Influence of Doping and Excitation Powers on Optical Thermometry in Yb³⁺-Er³⁺ doped CaWO₄

Optical thermometry has been widely studied to achieve an inaccessible temperature measurement in submicron scale and it has been reported that the temperature sensitivity depends mainly on host types. In this work, we propose a new method to improve the optical temperature sensitivity of Yb³⁺-Er³⁺ co-doped CaWO₄ phosphors by doping with Li⁺, Sr²⁺, and Mg²⁺ ions and by controlling excitation powers of 980 nm laser. It is found that the thermometric parameters such as upconversion emission intensity, intensity ratio of green-to-red emission, fluorescence color, emission intensity ratios of thermally coupled levels (²H_11/2/⁴S_3/2), and relative and absolute temperature sensitivity can be effectively controlled by doping with Li⁺, Sr²⁺, and Mg²⁺ ions in the Yb³⁺-Er³⁺ co-doped CaWO₄ system. Moreover, the relative sensitivity S_R and the absolute sensitivity S_A are proved to be dependent on the pump power of 980 nm laser. The sensitivities of S_R and S_A in Yb³⁺-Er³⁺ co-doped CaWO₄ increase about 31.5% and 12%, respectively, by doping with 1 mol% Sr²⁺.