A study of energy transfer phenomenon leading to photon up-conversion in Ho3+:Yb3+:CaF2 crystalline powders and its temperature sensing properties

When Ho³⁺:Yb³⁺:CaF₂ crystalline powders prepared by combustion synthesis were exposed to near-infrared (λ ～ 975 nm) radiation, intense photon up-conversion (UC) was observed at the visible with emission bands peaked at ～ 545, ～650 and ～750 nm identified as 4f-4f transitions from higher levels (⁵F₄, ⁵S₂) and ⁵F₅ to lower levels ⁵I₈ and ⁵I₇ of Ho³⁺. The emission bands at the green and red, in particular, have been demonstrated to be useful for temperature sensing based on luminescence intensity ratio technique. However, no model is available in literature to explain the change of the electronic populations of states (⁵F₄, ⁵S₂) and ⁵F₅ with temperature. The UC phenomenon was studied from both theoretical and experimental points of view. A rate equation model with temperature dependent parameters for Ho³⁺ and Yb³⁺ electronic populations considering a high sensitization of Ho³⁺ ions by Yb³⁺ ions was used. High Yb³⁺ → Ho³⁺ energy transfer efficiency was found (～88% at room temperature). The change with temperature predicted by the model for the luminescence intensity ratio of the UC green and red emission lines agrees well with the experimental data.