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.

We have developed a high-precision thermometry strategy based on 4I9/2–4I15/2 (I800 nm) and 4S3/2–4I15/2 (I551 nm) transitions of Er3+, after replacing the measurement of the 4I11/2–4I15/2 (I1000 nm) transition with the 4S3/2–4I15/2 transition, i.e., using visible light for detection instead of infrared. Through rate equation model analysis, (I1000 nm)2 and I551 nm can be substituted for each other under certain conditions. Further, because the 4I9/2 and 4I11/2 levels of Er3+ are thermally coupled, a new idea of ratiometric thermometry is proposed based on the ratio of (I800 nm)2 and I551 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.

In this work, the transition properties of wide-bandgap compounds are studied by way of the temperature dependence of fluorescence dynamic curves. The YVO4, CaWO4, and CaMoO4 samples are selected in this study owing to their strong absorption in the ultraviolet (UV) region. The fluorescence decay curves of the three samples, in the range of 173–373 K, are measured under excitation by deep UV light-emitting diodes (LEDs). The temperature dependence of transition rate (A) is composed of a constant term, as well as a mono-exponential term which follows the Boltzmann distribution law. The constant term reflects a spontaneous radiative transition rate (WR), while the mono-exponential term represents the temperature-dependent nonradiative transition rate (WNR). The nonradiative relaxation is attributed to the upward thermal population rather than the downward multiphonon relaxation (MPR). The thermal populating process is supposed to originate from the bottom of the excited state to the intersection of the ground state and excited state by overcoming the potential barrier of activation energy (Ea), and the Ea values of YVO4, CaWO4, and CaMoO4 samples are calculated.