Giant Third-Order Nonlinear Response Of Liquids At Terahertz Frequencies

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 zscan technique of the nonlinear refractive-index coefficient n2 at terahertz frequencies for several liquids with noncentrosymmetric molecules, specifically, water, ethanol, and ?-pinene. We describe how the value of n2 depends on the physical parameters of these molecules. The measured values of n2 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.