Origin of optical nonlinearity for PbO, TiO2, K2O, and SiO2 optical glasses

The nonlinear optical properties for the PbO, TiO2, K2O, and SiO2 system have been measured by the Z-scan method. The magnitude and sign of the nonlinear refractive index n2 were determined, as was the negative sign, which indicated a self-defocusing optical nonlinearity. Two optical absorption bands at 540 and 660 nm, respectively, are observed in the optical absorption spectra. The sources of the absorption bands are attributed to the 3d-shell electronic transitions of Ti3+ ions from the ground state to the excited states. The origin of the negative nonlinear refractive index was the contribution of resonant electronic transition processes, which can cancel the positive nonresonant refractive index that mainly resulted from the hyperpolarizabilities of the Pb–O and Ti–O pairs.