementation of a robust, practical, transportable ultraprecision clock system with broad applications in science and technology.