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 3F2,3 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.