From upconversion to thermal radiation: spectroscopic properties of a submicron Y2O3:Er3+,Yb3+ ceramic under IR excitation in an extremely broad temperature range

Along with the ongoing developments in the field of luminescence thermometry, especially concerning ratiometric methods, opportunity presents itself to broaden the scope of their application potential. In this work, the spectroscopic and temperature sensing properties of a Y2O3:1%Er3+,20%Yb3+ submicron ceramic were investigated over a wide temperature range. The fluorescence intensity ratio (FIR) between the emission bands corresponding to the H-2(11/2) -> I-4(15/2) and S-4(3/2) -> I-4(15/2) branches of Er3+ upconversion under 975 nm excitation was evaluated experimentally between 175 K and 895 K. The relative temperature sensitivity of the system at 300 K was established at 1.4% K-1. This model was subsequently used for the evaluation of the magnitude of laser-induced heating during the operation of the temperature sensing system under varying experimental conditions of surroundings (vacuum and air) and laser beam density (focused and defocused) as a function of laser diode power. The outcomes provided insight into the strong impact of the aforementioned conditions and yielded technical remarks regarding the proper evaluation of the temperature sensing properties, crucial during the initial calibration measurements. For sensing in a high temperature range, ratiometry based on Planck's law was successfully employed to evaluate the temperature, extending the measurement range up to ca. 2100 K. The agreement between the Planck-based and fluorescence-based ratiometric temperature readout models used in separate temperature ranges served as cross-validation for both methods. Based on these data, submicron ceramics of yttria co-doped with Yb3+ and Er3+ can be considered as a promising candidate for temperature sensing in a broad range, spanning over nearly two thousand degrees.