Nonperturbative low harmonics generation in low-frequency laser field

Solving numerically three-dimensional non-stationary Schrödinger equation, we find the atomic response to the quasi-static electric field We suggest a semi-phenomenological approximation of this response which describes well the {\it ab initio} numerical calculation result both for low and high intensities (up to $1.4 \cdot 10^{14}$ W/cm$^2$). In particular, this approximation describes the nonperturbative increase or the third harmonic generation efficiency with the laser intensity, as well as the dependence of the optical ratification signal in the two-color field on the phase difference between the fields. Making the calculations for the realistic laser frequencies, we find that our approach is applicable up to fundamental frequencies of about 1 eV.