Nonlinear optical properties of colloids have technological appeal, since nanoparticles with nonlinear optical properties can be combined with the fluidity of liquid carriers, in the emerging area of Optofluidics. Ferrofluids, especially, can be used in magnetically-controllable applications or in optical limiting devices, where nonlinear absorption is a key characteristic. Besides two-photon absorption, some phenomena are present in experimental studies in optical nonlinearities of colloids: the particles can absorb light and heat the liquid around it, giving rise to a temperature and a subsequent refractive index gradient, what originates a thermo-optical self-focusing; also, the temperature gradient can drive the particles inward or outward the illuminated region, what changes the refractive index and the absorption coefficient of the material. In this work, the z-scan technique is performed in ferrofluids and thin films made from ferrofluids to measure the two-photon absorption coefficient of magnetite and manganese ferrite nanoparticles and to determine their two-photon absorption cross-section (σ2PA). To avoid the influence of the cited thermo-optical effects in these measurements, the frequency of the pulsed Gaussian beam (pulse width of 196 fs) is decreased with an electro-optic modulator and a shutter is used to allow the measurement of the nonlinear effects, present at the first pulse illuminating the sample, after a period of 2 seconds without illumination. The z-scan curves with and without using shutter are compared in colloids and thin films. The achieved values of σ2PA at 800 nm are 50 GM and 107 GM, for the magnetite and manganese ferrite nanoparticles, respectively.
Daniel Espinosa, Eduardo S. Gonçalves, and Antonio M. Figueiredo Neto, "Determination of the two-photon absorption cross-section of magnetite and manganese ferrite nanoparticles in ferrofluids and thin films (Conference Presentation)," Proc. SPIE 10125, Emerging Liquid Crystal Technologies XII, 101250L (Presented at SPIE OPTO: February 01, 2017; Published: 21 April 2017); https://doi.org/10.1117/12.2250636.5398643902001.
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