Variation of nonlinear refractive index in dye-doped liquid crystals by local and nonlocal mechanisms

We report in this work on the simultaneous presence of both positive and negative nonlinear optical (NLO) refractive indexes measured in methyl-red dye doped 5CB liquid crystal (LC). Characterization of these materials was performed according to the Z-Scan technique under cw He–Ne laser irradiation, taking into account the polarization and temperature dependence of the NLO-refractive index, and how these parameters are related to local and nonlocal mechanisms. The experimental conditions and a possible explanation for the simultaneous presence of positive and negative nonlinear refractive indexes are carefully discussed.