Radiation shift from triple to quadruple frequency caused by the interaction of terahertz pulses with a nonlinear Kerr medium

Abstract

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.