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

The determination of the nonlinear optical coefficient ����2 of pure water is of great interests 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 rates 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 ����2 value of (8 ± 1.4) x10−20 m²/W. The influence of higher-order nonlinearities such as χ5 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.