The nonlinear response of liquids in the terahertz regime has recently attracted significant interest, even though very few measurements have been reported. Here, we report on our measurements based on a $z$-scan technique of the nonlinear refractive-index coefficient $n_2$ at terahertz frequencies for several liquids with noncentrosymmetric molecules, specifically, water, ethanol, and $\alpha$-pinene. We describe how the value of $n_2$ depends on the physical parameters of these molecules. The measured values of $n_2$ of the liquids in the terahertz region are as much as 6 orders of magnitude larger than their corresponding values in the visible or near-IR. Through a simple theoretical model, we confirm that the predominant source of this large third-order nonlinearity is the second-order perturbative component of the vibrational response of these molecules, which have resonances in the mid-IR.

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太赫兹频率下液体的巨型三阶非线性响应

太赫兹状态下液体的非线性响应引起了人们的极大兴趣，尽管已报道了很少的测量结果。在这里，我们基于$ž$折射率系数的扫描技术 ${ñ}_{\mathrm{2个}}$ 在太赫兹频率下，几种具有非中心对称分子的液体，特别是水，乙醇和 $\alpha$-pine烯。我们描述了${ñ}_{\mathrm{2个}}$取决于这些分子的物理参数。的测量值${ñ}_{\mathrm{2个}}$太赫兹区域中的液体比可见或近红外中的相应值大6个数量级。通过一个简单的理论模型，我们确认了这种大的三阶非线性的主要来源是这些分子的振动响应的第二阶微扰分量，这些分量在中红外具有共振。