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ALT'23
The 30th International Conference on Advanced Laser Technologies
N-I-2
Nonlinear THz optics
Melnik M.1, Nabilkova A. Artser I.1, Guselnikov M. Zhukova M. Ismagilov A. Tcypkin
A1., and Kozlov S. 1
1- Research and Educational Center for Photonics and Optoinformatics, ITMO University, Russia.
mmelnik@itmo. ru
Nonlinear optics of the THz frequency range is a rather young field of knowledge. This is due to the fact that high-power pulsed sources of THz radiation, which make it possible to observe nonlinear phenomena in various media without damaging them, appeared not so long ago [1]. The wide application of intense radiation in the THz range in various fields of knowledge elevates to the rank of topical the task of developing devices and technologies for controlling light by light, which can be based on nonlinear optics of this frequency range. Accordingly, the problem of systematic analysis of the mechanism of giant nonlinearity and study of the features of wave phenomena in media with such a nonlinearity has become urgent.
This work is devoted to a review of the achievements in the field of nonlinear optics of the THz frequency range over the past 5 years. For instance, it has been shown that for this spectral range the main low-inertia mechanism of the materials nonlinearity is the anharmonicity of molecular vibrations of optical media. Particularly large values of the nonlinear refractive index of crystals were predicted in the region of two-photon resonances with vibrational modes of crystals and in liquids [2]. The above theory has been modified to describe the nonlinear mechanisms of liquids with anisotropic molecules [3]. It has been shown theoretically and experimentally for the first time that the nonlinear refractive index of some liquids can be six orders of magnitude (million times) higher than that of the same liquids in the visible and near-IR spectral ranges [4].
In addition, the features of nonlinear wave phenomena in the field of THz radiation were studied. Such "surprises" of the nonlinear optics of pulsed THz radiation as the possibility of "disappearance" of self-focusing of radiation when its power exceeds the critical power of self-focusing due to the predominance of the phenomenon of their dispersion over diffraction in the field of few-cycle waves [5], or "disappearance" for single-cycle THz waves in media with cubic nonlinearity of another classical nonlinear phenomenon, third-harmonic generation, and the generation in the field of such an extremely short radiation pulse of quadruple frequencies with respect to the maximum of the main spectrum [6] have been shown theoretically and experimentally. It has been established that, due to the peculiarities of the spatiotemporal dynamics of single-cycle THz pulses, the use of known methods for estimating the nonlinear refractive index in this spectral range, for example, z-scan, requires their qualitative processing. Otherwise, the results obtained by the technique for quasi-monochromatic pulses may differ from the actual results for waves from a small number of oscillations by orders of magnitude [7].
The results obtained open up the possibility for detailed study of the nonlinear properties of materials caused by the interaction with intense THz pulses, which can help to reveal new features of nonlinear media, and to obtain the characteristics of the most promising materials for creating passive and active devices of THz photonics based on nonlinear phenomena.
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[3] A.N. Tcypkin, M.V. Melnik, M.O. Zhukova, I.O. Vorontsova, S.E. Putilin, S.A. Kozlov, X.C. Zhang, High Kerr nonlinearity of water in THz spectral range, Optics express, vol. 27(8), pp. 10419-10425, (2019)
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[7] M.V. Melnik, I.O. Vorontsova, S.E. Putilin, A.N. Tcypkin, S.A. Kozlov, Methodical inaccuracy of the Z-scan method for few-cycle terahertz pulses, Scientific Reports, vol. 9, pp. 9146, (2019)
