@article{2518,
  author = {D. Yost and Thomas Schibli and Jun Ye and Jennifer Tate and James Hostetter and Mette Gaarde and K. Schafer},
  title = {Vacuum-ultraviolet frequency combs from below-threshold harmonics},
  abstract = {Recent demonstrations of high-harmonic generation (HHG) at very high repetition frequencies (approx100 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.},
  year = {2009},
  journal = {Nature Physics},
  volume = {5},
  pages = {815-820},
  month = {2010-11},
  issn = {1745-2473},
  doi = {10.1038/nphys1398},
}