Eliminating the “decoupling” effect of fluorescence intensity ratio thermometry for an upconversion pumped system

To eliminate the “decoupling” effect, which causes the fluorescence intensity ratios (FIR) to deviate from the pure Boltzmann distribution law, a correcting method for an upvonversion pumped system was theoretical derived and experimentally verified. Theoretical results indicated that the double exponential decay of the lower level was entirely involved in the decay of the upper level, which could be used to determine the thermal population degree (η). Taking Tm³⁺ ions as examples, by analyzing the temperature dependence of the fluorescent dynamic curves originating from the thermally coupled pair, the η values at different temperatures were obtained and the corresponding correcting curve is given. The corrected FIR abides by the Boltzmann law, even in the lower temperature range.