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 in the optical range quadratic nonlinear effects in solids appear at relatively low radiation intensities, while the radiation intensity required for a significant manifestation of cubic effects is much higher and can reach the damage threshold values. In this regard, quadratic effects dominate over cubic ones. In this work, it is analytically demonstrated that in the terahertz range the intensities required for the manifestation of cubic effects are much lower than in the optical range. In addition, their contribution to nonlinear effects can exceed the quadratic one by several orders of magnitude.
High-intensity optical radiation propagation in a transparent dielectric medium causes the phenomena of pulse self-action and radiation generation at triple frequencies due to the cubic nonlinearity of the medium. However, quadratic nonlinear effects usually outshine the cubic ones in anisotropic nonlinear crystals. In this work, we demonstrate that for certain experimental parameters the nonlinear effect of the third order can be stronger than the second order one in the MgO:$$\hbox {LiNbO}_3$$
LiNbO
3
crystal for terahertz frequency range. We experimentally and theoretically show that this effect can lead to the significant modification of the classical phenomenon of radiation generation at triple frequencies in the case when the pulse represents only one complete oscillation of the optical field. The experiment demonstrated that the phenomenon of generation of radiation at triple frequencies with respect to the frequency of the maximum spectral density in a nonlinear medium of the pulse disappears, and it is replaced by the generation of radiation at quadruple frequencies. The analysis confirms that this effect is based on the asymmetry and large width of the initial spectrum of such extremely short pulses in terms of the number of oscillations.
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