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.

• Received 26 August 2020
• Revised 9 February 2021
• Accepted 7 April 2021

DOI:https://doi.org/10.1103/PhysRevApplied.15.054009