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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.

The problem of harmonic generation efficiency (as an example of non-linear optical phenomena) increasing by atomic media near the surface of a one-dimensional metal diffraction grating under conditions of plasmon resonance excitation is considered. The local field enhancement factor dependences on diffraction grating profile are studied. Spatial distribution of the femtosecond pulse electric field strength over the grating surface under conditions of plasmon resonance excitation is simulated. Generated harmonic efficiency enhancement and photoemission spectra enrichment (due to laser field strength amplification near the grating) are discussed.

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.

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.