title: Mechanisms of below-threshold harmonic generation in atoms.
abstract: Most previous studies have focused on high-order harmonic generation beyond the ionization threshold; mechanisms of below-threshold harmonics are less understood. We schematically study the harmonic emission process in this region by numerically solving the time-dependent Schrödinger equation of an atom in laser fields. We show that, besides the quantum path interference mechanism recently identified, the effects induced by the Coulomb potential also have a critical impact on these harmonics. These mechanisms can be distinguished in the structure of harmonic spectra by changing the laser wavelength and peak intensity. We find that the long quantum orbits can influence lower-order harmonics at a higher laser intensity. In addition, we show that the intensity-dependent steps of harmonic yield can disappear for certain harmonic orders, due to the trapping in the Rydberg states before recombination, which can explain recent experimental observations.
authors: State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, China.
authors: Wei-Hao
authors: WH
authors: Xiong
authors: State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, China.
authors: Ji-Wei
authors: JW
authors: Geng
authors: State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, China.
authors: Jing-Yi
authors: JY
authors: Tang
authors: State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, China and Collaborative Innovation Center of Quantum Matter, Beijing 100871, China.
authors: Liang-You
authors: LY
authors: Peng
authors: State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, China and Collaborative Innovation Center of Quantum Matter, Beijing 100871, China.
authors: Qihuang
authors: Q
authors: Gong