Comparison of visible fluorescence properties between sol-gel derived Er³⁺-Yb³⁺ and Er³⁺-Y³⁺ co-doped TiO₂ films

Both Er³⁺-Yb³⁺ and Er³⁺-Y³⁺ co-doped TiO₂ films were prepared on fused silica by sol-gel processes. The effect of doping concentration upon the up-conversion fluorescence of Yb³⁺-Er³⁺ and Er³⁺-Y³⁺ co-doped TiO₂ systems will be focused in this investigation. Although the influence of Yb³⁺ and Y³⁺ ions on the structural of Er³⁺-TiO₂ host material is almost the same, different up-conversion emissions are observed. Both Er³⁺-doped and Er³⁺-Y³⁺ co-doped TiO₂ films exhibit the up-conversion emission with green and red light. However, much stronger intensity of red light relative to green light is observed for the Er³⁺-Yb³⁺ co-doped TiO₂ system. For Er³⁺ (5 mol%)-Y³⁺ co-doped TiO₂ samples, the relative intensity ratio (I_R/I_G) rapidly decreases with the increase of Y³⁺ concentration above 10 mol%. On the contrary, the relative intensity ratio (I_R/I_G) increases with increasing Yb³⁺ concentration for Er³⁺ (5 mol%)-Yb³⁺ co-doped TiO₂ samples. The up-conversion emission in the Er³⁺-Y³⁺ co-doped TiO₂ system belongs to a two-photon absorption up-conversion process. However, the overall up-conversion efficiency in the Er³⁺-Yb³⁺ co-doped TiO₂ system could be contributed from the complicated mechanisms.