Extension of the high-order-harmonic-generation cutoff frequency by two-color excitation

We study high-order harmonic generation resulting from a two-color scheme involving a circularly polarized high-frequency field and a weak linearly polarized low-frequency field. Although the low-frequency field is not intense enough to ionize the atom, it can accelerate the electron more efficiently. As a result, the application of this field can generate harmonics with much higher energy compared to the monochromatic case. For a hydrogenlike atom irradiated by a Nd:YAG (yttrium aluminum garnet) laser of intensity $1.9\times {10}^{14}\hspace{0.5em}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ and a ${\mathrm{CO}}_2$ laser of intensity $7.6\times {10}^{12}\hspace{0.5em}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2,$ the cutoff occurs at the 241st harmonic-order corresponding to a photon energy of 281 eV, which is three times higher than that of the monochromatic case.