Recent demonstrations of high-harmonic generation (HHG) at very high repetition frequencies (∼100 MHz) may allow for the revolutionary transfer of frequency combs to the vacuum-ultraviolet range. This advance necessitates unifying optical frequency-comb technology with strong-field atomic physics. Whereas strong-field studies of HHG have often focused on above-threshold harmonic generation (photon energy above the ionization potential), for vacuum-ultraviolet frequency combs an understanding of below-threshold harmonic orders and their generation process is crucial. Here, we present a new and quantitative study of the harmonics 7-13 generated below and near the ionization threshold in xenon gas with an intense 1,070 nm driving field. We show multiple generation pathways for these harmonics that are manifested as on-axis interference in the harmonic yield. This discovery provides a new understanding of the strong-field, below-threshold dynamics under the influence of an atomic potential and allows us to quantitatively assess the achievable coherence of a vacuum-ultraviolet frequency comb generated through below-threshold harmonics. We find that under reasonable experimental conditions, temporal coherence is maintained. As evidence, we present the first explicit vacuum-ultraviolet frequency-comb structure beyond the third harmonic. © 2009 Macmillan Publishers Limited. All rights reserved.
Publication Source (Journal or Book title)
Yost, D., Schibli, T., Ye, J., Tate, J., Hostetter, J., Gaarde, M., & Schafer, K. (2009). Vacuum-ultraviolet frequency combs from below-threshold harmonics. Nature Physics, 5 (11), 815-820. https://doi.org/10.1038/nphys1398