Frequency shiftand control ofhigh-order harmonicsof H atom driven by anasymmetric laser pulse

A scheme of the large frequency shift for high-order harmonic generation (HHG) produced by atomic gasdriven by an asymmetric laser pulse is proposed in the tunneling ionization regime. By numerically solving thethree-dimensional time-dependent Schrodinger equation in the dipole approximation, we theoreticallyinvestigate the characteristics of HHG emitted from hydrogen atom driven by the laser pulse with differentrising and falling times. Our results show that the HHG spectra of atomic H in cutoff region present a strongredshift and blueshift. The shift can be adjusted by varying the rising time or falling time of the laser pulse.The time frequency analysis, reveals that the reason for the frequency shift comes from different contributionsfrom the rising time or falling time in the asymmetric laser pulse. If the contributed harmonics during thefalling time is larger than that during the falling time, the red shift of HHG occurs. otherwise the blue shiftappears. Therefore, by shaping the laser pulse waveform, the frequency of atomic HHG for a given order in thecutoff region in the tunneling ionization regime is tunable, which can cover the frequency range from the oddorder to the adjacent even order.