Multipath optical thermometry realized in CaSc₂O₄: Yb³⁺/Er³⁺ with high sensitivity and superior resolution

Design and fabrication of contactless optical thermometer with rapid and accurate performance has become a research hotspot in recent years. Herein, CaSc₂O₄: Yb³⁺/Er³⁺ is employed as the intermediary for temperature sensing under the excitation of 980 nm, which is proven to afford an ultra-sensitive and high-resolution optical thermometry in multiple ways based on the fluorescence intensity ratio (FIR) technology. The optimal thermal sensing behaviors are realized by the FIR of Er³⁺:²H_11/2 → ⁴I_15/2 to ⁴S_3/2 → ⁴I_15/2 transition, which has a relative sensitivity of 1184/T² and a minimal resolution of 0.03 K along with a maximal absolute error of 0.96 K. Besides that, the FIR between the thermally coupled Stark sublevels of Er³⁺:⁴F_9/2 manifold (FIR_R) as well as that of Er³⁺: ⁴I_13/2 manifold (FIR_N) can also provide excellent optical thermometry. The relative sensitivity of FIR_R-based and FIR_N-based optical thermometers are calculated to be 402/T² and 366/T², respectively, with a same minimal resolution of 0.09 K, which possess the potential to be used for biomedicine due to the inherent advantage of their operating wavelengths located in the biological window. The results demonstrate that CaSc₂O₄: Yb³⁺/Er³⁺ is a promising candidate for temperature sensing with multipath, high sensitivity, and superior resolution.