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) and (b) Enhancement of HHG spectra by varying CEP shift in gc- and ec-type three-level systems, respectively. For comparison, HHG spectra of the two-level system with Δ ϕ /2 π = 0.1 (off-resonance) and 0.168295 (near-resonance) are plotted with thin red dashed and solid lines, respectively. All comb peaks are connected by a line for clarity. (c) and (d) P g → e as a function of Δ ϕ in gc- and ec-type three-level systems, respectively. For comparison, P g → e in the two-level system is plotted with a red dashed line. The energy separation ħω gc = 0.23 a.u. The parameters used are peak intensity 1 × 1014 W/cm2, carrier frequency 563.5 THz, and repetition frequency 10 THz of 20 fs FWHM Gaussian pulses.