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Influence of an external magnetic field in the two-photon absorption coefficient of magnetite nanoparticles in colloids and thin films (Conference Presentation)
Author(s): Antonio M. Figueiredo Neto; Daniel Espinosa; Dennys Reis; Cristiano L. P. de Oliveira

Paper Abstract

Magnetic fluids or ferrofuids (FF) are colloidal suspension of magnetic nanoparticles in a liquid carrier. When a material is illuminated with a high-intensity light, typically nanosecond, picosecond and femtosecond pulsed laser beam, its refractive index n2 and absorption coefficient β depend on the light intensity I. The Z-Scan (ZS) nonlinear optical and the Small-Angle X-Ray Scattering (SAXS) techniques are used to investigate the structure and nonlinear optical properties of magnetite nanoparticles dispersed in a colloid and trapped in thin films. n2 and β were measured as a function of the intensity of an external applied magnetic field H. Different relative orientations of the field with respect to the light-polarization direction were investigated. When the external magnetic field is applied to the colloidal sample (H parallel to the light-polarization direction), β increases with the field, ranging from 1.5 cm/GW (without field) to 2.4 cm/GW (2700 Oe). For the field direction perpendicular to the light polarization direction, β decreases to 1.0 cm/GW (2700 Oe) and after remains stable. These values allowed us to evaluate some elements of the third-order nonlinear optical susceptibility tensor χ^((3)). The SAXS experiments revealed that when the eld is applied, small linear aggregates are formed in the direction of H. Considering that the nanoparticles rotate to align their magnetic moment parallel to the applied field direction, and the particle's magnetic moment is aligned along the ⟨111⟩ lattice direction of the nanoparticle’s crystalline structure, our results indicate an optical anisotropy in magnetite. The calculated third-order nonlinear optical susceptibility, along the ⟨111⟩ direction, is Imχ_xxxx^((3))=2.0(3)×〖10〗^(-20) m^2/V^2, while its average along the other two orthogonal directions is 1/2 (Imχ_yyyy^((3))+Imχ_zzzz^((3)) )=0.9(3)×〖10〗^(-20) m^2/V^2. For the thin-film sample, however, the n2 and β values do not change when the field of 1600 Oe is applied. Within the experimental error, n_2 does not seem to change with field for the colloidal samples. CNPq, FAPESP, CAPES, INCT-FCx and NAP-FCx.

Paper Details

Date Published: 13 March 2019
Proc. SPIE 10941, Emerging Liquid Crystal Technologies XIV, 1094103 (13 March 2019); doi: 10.1117/12.2502912
Show Author Affiliations
Antonio M. Figueiredo Neto, Univ. de São Paulo (Brazil)
Daniel Espinosa, Univ. de São Paulo (Brazil)
Dennys Reis, Univ. de São Paulo (Brazil)
Cristiano L. P. de Oliveira, Univ. de São Paulo (Brazil)

Published in SPIE Proceedings Vol. 10941:
Emerging Liquid Crystal Technologies XIV
Liang-Chy Chien; Dirk J. Broer; Igor Muševič; Byoungho Lee, Editor(s)

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