Upconversion luminescence properties and optical thermometry based on non-thermal coupling levels of Ho3+, Yb3+, and Tm3+ codoped 12CaO·7Al2O3 single crystals

Compared with the fluorescence intensity ratio (FIR) temperature measurement technology based on the thermal coupling levels (TCLs) of rare earth (RE) ions, non-TCL (NTCL) FIR technology can greatly improve temperature measurement sensitivity because it is not limited by Boltzmann distribution. In this paper, a Ho3+/Yb3+/Tm3+ co-doped 12CaO⋅7Al2O3 (C12A7) single crystal was grown by the Czochralski method. As the temperature increased from 363 K to 523 K, the upconversion luminescence color of the Ho3+/Yb3+/Tm3+/C12A7 crystal changed from white to yellow, and exhibited a large temperature dependence under 980 nm excitation. In the temperature range of 363-523 K, the FIR temperature measurement based on different NTCLs exhibited high temperature sensitivity; the maximum absolute sensitivity and relative sensitivity values were 0.0207K-1 and 2.82%K-1, respectively, which are higher than those previously reported based on TCLs of Ho3+ and Tm3+. This provides a strategy to achieve accurate sensitivity of FIR technology. The RE ion doped C12A7 single crystal material has good research and application prospects in the field of temperature sensing and optoelectronics.