Non-dipole effects on high-order harmonic generation towards the long wavelength region

The non-dipole (ND) effects on high-order harmonic generation (HHG) with the laser wavelength increasing towards the long wavelength region are investigated. Two major phenomena due to the ND effects, the decrease of the HHG intensity and the shift of the harmonic spectrum, are discussed. It is shown that, for the commonly used laser intensity I∼1014 W/cm2 and target with ionization potential Ip∼0.5a.u., the ND effects become nonnegligible when the laser wavelength is increased to the mid-infrared region of several thousand nanometers. It is also found that the variation of the ND effects presents different rules compared with those towards the high intensity region. Two fitting formulas are proposed to describe the variation rules. The physical meanings of the fitting results are discussed with classical calculation.