Doping-Site Dependence of Upconversion Emission of Ho 3+ Ion in BaHfO 3

We investigated the doping-site dependence of photoluminescence and upconversion (UC) emission characteristics of Ho3+ and Yb3+ co-doped BaHfO3 polycrystals. Under 980-nm near-infrared excitation, we detected sizable green (550 nm) and red (660 and 760 nm) UC emissions from Ho3+ ions based on the assistance of Yb3+ ions as sensitizers. These emission features showed strong temperature-dependent variation and became more intense with decreasing temperature. Notably, the UC emission from Ho3+ was stronger in the samples in which the Ho3+/Yb3+ dopants were substituted for Hf4+ ions rather than for Ba2+ ions. The results suggest that the asymmetry ratio of the crystal field surrounding the Ho3+ ions should be an important factor for the UC emission properties.