Fig. 1. Laser electric field and high-order harmonic emission spectra. (a) Electric field as a function of time; (b) high-order harmonic spectra from helium atom irradiated by electric fields

Fig. 2. Semi-classical motion of electrons in an orthogonally polarized two-color field. (a) Evolution of the harmonic order with ionization and recombination time ; (b) variations of the amplitude of x-polarized and y-polarized fields and the ionization rate of the helium atom with time; (c) for the 125th harmonic, the short trajectory of two-dimensional classical motion of the electron from ionization to return to parent ion; (d) for the 125th harmonic, long trajectory of two-dimensional classical motion of the electron from ionization to return to parent ion

Fig. 3. Time-frequency spectra of high-order harmonic emission

Fig. 4. Time-domain spectra generated by attosecond pulse. (a) 800 nm laser field of Ti sapphire; (b) orthogonally polarized two-color field

Fig. 5. Attosecond pulse time-domain spectra generated by superimposing harmonics from 120th to 180th order in the case of orthogonally polarized two-color field. (a) Relative phase Δφ takes different values; (b) intensity of gating field takes different values

Fig. 6. High order harmonic spectra from helium atom irradiated by two-color field