A high-precision thermometry strategy by replacing the infrared with visible light for detection

We have developed a high-precision thermometry strategy based on ⁴I_9/2-⁴I_15/2 (I_{800 nm}) and ⁴S_3/2-⁴I_15/2 (I_{551 nm}) transitions of Er³⁺, after replacing the measurement of the ⁴I_11/2-⁴I_15/2 (I_{1000 nm}) transition with the ⁴S_3/2-⁴I_15/2 transition, i.e., using visible light for detection instead of infrared. Through rate equation model analysis, (I_{1000 nm})² and I_{551 nm} can be substituted for each other under certain conditions. Further, because the ⁴I_9/2 and ⁴I_11/2 levels of Er³⁺ are thermally coupled, a new idea of ratiometric thermometry is proposed based on the ratio of (I_{800 nm})² and I_{551 nm}, which has the advantages of anti-interference of excitation light source fluctuation and background-free detection. The feasibility of the idea was verified by researching the power-dependent emission spectra at different temperatures and temperature-dependent emission spectra of a CaWO₄:Er³⁺,Yb³⁺ sample under 980-nm laser excitation. The maximum relative sensitivity for the new ratiometric thermometry reaches up to 7.4% K^-1 and the optimal temperature uncertainty calculated is 0.03 K at 303 K. This study provides guidance for solving the problem of a weak response of an infrared detector.