To create self-controlled radiation photonics systems, it is necessary to have complete information about the nonlinear properties of the materials used. In this Letter, the vibrational mechanism of the giant low-inertia cubic nonlinearity of the refractive index of water in the terahertz (THz) frequency range is experimentally proven. Its dominance, which manifests itself when the temperature of the liquid changes, is demonstrated. The measured nonlinear refractive index in the THz frequency range for a water jet at temperatures from 14°C to 21°C demonstrates a correlation with the theoretical approach, varies in the range 4–10 × 10−10 cm2/W, and is characterized by an inertial time constant of less than 1 ps.

It is known that such features of pulsed THz radiation as a small number of periods and an ultra-wide spectrum modify the appearance of linear effects like absorption, dispersion, and diffraction. In this work, we study the influence of the spatial localization of a THz pulse during its generation. It is shown that this leads to a spatial chirp of the pulse. Based on the results obtained, recommendations are formulated for working with such pulses, which make it possible to guarantee that spectral information will not be lost during the propagation of such pulses.

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The dependences of the refractive index of a congruent LiNbO3 crystal cut perpendicular to the x and z axes on the radiation frequency in the range of 0.25–1.25 THz are presented. These dependences are presented for different values of the crystal thickness - 0.52 mm, 1 mm and 2.21 mm. A comparative analysis of the obtained dispersion curves with the results from other works is presented. The comparison was carried out by estimating the dispersion broadening of a THz pulse with time in the process of simulating its propagation in a medium with a given dispersion. It is shown that a 1.5-cycle THz pulse is broadened in media with dispersions found in other works, which does not correspond to experimental data. In accordance with this, it was concluded that the dispersion curves for congruent LiNbO3 from the considered works do not agree with the real values of the refractive index in the THz frequency range.