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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.

We present a new interpretation of high-order optical harmonic generation (HHG) in atomic gases interacting with laser fields. The mechanism of harmonic generation is due to the temporal evolution of the atomic electron density spatial distribution in the external laser field, and the integral energy of a high harmonic emission and an electron excitation is equal to the work executed by the laser field for an electron motion in the intra-atomic potential field. The proposed interpretation is based on the use of the basis of eigenfunctions of the boundary value problem for “an atom in the external field.” As an illustration, we present the original results of computer simulations of an argon atom interaction with the two-color laser field formed by the fundamental and second harmonics of the Ti:sapphire laser.