Influence of Magnetic Field on the Two-Photon Absorption and Hyper-Rayleigh Scattering of Manganese-Zinc Ferrite Nanoparticles

摘要

Magnetic nanoparticles based on manganese-zinc ferrite presenting spherical and cubic shapes, verified by small-angle X-rays scattering, were studied by means of the hyper-Rayleigh scattering, the Z-Scan technique in the open-aperture configuration, and a spectrally resolved femtosecond transient absorption setup. Hyper-Rayleigh scattering and Z-Scan experiments were performed with applied magnetic field parallel to the laser polarization state and in the perpendicular direction. The hyperpolarizability of spherical nanoparticles was greater than the cubic ones due to the higher volume of spherical nanoparticles and, therefore, the increased influence of the spin-disoriented layer at the surface on cubic nanoparticles, resulting in a smaller orientational average contribution to the hyperpolarizability. The two-photon absorption cross section was the same for both samples, since the basic constituents are the same and the light absorption is not influenced by the surface anisotropy present on magnetic nanoparticles. For both nanoparticles, the measured optical second harmonic and the nonlinear absorption increased during experiments performed in the presence of external field in the parallel configuration, while a decrease was verified for experiments in the perpendicular case, which demonstrates a crystalline anisotropy on the nonlinear optical properties. Transient absorption measurements revealed an ultrafast relaxation process containing at least two separated dynamical processes, the faster with characteristic time below 1 ps and a longer one, much higher than 100 ps.