Crocin has a strong Two-photon Absorption (TPA) capability that makes it suitable for medicine and photonic devices. To study the linear and nonlinear optical properties of crocin in solvent and Microemulsion (MEs), Z-scan instruments, Uv-Vis, and fluorescence were used. Nonlinear absorption coefficient (β), nonlinear refractive index (n2), third-order susceptibility (χR), and second-order molecular hyperpolarizability (γR) values were extracted from the Z-scan instrument. The β and n2 reduce with the increase in solvent’s dielectric constant (ε). In crocin mixed with MEs, the nonlinear optical values depend on the size and concentration of the droplet at constant crocin concentration. The MEs can enhance the value of β as compared with other solvents. A red shift in absorption spectra of crocin was observed with an increase in ε. However, the absorption peak was constant in the MEs. The ratios of the excited state to the ground state of the dipole moments (μe/ μg) of crocin in the solutions were determined by the quantum perturbation theory. It was shown that μe/ μg has similar behavior with 1/ γR. The value of μg and γR are reduced with an increase in ε.

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