A closed-form analytic formula for high-order harmonic generation (HHG) rates for atoms (that generalizes an HHG formula for negative ions [M. V. Frolov et al., J. Phys. B 42, 035601 (2009)]) is used to study laser wavelength scaling of the HHG yield for harmonic energies in the cutoff region of the HHG plateau. We predict increases of the harmonic power for HHG by Ar, Kr, and Xe with increasing wavelength $\lambda$ over atom-specific intervals of $\lambda$ in the infrared region, $\lambda \sim (0.8-2.0)\mu \mathrm{m}$.

High-order harmonic generation (HHG) by an atom in an elliptically polarized laser field is studied numerically and theoretically. We derive equations describing the effect of the driving ellipticity on the HHG efficiency, harmonic ellipticity, and rotation angle for the cases of the ground state with the orbital angular momentum $l=0$ and $l\ne 0$. These equations provide good agreement with the results of our three-dimensional time-dependent Schrödinger equation calculations, as well as with the published experimental data. We show that HHG sensitivity to the driving laser ellipticity increases with the drive wavelength. This predicts high efficiency of the polarization gating schemes using midinfrared laser fields. Furthermore, our study shows significant enhancement of the harmonic ellipticity near the Cooper minimum.

We investigate numerically and analytically the polarization properties of high-order harmonics generated by an atom in intense elliptically polarized laser field. The offset angle of the harmonic polarization ellipse can be well described with the semiclassic ``simple-man'' high-harmonic generation model. The harmonic ellipticity itself, however, can be hardly understood within this model. We show that this ellipticity originates from quantum-mechanical uncertainty of the electron motion. We develop a theoretical approach describing this ellipticity and, more generally, the time evolution of the high-harmonic polarization state within the laser cycle. The analytical results are verified with the exact numerical solution; to make the comparison accurately, we develop a specific technique for separating the contributions of quantum paths in the numerical calculation.

In this article, we address two strong-field phenomena which attract much attention and can be observed under similar experimental conditions, namely when a gas is ionized by ultrashort laser pulses. The first phenomenon is the excitation of high-order harmonics of the driving field frequency in the electron current, which leads to the generation of extreme UV and soft-x-ray radiation. The second phenomenon is the excitation of a quasi-dc residual current in the laser-produced plasma, which results in the generation of radiation having a frequency below the laser one, e.g., THz waves. We present alternative one- and two-dimensional quantum-mechanical models for the description of such phenomena for the gas consisting of H atoms and the generalization of these models to the various noble gases. The shape of the electrostatic potential produced by an atomic ion is shown to influence significantly the rates of the processes in the dynamics of atomic electron and, even more, the rate of tunneling ionization, which is of utmost importance for the considered phenomena. The results of solving the time-dependent Schrödinger equation with the proposed model potentials are compared with the results of the ab initio three-dimensional calculations for the H atom. We find that in a wide range of laser pulse parameters the results obtained with proposed models have much better accuracy than the results provided by the models used earlier.

High-order harmonic generation (HHG) is limited by the probability of recombination of the returning electron with the parent ion. We show here that the internuclear distance of simplest diatomic molecules, determining differently delocalized initial electronic states, can be used to enhance the HHG conversion efficiency exceeding the atomic one. We propose an experiment based on a double-pulse pump-drive scheme where a strong transient enhancement of the HHG yield is predicted.

We present a numerical study of the resonant high-harmonic generation by tin ions in an elliptically polarized laser field along with a simple analytical model revealing the mechanism and main features of this process. We show that the yield of the resonant harmonics behaves anomalously with the fundamental field ellipticity, namely, the drop of the resonant harmonic intensity with the fundamental ellipticity is much slower than for high harmonics generated through the nonresonant mechanism. Moreover, we study the polarization properties of high harmonics generated in elliptically polarized field and show that the ellipticity of harmonics near the resonance is significantly higher than for ones far off the resonance. This introduces a prospective way to create a source of the quasimonochromatic coherent XUV with controllable ellipticity potentially up to circular.