Second and third order nonlinear optical properties of magnetic colloids

Author(s)

Eduardo Sell Gonçalves

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI)
• Defense date: 2019-12-02

Examining board members

Antonio Martins Figueiredo Neto; Alex Fabiano Cortez Campos; Lino Misoguti; Paulo Alberto Nussenzveig; Ricardo Elgul Samad

• Advisor: Antonio Martins Figueiredo Neto

Abstract

In this work, second and third order nonlinear optical properties of monocrystalline magnetic nanoparticles in colloidal dispersions were studied. Commercial solutions of magnetite-based particles Fe3O4 as well as manganese-zinc ferrite nanoparticles Mn0.5Zn0.5Fe2O4, synthesized at the Institute of Physics, University of São Paulo in collaboration with researchers at Charotar University of Science and Technology, India, were investigated. In the latter case, solutions were composed of spherical or cubic nanoparticles. Second order nonlinear properties were studied using the hyper-Rayleigh scattering technique to determine first order hyperpolarizability. For the characterization of third order nonlinear optical properties, the absorption of two photons was measured using the Z-scan technique in the open-aperture configuration. In addition, linear optical properties were characterized by linear transmittance spectroscopy, in the range that extends from infrared to near ultraviolet. During the experiments, magnetic fields with magnitudes between 0 Oe and 3100 Oe were applied to the samples. The experiments were performed with the polarization state of the incident beam in the same direction as the magnetic field lines, herein called parallel configuration, and in the case where the polarization and magnetic field directions were orthogonal, the perpendicular configuration. Structural characteristics of nanoparticles, as well as their organization in the presence of magnetic field, were studied by small angle X-rays scattering measurements. By means of the hyper-Rayleigh scattering experiments it was verified that, for all particles, measurements made with the magnetic field applied parallel to the laser polarization resulted in an increase of the hyperpolarizability. In measurements with the field applied in the perpendicular direction, a systematic decrease of the measured values was observed. The two-photon absorption cross-section followed the same trend, increasing for measurements in the parallel configuration and being reduced in measurements in the perpendicular case. Since the nanoparticles are in the superparamagnetic state and free to rotate within the solution, the presence of magnetic field induces the orientation of the particles, aligning the axis of easy magnetization of each particle to the external field. Thus, there was the alignment of crystallographic planes of the material. Therefore, measurements in the parallel configuration result in values of nonlinearities stimulated in that direction, while in the perpendicular configuration, the average of the projections on the other two orthogonal directions were measured. Finally, in the absence of magnetic field, the result corresponds to the average over all possible orientations. (AU)

• FAPESP's process: 15/20555-2 - Second harmonic generation by centrossymetric nanoparticles in magnetic colloids
• Grantee: Eduardo Sell Gonçalves
• Support Opportunities: Scholarships in Brazil - Doctorate