Tcypkin Anton N, Melnik Maksim V, Zhukova Maria O, Vorontsova Irina O, Putilin Sergey E, Kozlov Sergei A, Zhang Xi-Cheng
Opt Express. 2019 Apr 15;27(8):10419-10425. doi: 10.1364/OE.27.010419.
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 cm/W in the THz frequency range, which exceeds the values for the visible and NIR ranges by 6 orders of magnitude. To estimate n, 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.
太赫兹频率范围内各种材料的非线性折射率系数值比可见光和近红外范围内的系数值高出几个数量级。这使得在太赫兹频率范围内能够制造出非线性开关、调制器以及需要更低控制能量的系统。我们报告了通过使用宽带脉冲太赫兹光束的Z扫描方法对液态水的非线性折射率系数进行的直接测量。我们的实验结果表明，水中的非线性折射率系数为正，在太赫兹频率范围内可高达7×10 cm/W，比可见光和近红外范围内的值高出6个数量级。为了估算n，我们使用了考虑离子振动对三阶极化率贡献的理论模型。我们表明，观察到的非线性起源是分子振动的非谐性。
