(color online) (a) Plots of quasienergies and (b) time-averaged transition probabilities as a function of the CEP shift for the two-level system driven by fundamental and third-harmonic frequency-comb fields with their relative envelope delay . It has five resonance positions separated by due to the five-photon dominant resonance. (c) Enlarged plots of the time-averaged transition probabilities as a function of the CEP shift, with envelope delay (black solid line), (red dash line), and (blue dotted line). The fundamental field has peak intensity , carrier frequency 374.7 THz, and repetition frequency 10 THz, and is generated from a train of Gaussian pulses with 20 fs FWHM. The third-harmonic field has peak intensity , carrier frequency 1124.1 THz, and repetition frequency 10 THz, and is generated from another train of Gaussian pulses with 20 fs FWHM. is set to the fulfill the matching condition. The relative carrier phase is set as φ = 0.