## Abstract

We report on the experimental observation of the diffraction pattern formed in the far-field region when a high-power continuous-wave laser convergent or divergent Gaussian beam passes through a cuvette with ferrofluid. Two different types of diffraction rings with opposite light-intensity distribution are shown in the far field. The difference between the diffractive patterns is attributed to the interaction of the strong spatial self-phase modulation caused by the refractive index change of the medium with wavefront curvature of the input Gaussian beam. The observed behavior of the diffraction pattern dynamics is interpreted theoretically based on the Fresnel–Kirchhoff integral. The negative polarity of nonlinear refraction can be identified by the central interference profiles and the diffraction pattern. At the same time, the self-defocusing phenomena of the ferrofluid can be determined by the type of pattern. The nonlinear refraction coefficients of the ferrofluid were estimated to be $\sim{-}{2.89} \times {{10}^{- 5}}\;{{\rm cm}^2}/{\rm W}$ (convergent Gaussian beam) and $\sim{-}{3.53} \times {{10}^{- 5}}\;{{\rm cm}^2}/{\rm W}$ (divergent Gaussian beam). In addition, the corresponding third-order nonlinear optical susceptibility of the sample was also estimated as $\sim{1.43} \times {{10}^{- 5}}\;{\rm esu}$ and $\sim{1.75} \times {{10}^{- 5}}\;{\rm esu}$, respectively. The experimental results imply a novel potential application of ferrofluid in nonlinear phase modulation devices.

© 2020 Optical Society of America

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