Light-induced anisotropy of atomic response: prospects for emission spectrum control

We present the quantum mechanical theory of the nonlinear atomic response in a strong laser field. The proposed approach provides the self consistent description of the atomic response both in the long (THz) and the short (XUV) wavelength regions. At the subrelativistic intensity of a laser pulse, our approach is valid for the arbitrary ratio between the laser and the intra-atomic field strengths. Instead of the traditional basis of the free atom eiegenfunctions, the basis of eiegenfunctions of the boundary value problem for the atom in an external field is used to calculate the matrix elements of the quantum mechanical operators. In this case the matrix elements of the generalized momentum operator and the Hamiltonian of the time-dependent Schrodinger equation are directly related with the experimentally measured dipole matrix elements and the energy spectrum of a free atom. The response of the atomic argon in the two-color laser field of arbitrary polarized components has been calculated numerically. The results of computer simulations show that the efficiency of the atomic response depends non-monotonically on the angle between the polarization vectors of the laser pulses. We also show that the most effective mechanism of THz emission in the two-color laser field appears mainly due to the atomic non-linearity, and not to the plasma non-linearities playing the dominant role in the case of the monochromatic laser field. The predictions of our theory coincide with the numerical and experimental results. This fact gives us a reliable base to provide the straightforward explanation of the observed dependencies, and to propose the methods of enhancement of efficiency of the nonlinear transformations both in the long and short wavelength spectral regions.