Two-photon absorption cross section of magnetite nanoparticles in magnetic colloids and thin films

We present z-scan measurements of magnetic nanoparticles made from magnetite in both thin film form and colloidal solutions. In order to avoid heating and, thus, spurious effects that could lead to misinterpretation of the z-scan results, an electro-mechanical shutter was added along the beam path in order to guarantee samples thermal relaxation. Two photon absorption coefficient β and nonlinear refractive index n 2 were measured as a function of concentration N of absorbing units (Fe3O4). Our magnetite samples presented n 2 ≈ − 1.5 × 10 − 14 cm 2 / W , similar to that of the liquid carrier, for concentrations below 2 × 10 20 cm − 3 . n 2 increases, in absolute value, to about − 10 × 10 − 14 cm 2 / W for a sample three times more concentrated and then decreases with N until about − 7 × 10 − 14 cm 2 / W for the most concentrated sample. β presented a linear dependence with N and the two-photon absorption cross section σ 2 PA was calculated, resulting in σ 2 PA = 50 ( 2 ) GM for magnetite nanoparticles.