We theoretically investigate the high-order harmonic generation (HHG) of helium atom driven by bichromatic counter-rotating circularly polarized laser fields. By changing the intensity ratio of the two driving laser fields, the spectral chirality of the HHG can be controlled. As the intensity ratio increases, the spectral chirality will change from positive- to negative-value around a large intensity ratio of the two driving fields when the total laser intensity keeps unchanged. However, the sign of the spectral chirality can be changed from positive to negative around a small intensity ratio of the two driving fields when the total laser intensity changes. At this time, we can effectively control the helicity of the harmonic spectrum and the polarization of the resulting attosecond pulses by adjusting the intensity ratio of the two driving laser fields. As the intensity ratio and the total intensity of the driving laser fields increase, the relative intensity of either the left-circularly or right-circularly polarized harmonic can be enhanced. The attosecond pulses can evolve from being elliptical to near linear correspondingly.

The control of the spatial distribution in the high-order harmonic generation (HHG) of a H2+ molecule is theoretically investigated by the combination of a mid-infrared laser pulse and a terahertz (THz) field. We use a THz pulse to steer the electron motion, and the numerical results show that the cutoff of the harmonic from the recombination of the electron with the nucleus along the negative-z direction is enhanced and the case along the positive-z direction is suppressed when a THz field is added. The underlying physical mechanism is illustrated by the semi-classical three step model and the ionization probability. The time-frequency analysis further demonstrates the asymmetric spatial distribution in a HHG controlled by adding a THz field.

Molecular high-order harmonic generation of H + 2 and its isotopes is investigated by numerical simulations of the non-Born-Oppenheimer time-dependent Schrödinger equations. The general characteristic of the typical high-order harmonic generation (HHG) spectra for the H + 2 molecule indicates that only the odd harmonics can be generated. Here we show that how the initial vibrational states and nuclear dynamics break down this standard characteristic, i.e. a redshift or blueshift of the harmonics appears. We investigate the effect of the initial vibrational states on the redshift or blueshift of the HHG spectrum under trapezoidal laser pulses. The ionization probability and time-frequency analysis are used to illustrate the physical mechanism of the shift of the harmonics. We also show the HHG spectra from the different isotopes of H + 2 molecule with different initial vibrational states.

We theoretically investigate the macroscopic high-order harmonic generation in argon gaseous medium by numerically solving the three-dimensional macroscopic propagation equation, and the results show that the harmonic spectral and spatial profiles of harmonics are gradually splitting with the increasing of the driving laser intensity. This splitting is mainly due to the distortions which the driving field suffers during propagation and the consequence is also on phase matching. To illustrate the physical mechanisms of harmonic splitting, a theoretical analysis of the phase matching is presented. The harmonic spectra from different focus positions reveal that the split is dependent on the focus-gas-jet relative position. Moreover, we demonstrate that the spectral splitting of high harmonics can hardly be observed for neon gas jet due to the high ionization energy.

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We theoretically investigate the high-order harmonic generation (HHG) from laser-solid interaction in a doped semiconductor by solving the one-dimensional time-dependent Schrödinger equation within the single active electron model. The results show that the energy band of the valence and the low conduction band can be split into a small band in the doped semiconductor. The splitting band gap can be controlled by changing the depth of the potential well of the doped semiconductor, and the second HHG plateau can be enhanced. We also demonstrate the energy band structures in coordinate space with different numbers of dopant. Time-dependent population imaging is used to illustrate the change of the splitting band gap with different depths of the potential well of the doped semiconductor.

The spatial distribution in high-order harmonic generation (HHG) from the asymmetric diatomic molecule HeH 2+ is investigated by numerically solving the non-Born-Oppenheimer time-dependent Schrödinger equation (TDSE). The spatial distribution of the HHG spectra shows that there is little contribution in HHG around the geometric center of two nuclei (z = 1.17 a.u.) and the equilibrium internuclear position of the H nucleus (z = 3.11 a.u.). We demonstrate the carrier envelope phase (CEP) effect on the spatial distribution of HHG in a few-cycle laser pulse. The HHG process is investigated by the time evolution of the electronic density distribution. The time-frequency analysis of HHG from two nuclei in HeH 2+ is presented to further explain the underlying physical mechanism.

The generation of high-order harmonic and the attosecond pulse of the N 2 molecule with an orthogonally polarized two-color laser field are investigated by the strong-field Lewenstein model. We show that the control of contributions to high-order harmonic generation (HHG) from different nuclei is realized by properly selecting the relative phase. When the relative phase is chosen to be ϕ = 0.4π, the contribution to HHG from one nucleus is much more than that from another. Interference between two nuclei can be suppressed greatly; a supercontinuum spectrum of HHG appears from 40 eV to 125 eV. The underlying physical mechanism is well explained by the time-frequency analysis and the semi-classical threestep model with a finite initial transverse velocity. By superposing several orders of harmonics, an isolated attosecond pulse with a duration of 80 as can be generated.