Phase-coherence dynamics of frequency-comb emission via high-order harmonic generation in few-cycle pulse trains


 Frequency-comb emission via high-order harmonic generation (HHG) provides an alternative method for the coherent vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) radiation at ultrahigh repetition rates. In particular, the temporal and spectral features of the HHG were shown to carry profound insight into frequency-comb emission dynamics. Here we present an ab initio investigation of the temporal and spectral coherence of the frequency comb emitted in HHG of He atom driven by few-cycle pulse trains. We find that the emission of frequency combs features a destructive and constructive coherence caused by the phase interference of HHG, leading to suppression and enhancement of frequency-comb emission. The results reveal intriguing and substantially different nonlinear optical response behaviors for frequency-comb emission via HHG. The dynamical origin of frequency-comb emission is clarified by analyzing the phase coherence in HHG processes in detail. Our results provide fresh insight into the experimental realization of selective enhancement of frequency comb in the VUV-XUV regimes.