8:00 PM - MT07.05.07
Nanoparticle Shape Effect on the Determination of the First-Order Hyperpolarizability Anisotropy of Magnetic Colloids
Leonardo de Boni2,Eduardo Goncalves1,Leandro Cocca2,Wagner Wlysses1,Kinnari Parekh3,Cristiano Oliveira1,Antônio Figueiredo Neto1
University of São Paolo1,University of São Paulo2,Charotar University of Science & Technology3
Colloidal dispersions of magnetic nanoparticles were synthesized at the University of São Paulo, Brazil, and at Charotar University of Science and Technology, India, based on manganese-zinc ferrites, (Mn0.5Zn0.5Fe2O4). Solutions containing nanoparticles with distinct shapes (MZS, containing spherical nanoparticles, and MZC, composed of cubic NP's) were studied by means of the hyper-Rayleigh scattering (HRS) technique, allowing the determination of the optical second harmonic generation, defined as β. The size distribution and shape of nanoparticles were determined through small angle x-rays scattering (SAXS). Since the produced nanoparticles are free to rotate within the liquid carrier, an external magnetic field was applied in order to orient the particles in solution. Experiments with applied magnetic field were performed both in the parallel configuration (when the incident laser beam polarization was parallel to the magnetic field lines) and in the perpendicular case (where the polarization and the field directions were orthogonal). Furthermore, the linear attenuation spectrum was measured in the presence and absence of external magnetic field and it shows no changes regardless the field presence, indicating that the nanoparticles are being oriented but not forming bigger structures that would scatter the light. This is supported by the SAXS data, also measured in the presence of magnetic field, that demonstrate the formation of small linear aggregates, composed by a few nanoparticles. In the absence of magnetic field the linear attenuation spectrum of both samples is the same, as expected since the NP's have the same constituents. In this case, the hyperpolarizability was measured, for spherical nanoparticles, as β○=9.5(2)×10-28 cm5/esu, while for cubic ones, β■=7.8(1)×10-28 cm5/esu. For both studied nanoparticles, optical second harmonic experiments were performed in the presence of external field in the parallel configuration. In this condition, an increase in the value was verified, that is, for spherical nanoparticles (MZS), β○//=10.1(2)×10-28 cm5/esu, and for MZC sample, β■//=8.1(2)×10-28 cm5/esu. On the other hand, a slight decrease on the hyperpolarizability value was measured for experiments performed on the perpendicular configuration, β○⊥=9.3(3)×10-28 cm5/esu for MZS and β■⊥=7.4(2)×10-28 cm5/esu for the cubes. For cubic nanoparticles the anisotropy in the first-order hyperpolarizability δβ = (β// - β⊥)/β// was 8.2 while for the spherical nanoparticles, 7.6%. In the presence of magnetic fields, nanoparticles tend to align their magnetic momentum to the external field, in consequence, there is the alignment of crystallographic planes of the material. Therefore, in the parallel configuration the first-order hyperpolarizability was measured alongside the magnetic momentum direction, while in the perpendicular configuration, in the two dimensions perpendicular to that direction, that are equivalent. When there is not magnetic field applied, the nanoparticles are randomly oriented and the measured hyperpolarizability corresponds to an average over the three orthogonal directions. This can be verified calculating the weighted average of the hyperpolarizability in the presence of external field, that is, <β> = 1/3(β// + 2β⊥), so <β○>=9.6(3)×10-28 cm5/esu and <β■>=7.7(2)×10-28 cm5/esu, in both cases compatible with the measured value for the system without magnetic field. This demonstrates the shape dependent anisotropy in the second-order nonlinear properties of ferrite NP's, opening the possibility of the development of optofluidic devices, associating magnetic nanoparticles in solution and optical elements as i.e. optical fibers, as optical magnetometers, that would allow the determination of the field magnitude and direction, as well as in information propagation since there are three possible states and light intensities for a system composed by magnetic NP and an external magnetic field.