Fluorescence Upconversion Properties of Er<SUP>3+</SUP>-Doped TiO<SUB>2</SUB> and BaTiO<SUB>3</SUB> Nanocrystallites

The effects of the erbium concentration, crystal size, crystal phase, and different processing temperatures on the upconverted emission of Er3+ in BaTiO3 and TiO2 nanocrystals are reported. A cw diode laser at 975 nm was used as a pump source for resonant sequential excitation of the 4&#8739; 11/2 and 4F7/2 levels. Green and red upconversion emission at 550 and 670 nm were observed from these oxide nanocrystals with 975 nm excitation. With the same Er3+ concentration, the upconversion emission intensity from BaTiO3 was higher than that observed in the TiO2 host. In the TiO2 matrix, the maximum upconversion emission intensity of Er3+ was found for samples calcined at 800 °C where both the anatase and the rutile phases were present. The observed emission characteristics and the pump intensity dependence of the luminescence intensity confirm that the upconverted emission in these materials is produced by two-photon excited-state absorption (ESA) processes