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In high-order harmonic generation in low-dense media, the application of two-colored pump has been shown to enhance odd-order harmonics, as well as the generation of strong even harmonics. Here, we apply this approach to compare high-order harmonic generation from laser-produced plasmas and gases using different mechanisms of harmonic emission enhancement. We generate two-color pump (TCP, 806 nm + 403 nm and 1030 nm + 515 nm) using two nonlinear crystals of different thickness (0.2 and 0.4 mm in the case of experiments with plasmas using 806 nm pump; and 0.1 mm and 0.4 mm in the case of experiments with gases and plasmas using 1030 nm pump) to demonstrate the enhancement of odd and even harmonics in laser-produced plasmas and by applying weak second field (403 or 515 nm) at different overlaps of pump pulses. The high-order harmonic generation in two media using TCP pulses is optimized. We demonstrate the wavelength dependence and the prevailing role of the partial overlap between the two orthogonally polarized waves in laser-produced plasmas and gases over large ratios of the second-order and fundamental photons. Our results are supported by the non-perturbative single atom theory and the interference model simulations.

Within the framework of the non-perturbative light–atom interaction theory, we consider the population dynamics of the ground-state sublevels corresponding to the different values of the orbital quantum number (l) projections and its influence on the high optical harmonics. We study the problem of high optical harmonics generation in two geometries: (1) two linearly polarized fields at frequencies ω and 2ω for different angles of azimuthal projection difference and arbitrary orientation with respect to the angular momentum of the atom direction; (2) two elliptically polarized fields at frequencies ω and 2ω rotating in opposite directions with variable mutual ellipticity. We show that the unequal population dynamics of the levels with different projections of the atom angular momentum (magnetic quantum number) has a significant impact on the value of the ellipticity of the generated radiation.

We develop the theory of interaction of the electromagnetic field and a single atom being in an arbitrary state and having an arbitrary direction of the angular momentum of the atomic electron with respect to the direction of the field polarization vector. It is shown that the atom response current has a tensor structure and depends on both the direction of the angular momentum of the atom, and the polarization vector of the external field. The tensor character of the response is determined by the externally induced anisotropic distribution of the probability density of spatial localization of the atomic electron. It is shown that the induced-anisotropy effects clarify the harmonic generation mechanism at play during the non-resonance interaction of laser radiation with atomic media. The developed theory is applied to the analysis of the problem about the generation of terahertz waves in a two-color laser field. It is shown that the change in the mutual orientation of wave polarization vectors leads to a significant increase in the efficiency of conversion of high-frequency fields to low-frequency ones. It is shown for the first time that the generation of terahertz waves is possible in the preionization regime, when the generation mechanism is related to atomic nonlinearity.

Ionization and high-harmonics generation in a single hydrogen-like atom driven by a laser pulse of near-atomic field strength is the subject of this paper. We use exact solutions of the eigenvalue problem on particle motion in the cylindrically symmetric field (CSF) as a basis for the wave-function expansion. The superposition of the spherically symmetric intra-atomic field and linearly polarized laser field has the cylindrical symmetry. Hence, the use of the free-atom eigenfunctions as a basis for the wave-function expansion requires an infinitely increasing number of spherical harmonics (i.e., free-atom eigenfunctions) when the laser field strength approaches the intra-atomic field value. The eigenfunctions of the CSF problem depend on the laser field strength; therefore, the appropriate matrix elements show how the spectral width of atomic response and angular selection rules vary with increase in the laser field strength. The introduction gives a phenomenological semiclassical illustration of the problem.

The results of theoretical and experimental study of high-order optical harmonic generation in the ensemble of silver atoms irradiated by intense femtosecond pulses of Ti:Sapphire laser are presented. It is shown that the photoemission spectra exhibit unusual behavior when the laser field strength approaches near-atomic values. In subatomic field strength the cut-off frequency increases linearly with laser pulse intensity. However, when the field strength approaches near-atomic region firstly cut-off frequency slows down and then saturates. To give new interpretation of such kind of photoemission spectrum behavior we have proposed the light-atom interaction theory based on the use of eigenfunctions of boundary value problem for “the atom in an external field” instead of the traditional basis of the “free atom” eigenfunctions. The results of computer simulations clearly demonstrate saturation of the cut-off frequency at near-atomic strength of a laser field. The problem of angular divergence of a harmonic emission is analyzed.

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.