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Using a three-dimensional classical ensemble model, we investigate the dependence of relative frequency and relative initial phase for nonsequential double ionization (NSDI) of atoms driven by orthogonal two-color (OTC) fields. Our findings reveal that the NSDI probability is clearly dependent on the relative initial phase of OTC fields at different relative frequencies. The inversion analysis results indicate that adjusting the relative frequency of OTC fields helps control returning probability and flight time of the first electron. Furthermore, manipulating the relative frequency at the same relative initial phases can vary the revisit time of the recolliding electron, leading that the emission direction of Ar2+ ions is explicitly dependent on the relative frequency.

Circularly and elliptically polarized high-order harmonics have unique advantages when used in studying the chiral and magnetic features of matters. Here, we studied the polarization properties of high-order harmonic generated from alignment nitrogen molecules driven by cross-linearly-polarized two-color laser fields. Through adjusting various laser parameters and targets, such as the relative phase, the crossing angle, the intensity ratio of driving fields, and the molecular alignment angle, we obtained highly elliptically polarized high-order harmonics with the same helicity in a wide spectral range. It provides a possible and effective way to generate elliptically polarized attosecond pulses. Finally, we show the probability of controlling the spectral range of elliptically polarized harmonics.

We theoretically investigate the frustrated double ionization (FDI) of molecules with different alignment-dependence using a three-dimensional classical ensemble method. The numerical results show that the FDI probability decreases with increasing wavelength, which is similar to the wavelength dependence of the FDI probability of atoms. Tracing the classical trajectories reveals that the contributions to molecular FDI from single-recollision and multiple-recollision mechanisms are equal in the short wavelength regime. In the long wavelength regime, the single-recollision FDI channel dominates in FDI. The nature in which molecular FDI occurs is identified and explained.

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