We theoretically investigate the high-order harmonic generation (HHG) with an intense two-color femtosecond laser field, in which both fundamental field and its second harmonic are linearly polarized. In contrast to usual HHG with a linearly polarized fundamental field, the ionized electron moves in a plane and there enter new degrees of freedom: angle between polarization axes, relative phase, and relative intensities. The dependence on these parameters are investigated comprehensively with quantum mechanical simulation of HHG. Furthermore, by using the semiclassical model of HHG, this dependence is explained in terms of electron paths contributing to HHG. The symmetry of the interacting system is also analyzed that the possible orders depending on polarization and the π-periodic dependence on the relative phase are derived.
Considering the present status of HHG as an ultrashort soft x-ray source, the utilization of the new degrees of freedom to generate attosecond pulses is investigated. With proper choice of the relative phase, short paths are found to be enhanced compared to case with the one-color field of the same intensity, and this results in strong and regular attosecond pulse train. By adjusting the polarization directions, the amount of ionization can be reduced, which is promising for better phase matching in macroscopic generation. The strong attosecond pulses obtained in this way can be useful for applications such as time-resolved studies of ultrafast phenomena in atoms and molecules.