Ionization and nonlinear upconversion of xenon in intense multichromatic fields

The nuclear isomeric transition in ^229mTh represents the frontier of ultra-precise timekeeping, with the potential to surpass the precision of current optical lattice clocks by more than an order of magnitude. A primary barrier to practical implementation has been efficient excitation of the narrow linewidth nuclear transition, however, recent breakthroughs at JILA¹ demonstrated excitation using the 7th harmonic of a vacuum ultraviolet (VUV) frequency comb generated in xenon gas. This experiment utilized a single-frequency, single-shot high harmonic generation (HHG) process to generate the VUV comb, but a higher power source of VUV is needed. In response, the Kapteyn-Murnane group at JILA has introduced a four wave mixing process², which allows generation of the 7th harmonic via the nonlinear process 4ω + 4ω - ω -> 7ω. Our work utilizes a numerical time-dependent Schrodinger equation (TDSE) simulation to study the effects of relative delay, pulse duration, and intensity of multichromatic laser fields on nonlinear upconversion and few-photon ionization processes in xenon. The results of our study will contribute to the understanding of VUV generation efficiency.

1. Zhang C, Ooi T, Higgins JS, Doyle JF, von der Wense L, Beeks K, Leitner A, Kazakov GA, Li P, Thirolf PG, Schumm T, Ye J. Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock. Nature. 2024 Sep;633(8028):63-70. doi: 10.1038/s41586-024-07839-6.

2. Thurston J, Dickson I, Burton E, Finger K, Shearer B, Morrill D, Lebrat L, Becker A, Murnane M, Kapteyn H. Highly Efficient 80 mW VUV Laser Source at MHz Repetition Rates using Four-Wave Mixing in Structured Fibers. In preparation.