Influence of Magnetic Field on the Two-Photon Absorption and Hyper-Rayleigh Scattering of Manganese-Zinc Ferrite Nanoparticles
2020
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.
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