Below-threshold harmonic generation in gas-jets for Th-229 nuclear spectroscopy

The generation of below-threshold harmonics in gas-jets constitutes a promising path towards optical frequency combs in the vacuum ultra-violet (VUV) spectral range. Of particular interest is the 150 nm range, which can be exploited to probe the nuclear isomeric transition of the Thorium-229 isotope. Using widely available high-power, high-repetition-rate Ytterbium-based laser sources, VUV frequency combs can be generated through the process of below-threshold harmonic generation, in particular 7th harmonic generation of 1030 nm. Knowledge about the achievable efficiencies of the harmonic generation process is crucial for the development of suitable VUV sources. In this work, we measure the total output pulse energies and conversion efficiencies of below-threshold harmonics in gas-jets in a phase-mismatched generation scheme using Argon and Krypton as nonlinear media. Using a 220 fs, 1030 nm source, we reach a maximum conversion efficiency of 1.1 x 10-5 for the 7th harmonic (147 nm) and 0.78 x 10-4 for the 5th harmonic (206 nm). In addition, we characterize the 3rd harmonic of a 178 fs, 515 nm source with a maximum efficiency of 0.3%.Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Automated summary

The generation of below-threshold harmonics in gas-jets offers a promising route to optical frequency combs in the vacuum ultra-violet (VUV) range. Of particular interest is the 150 nm region, which can be used to probe the nuclear isomeric transition of the Thorium-229 isotope. Using readily available high-power, high-repetition-rate Ytterbium-based lasers, VUV frequency combs can be generated through below-threshold harmonic generation, specifically the 7th harmonic of 1030 nm. Understanding the achievable efficiencies of this process is crucial for the development of suitable VUV sources.