Abstract-High Kerr nonlinearity of water in THz spectral range

Anton N. Tcypkin, Maksim V. Melnik, Maria O. Zhukova, Irina O. Vorontsova, Sergey E. Putilin, Sergei A. Kozlov, and Xi-Cheng Zhang

Fig. 1 (a) The experimental setup for measuring the nonlinear refractive index (n2) of a liquid jet in the THz spectral range. Two parabolic mirrors (PM1 and PM2) with a focal length of 12.5 mm form the caustics area where the water jet (jet) is scanned along the z axis. The synchronization is performed using the mechanical modulator (M) located between the lens and the Golay cell (GC). The aperture (A) is moved from open to closed position to change the geometry of Z-scan from open to closed aperture. Insert - Geometrical position of the jet moved along the z axis relative to the THz radiation. The temporal waveform (b) and its spectrum (c) of the THz pulse generated by the TERA-AX system.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-8-10419

The values of the nonlinear refractive index coefficient for various materials in the terahertz frequency range exceed the ones in both visible and NIR ranges by several orders of magnitude. This allows to create nonlinear switches, modulators, systems requiring lower control energies in the terahertz frequency range. We report the direct measurement of the nonlinear refractive index coefficient of liquid water by using the Z-scan method with broadband pulsed THz beam. Our experimental result shows that nonlinear refractive index coefficient in water is positive and can be as large as 7×10−10 cm2/W in the THz frequency range, which exceeds the values for the visible and NIR ranges by 6 orders of magnitude. To estimate n2, we use the theoretical model that takes into account ionic vibrational contribution to the third-order susceptibility. We show that the origins of the nonlinearity observed are the anharmonicity of molecular vibrations.

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