Vibrational Nonlinear Response of Complex Molecules: Nature and Measurements in the THz Range

The nonlinear optical response of materials under high-intensity electromagnetic fields is key to advancing THz optical technologies. This study introduces a theoretical approach to estimate the nonlinear refractive index coefficient n2 of complex molecules by summing contributions from individual vibrational bonds, which are extracted from structurally simpler molecules predominantly consist of corresponding type of bond. Using water, alpha-pinene, and CO2 as reference materials, we predict n2 for isopropanol as 4.7*10^(-9) cm^2/W. Z-scan measurements with pulsed THz radiation yield n2 = (2.5+-0.5)*10^(-9) cm^2/W, validating our model. This bond-based decomposition links microscopic bond vibrations and macroscopic nonlinear coefficients, offering a framework for understanding molecular nonlinear optics and guiding the design of THz nonlinear materials.