IC-scan : uma nova técnica para medir o índice de refração não linear de meios espalhadores

Abstract

Light scattering is one of the most common optical phenomena observed due to the interaction of light with particles suspended in a solution. The relevance of scattering is evidenced by the various non-invasive techniques developed to measure particle size and colloidal stability. However, because scattering is also caused by system imperfections, its contribution can be critically detrimental to the precision and sensitivity of many optical techniques. In nonlinear (NL) optical spectroscopy, the measurement of the refractive index change as a function of intensity is essential to characterize the NL response of materials, allowing to identify their applications in optical and photonic devices. In this work, we present the development of a new NL technique to measure the NL refractive index (𝑛2) mainly in scattering media, based on the analysis of the intensity correlation function (𝑔(2)), at different sample positions, along the propagation direction, around the beam waist. The new technique, which we name IC-scan (Intensity Correlation Scan), uses light scattering to generate speckle patterns that are sensitive to wavefront changes induced by the self-focusing and/or self-defocusing effects. This new methodology allows the measurement of phase modulation effects without being affected by linear scattering when intensity cross-correlation functions between linear and NL regimes are analyzed. As a proof-of-principle, measurements were performed on NL colloids, with high concentrations, containing silica nanospheres and gold nanorods as light scatterers, using a Titanium-Sapphire laser (788nm, 100fs, 76MHz). The results showed that the IC-scan technique to measure 𝑛2 is more robust in scattering media than the Z-scan and D4σ scan techniques, which are well established and widely used, but strongly influenced in media that have high degree of scattering. The values of 𝑛2 obtained show the potential of the new technique, at the same time they show that the results obtained through Z-scan and D4σ are underestimated (or overestimated) due to light scattering effects.