Nonlinear optical refractive index measurements of pure water via Z-scan technique at 800 nm

The determination of the nonlinear optical coefficient \( n_2 \) of pure water is of great interest for applications such as imaging biological systems, performing femtosecond laser surgery, and underwater ablation. Traditionally, probing third-order nonlinearity while eliminating the thermo-optic effect requires high pulse energy and low repetition rate laser sources. In this article, a low-cost, simple, and versatile third-order nonlinear characterization technique for the Z-scan closed-aperture mode has been developed to determine the nonlinear refractive index of pure water at 800 nm, with a pump duration of 140 fs and a repetition rate of 80 MHz. Experimental results reveal an \( n_2 \) value of \( (8 \pm 1.4) \times 10^{-20} \) m\(^2\)/W. The influence of higher-order nonlinearities such as \( \chi^5 \) was assessed, showing no significant contribution within the examined intensity range. This work paves the way for the use of low-power and high-repetition-rate systems for characterizing third-order nonlinear optical materials.