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 (VUV). This advance necessitates unifying optical frequency comb technology with strong-field atomic physics. While strong-field studies of HHG have often focused on above-threshold harmonic generation (photon energy above the ionization potential), for VUV 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. 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 VUV 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 VUV frequency comb structure beyond the 3rd harmonic.
Harmonics of femtosecond and continuous-wave radiation for precision spectroscopy: generation efficiency and spectral line shape
P.N. Lebedev Physical Institute of the Russian Academy of Sciences（俄罗斯科学院列别杰夫物理研究所）；Moscow Institute of Physics and Technology（莫斯科物理技术学院）；Russian Quantum Center（俄罗斯量子中心）
A solid-state nuclear clock based on VUV absorption spectroscopy of $^{229}$Th
Hefei National Laboratory, University of Science and Technology of China（中国科学技术大学合肥国家实验室）；School of Physical Sciences, University of Science and Technology of China（中国科学技术大学物理学院）
Generation of bright quantum high-order harmonic driven by combined coherent and bright squeezed vacuum light
Institute of Atomic and Molecular Physics, Jilin University（吉林大学原子与分子物理研究所）；Jilin Provincial Key Laboratory of Transient Quantum Process Control and Application, Jilin University（吉林大学瞬态量子过程控制与应用吉林省重点实验室）；College of Physics, Jilin University（吉林大学物理学院）
Tsinghua University（清华大学）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences（中国科学院上海光学精密机械研究所）；University of Chinese Academy of Sciences（中国科学院大学）
Molecular cooling via Raman transitions and a harmonic spectral modulation of an optical frequency comb: The role of the parity of the chirp in quantum control
