Multi-level effects in the high-order harmonic generation driven by intense frequency-comb laser fields*
Zhao Di†, Jiang Chen-Wei, Li Fu-Li
       
(a) The two-level system. The energy separation between ground and excited levels is ħω ge = ε e – ε g = 0.25 a.u. (b) The gc-type three-level system in which the third level | c 〉 is dipole-allowed with the ground level | g 〉. (c) The ec-type three-level system in which the third level | c 〉 is dipole-allowed with the excited level | e 〉.