Effect of the relative phase between two-colour pump pulses on structure of harmonic spectra

Abstract

We numerically calculate the high-order harmonic generation (HHG) power spectra from a one-dimensional model atom irradiated by linearly polarised 12 fs two-colour laser pulses composed of a fundamental pulse from Ti:sapphire laser and its second harmonic. It is found that a distinct double plateau structure appears when the relative phase of the two pulses is set as π/8, 2π/8 or 3π/8, and the double plateau structure disappears when the relative phase is set as 4π/8, 5π/8, 6π/8 or 7π/8. The relative-phase-dependent plateau structure is explained by the temporal profile of the synthesised electric fields as well as the semi-classical “three-step” model. Moreover, our numerical result shows that cut-off frequencies of the two-colour pulse HHG spectra can be exactly predicted by use of the semi-classical “three-step” model.