ИСТИНА

Here we present the result of theoretical investigations of a single atom interaction with multicolor laser field formed by arbitrary number of the components with arbitrary orientation of polarization vectors. The developed theory is non-perturbative and valid in the whole sub-relativistic region of laser field strength. The most specific feature is that the eigenfunctions of the cylindrically symmetric boundary value problem for atom in external field is used for wave function expansion (contrary to the traditional approaches where the free atom eigenfunctions are usually used as basis functions). It results in new mechanism of nonlinear atomic response which is mainly due to the light-induced anisotropy of electron wave function but not by electron transitions between the free atom eigenstates. As an example the terahertz (THz) emission in ionization-free regime is interpreted without exploiting the plasma nonlinearities (usually, four-wave mixing rectification process). Numerical results have a good agreement with the latest experimental results [K. Y. Kim et al, Nature Photonics, 2, 605 (2008)]. Numerical experiment simulates the argon atom interaction with fundamental and second harmonic of Ti:Sapphire laser. The parameters of the laser pulse can be easily achieved in modern lasers systems. It is shown that spectral width and efficiency of THz emission depend nonlinearly on the angle between the polarization vectors, delay time between the pulses, and chirp of the laser field components. Furthermore, relative amplitudes and widths of the laser pulses influences on the THz emission too. Numerical simulations shows that chirp of the fundamental frequency leads to the harmonic shifting, which is agree with the latest experimental data [R.A.Ganeev et al, Phys. Rev. A, 80, 033845 (2009)]. Optimization of the laser pulse parameters provides the THz emission in wide spectral region.