High-intensity optical radiation propagation in a transparent dielectric medium causes the phenomena of self-action of the pulse and the generation of radiation at triple frequencies due to the cubic nonlinearity of the medium. However, usually the quadratic nonlinear effects outshine the cubic ones in the anisotropic nonlinear crystals. In this work, we demonstrate that for certain experimental parameters the nonlinear effect of the third order can be stronger than one of the second order in MgO:LiNbO3 crystal for terahertz frequency range. We experimentally and theoretically show that it 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. In the experiment it was shown 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 demonstrates that this effect is based on the asymmetry of the initial spectrum of such extremely short pulses in terms of the number of oscillations.

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.

Experimental results and theoretical analysis of single-cycle terahertz pulse interaction with cubic nonlinear medium shows that the generation of radiation at triple frequencies disappears. It is replaced by the generation of radiation at quadruple frequencies.

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