Sensitivity Enhancement of Cubic Nonlinearity Measurement in THz Frequency Range

The study applies for the first time the eclipse Z-scan technique to measure LiNbO $_{3}$ crystal's nonlinear refractive index coefficient in the THz range. This technique is by one order of magnitude more sensitive than the conventional Z-scan method. The obtained value of LiNbO $_{3}$ nonlinear refractive index coefficient is estimated to be 5 $\pm \, 2\times 10^{-11}$ cm $^{2}$ /W, which is commensurate with the established 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 refractive index change induced by temperature $\Delta n{\text{th}}$ equals 2.6 $\times\, 10^{-5}$ , while the contribution $\Delta n_{nl}$ from optical nonlinearity is 2.9 $\times\, 10^{-3}$ . The demonstrated heightened sensitivity of eclipse Z-scan holds promise for the evaluation of properties of materials with lower nonlinear refractive index coefficients, thus expanding the method applicability in characterizing various nonlinear optical materials.