Ratiometric optical thermometry based on upconversion luminescence with different multi-photon processes in CaWO4:Tm3+/Yb3+ phosphor

Ratiometric optical thermometry based on upconversion (UC) luminescence with different multi-photon processes in CaWO₄:Tm³⁺,Yb³⁺ phosphor was developed. A new fluorescence intensity ratio (FIR) thermometry, utilizing the ratio of the cube of ³F_2,3 emission to the square of ¹G₄ emission of Tm³⁺ and retaining the feature of anti-interference of excitation light source fluctuations, is proposed. Under the hypotheses of the UC terms being neglected in the rate equations and the ratio of the cube of ³H₄ emission to the square of ¹G₄ emission of Tm³⁺ being a constant in a relatively narrow temperature range, the new FIR thermometry is valid. The correctness of all hypotheses was confirmed by testing and analyzing the power-dependent emission spectra at different temperatures and the temperature-dependent emission spectra of CaWO₄:Tm³⁺,Yb³⁺ phosphor. The results prove that the new ratiometric thermometry based on UC luminescence with different multi-photon processes is feasible through optical signal processing, and maximum relative sensitivity of the thermometry is 6.61% K^-1 at 303 K. This study provides guidance in selecting UC luminescence with different multi-photon processes to construct ratiometric optical thermometers with anti-interference of excitation light source fluctuation.