Title: Sensitivity Enhancement of Third-order Nonlinearity Measurement in THz Frequency Range

Abstract: In this article for the first time we applied the eclipse Z-scan technique which is by one order of magnitude more sensitive than the conventional Z-scan technique to measure LiNbO3 crystal nonlinear refractive index in the THz range. The obtained value of LiNbO3 nonlinear refractive index is estimated to be order of 10^-11 cm^2/W which is commensurate with known results. This value correlates with the theoretically calculated nonlinear refractive index coefficient of vibrational nature. The influence of thermal nonlinearity on the experimental results can be neglected, since the estimated temperature induced refractive index change equals to 2.6*10^-5, while addition from optical nonlinearity is 2.9*10^-3. The demonstrated heightened sensitivity of eclipse Z-scan allows to hold promise for the properties evaluation of materials exhibiting lower nonlinear refractive indices, thus expanding its applicability in characterizing diverse nonlinear optical materials.