7th harmonic generation in gases for coherent 150 nm light production

High-harmonic-generation in gases (HHG), along with crystal-based nonlinear frequency conversion, represent well established methods for laser-light production, covering spectral regions from soft X-rays down to the terahertz regime. A less-explored intermediate spectral regime can be identified in-between the spectral regions covered by these two methods, characterized by the cut-off wavelength of nonlinear crystals (e.g. KBBF around 160 nm) and the ionization potential of nonlinear gas media (e.g. Xenon at 102 nm). This wavelength regime is currently attracting increasing attention, brought along by measurements of the nuclear clock transition energy of the Thorium-229 isotope at a wavelength of 149.7 ± 3.1 nm [1] . Compared to alternative approaches such as cascaded frequency up-conversion in a hollow capillary [2] or four-wave-mixing processes, the simplest method to producing 150 nm light is the direct generation of the 7 th harmonic of a 1030 nm Ytterbium (Yb) laser source. In particular, it is easily compatible with demonstrated methods for vacuum-ultraviolet frequency comb production via intra-cavity harmonic generation [3] . However, in contrast to HHG, where phase-matching is commonly reached, for 7 th harmonic generation in gases all key phase-mismatch contributions (Gouy phase, neutral- and plasma dispersion) have the same sign. In this work, we demonstrate 7 th harmonic generation in a noble gas-jet driven by a 1025 nm Yb laser source. We measure a maximum conversion efficiency of η ≅ 5 × 10 -6 for two different nozzle orifice diameters for Krypton and Argon, limited mainly by phase-matching effects.