Nonlinear optical properties of ionic magnetic colloids in the femto- and millisecond time scales: change from convergent-to-divergent lens-type behaviors

The Z-scan technique is used to measure the nonlinear refractive index n2 and the nonlinear absorption coefficient β of ionic magnetic colloids as a function of the nanoparticles' diameter (Drx) in the femto- and millisecond time-scale regimes. A change from a convergent-lens type (at a femtosecond time scale) to a divergent-lens type behavior (at a millisecond time scale) is observed. From the values of n2 and β measured in the femtosecond time-scale regime, the third-order susceptibility χ(3) is calculated and is shown to depend on Drx: the smaller the Drx, the bigger the χ(3). A possible origin of the high values of χ(3) found may be confining effects related to the physical nanoscale of the magnetic particles.