Threshold-tunable optical limiters based on nonlinear refraction in ferrosols

The Fe2O3/SiO2 sols, referred as the ferrosols. are prepared by the sol-gel techniques. The addition of CTAB surfactant used as stabilizer can considerably enhance the refractive optical nonlinearity of the ferrosols, though it reduces the optical transparency. The divergence and annular rings formation of the laser beam transmitted the ferrosols are assigned to the self-defocusing and transverse self-phase modulation within the ferrosols and are investigated experimentally. By numerically solving the Kirchhoff diffraction integral, the value of the nonlinear refraction (n(2)) of the CTAB-stabilized ferrosols is found to be -2.3 x 10(-9) m(2)/W or so, which is reasonably large. The aperture-limited optical limiters based on the nonlinear refraction of the stabilized ferrosols are presented. The tunability of limiting threshold of the proposed optical limiters can be accomplished in on-line mode by engineering the experimental geometry, e.g. the aperture size and position behind the sample. The mechanisms of the optical limiting effects and the tunable limiting threshold are clarified. The results presented in this work provide cost-effective means and materials for designing the threshold-tunable optical limiters. (C) 2008 Elsevier B.V. All rights reserved.