Ratiometric optical thermometry based on upconversion luminescence with different multi-photon processes in CaWO:Tm/Yb phosphor.

Ratiometric optical thermometry based on upconversion (UC) luminescence with different multi-photon processes in CaWO:Tm,Yb phosphor was developed. A new fluorescence intensity ratio (FIR) thermometry, utilizing the ratio of the cube of F emission to the square of G emission of Tm 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 H emission to the square of G emission of Tm 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 CaWO:Tm,Yb 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 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.