Optical High Temperature Sensor Based on Enhanced Green Upconversion Emissions in Er³⁺–Yb³⁺–Li⁺ Codoped TiO₂ Powders

The Er³⁺–Yb³⁺–Li⁺ codoped TiO₂ powders have been prepared by sol–gel method. The strong enhancement of green and red upconversion emissions were obtained for Er³⁺–Yb³⁺ codoped TiO₂ by additional Li⁺ codoping and investigated using 976 nm semiconductor laser diode excitation. The enhanced upconversion emissions by the addition of Li⁺ resulted from the formation of Li compound with lower crystal field symmetry. The fluorescence intensity ratio (FIR) of green upconversion emissions from the transitions of ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 of Er³⁺ in the Er³⁺–Yb³⁺–Li⁺ codoped TiO₂ has been studied as a function of temperature in the range of 300–925 K, and the maximum sensitivity was determined to be 0.0025 K⁻¹. Er³⁺–Yb³⁺–Li⁺ codoped TiO₂ material with the highest operating temperature up to 925 K, has higher temperature sensitivity and fluorescence efficiency being a promising candidate for applications in optical high temperature sensor.