title: Dynamical origin of near- and below-threshold harmonic generation of Cs in an intense mid-infrared laser field.
abstract: Near- and below-threshold harmonic generation provides a potential approach to generate vacuum-ultraviolet frequency comb. However, the dynamical origin of in these lower harmonics is less understood and largely unexplored. Here we perform an ab initio quantum study of the near- and below-threshold harmonic generation of caesium (Cs) atoms in an intense 3,600-nm mid-infrared laser field. Combining with a synchrosqueezing transform of the quantum time-frequency spectrum and an extended semiclassical analysis, the roles of multiphoton and multiple rescattering trajectories on the near- and below-threshold harmonic generation processes are clarified. We find that the multiphoton-dominated trajectories only involve the electrons scattered off the higher part of the combined atom-field potential followed by the absorption of many photons in near- and below-threshold regime. Furthermore, only the near-resonant below-threshold harmonic is exclusive to exhibit phase locked features. Our results shed light on the dynamic origin of the near- and below-threshold harmonic generation.
authors.affiliation: 1] Center for Quantum Science and Engineering, and Center for Advanced Study in Theoretical Sciences, Department of Physics, National Taiwan University, Taipei 10617, Taiwan [2] College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China [3] Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
authors.forename: Peng-Cheng
authors.initials: PC
authors.lastname: Li
authors.affiliation: Center for Quantum Science and Engineering, and Center for Advanced Study in Theoretical Sciences, Department of Physics, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: Yae-Lin
authors.initials: YL
authors.lastname: Sheu
authors.affiliation: School of Mathematical Science, University of Nottingham, Nottingham NG7 2RD, England.
authors.forename: Cecil
authors.initials: C
authors.lastname: Laughlin
authors.affiliation: 1] Center for Quantum Science and Engineering, and Center for Advanced Study in Theoretical Sciences, Department of Physics, National Taiwan University, Taipei 10617, Taiwan [2] Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
authors.forename: Shih-I
authors.initials: SI
authors.lastname: Chu