The enhancement of fluorescence intensity and nonlinear absorption of a dye has application in Photodynamic therapy and biological detection. In this work, it is shown that the Fluorescein sodium salt -FSS- have dianion structure in water and anion structure in microemulsion -ME-. The optical nonlinearity of FSS at different concentrations in aqueous solution and ME has been studied. The nonlinear absorption -ß-, nonlinear refractive index -n2-, hyperpolarizability -- and the nonlinear susceptibility --3-- were characterized by measuring the intensity dependent of open and close aperture. The FSS in water at 1 mM concentration exhibited, ß = 9.03×10−8 mW−1, n2 = 2.88×10−15 m2W−1 and = 5.2×10−26 m−5V−2 was founded at 0.25 mM concentration showed. Moreover, the value of nonlinear optical properties of FSS-ME is higher than an aqueous solution of FSS at below 0.25 mM. By perturbation theory and Raman spectra, it is indicated that the formation of ME cannot change the polarity of ME but it can change the ratio of the dipole moment. The molecular structure as well as the FSS dipole moment control by different formation of ME.

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