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This article studies the generation of terahertz (THz) radiation in carbon dioxide ( ${\bf CO_2}$ ) at different levels of gas pressure. It was demonstrated that under low gas pressure, optical-terahertz conversion can be adequately described by a simplified photocurrent model. However, with the increase of gas pressure, the contribution of intra-atomic nonlinearities as a source of terahertz response irradiated by a two-color laser field is growing. Moreover, at high levels of gas pressure, the interference and dispersive effects should be taken into consideration as well. The proposed model allows for the optimization of experimental conditions aimed at flexible control of intensity and spectral properties of THz radiation.

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.

The process of high harmonic generation (HHG) has a wide area of different applications, in particular, in nanoscale imaging, study the nanoscale energy transport in complex molecules and solids, probe the charge and spin dynamics with the femtosecond time resolution, etc. Here we discuss a possible application of HHG effect related with the study of nuclear transition dynamics. We analyse the efficiency of excitation of the isomeric nuclear state 229mTh by the fifth harmonic of Ti: Sa laser generated in thorium by two-color femtosecond pulse consisting of the fundamental and thensecond harmonic of Ti: Sa laser (ω+2ω). It is shown that the rate of isomeric state excitation can be enhanced significantly with respect to other nucleus excitation processes in laser plasma or by an external coherent source at the resonance wavelength. This enhancement is due to the discussing process of “nonlinear laser nuclear excitation”.

One of the promising methods of the terahertz (THz) radiation generation is the use of the two-color laser field (ω-2ω) focused in the gas medium [1]. Typically, the polarization of the low frequency THz field generated in gas is linear [2], but, in some conditions, it can be elliptical due to modulation of the laser pulse phase and its polarization in the gas plasma [3]. For some practical applications, it is useful to have the elliptically polarized THz radiation. Usually, methods based on the mixing of the elliptically polarized ω with a linear (or elliptically polarized) 2ω, producing low-frequency elliptically polarized THz emission (

We compare the results of experimental and theoretical studies of the quasi-phase matching (QPM) of high-order harmonic generation (HHG) during propagation of two-color femtosecond pulses through the indium multi-jet plasmas. The role of number of coherent zones and size of plasma jets on the HHG efficiency at QPM conditions is analysed. We demonstrate the enhancement of the HHG efficiency due to QPM during laser-plasma interaction.

We present the results of the theoretical study of the high-order harmonic generation (HHG) in atomic gases. 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 interpret 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 “an atom in the external field” instead of the traditional basis of the “free atom” eigenfunctions.

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.

The interaction between high intensity ultrashort laser pulses and the atomic gas in the ionization-free regime leads to the emission of coherent, short pulse radiation at terahertz frequency. In this work we discuss a model for this effect and its experimental realization.

The specific features of the terahertz (THz) emission from the extended media illuminated by the two-color laser fields is studied. It is shown that due to dispersion effects the amplitude of the response THz field varies periodically with the travelled distance. These spatial variations manifest themselves in the angular spectra of THz emission and, as a result, the integral efficiency of the THz emission depends significantly on the medium geometry. The origin of such behavior is in the variation of the relative phase of ω and 2ω components of the laser field. The THz intensity dependence on the relative phase between the fundamental and the second harmonics for two gases (Ne and Ar), two seed lasers (Ti:Sa and Cr:F) and two angles between the polarization of two-color laser field components (0 and π/2) is investigated quantum-mechanically, and it is confirmed that the optimal phase for the THz generation is π/2 for all described cases.