Strong reverse saturable absorption and negative nonlinear refractive index in NiFe₂O₄ nanoparticles.

Transition metal oxides (AB₂O₄) are an important class of materials due to their wide potential applications, such as energy storage, sensors, high-density magnetic recordings. We study the third-order nonlinear optical response of NiFe₂O₄ (NFO) by using the open and close aperture Z-scan technique to get a comprehensive study of nonlinear absorption and refraction. NFO has an indirect bandgap around 1.48eV and two direct bandgaps around 2.49eV and 3.12eV respectively. We excite the sample with a nanosecond laser at a wavelength of 532nm, and a pulse width of 7ns. We observe reverse saturable absorption as a nonlinear optical response of NFO. From the study of nanosecond excitation in NFO, we get a strong nonlinear optical response, a nonlinear absorption coefficient of around 1.3×10² cm GW^-1, and a nonlinear refractive index of 3.36 × 10^-4cm² GW^-1 at intensity 0.31GWcm^-2The observation of the negative nonlinear refractive index reveals the occurrence of self -defocusing effect. From the strong nonlinear optical response, it is clear that NiFe₂O₄ can be used for optical limiting applications.