Up-Conversion Luminescence and Optical Temperature Sensing Properties of NaLuF₄:Yb³⁺/Ho³⁺ Micron-Sized Crystals at...

Non-contact temperature sensors utilising the fluorescence intensity ratio and the unique up-conversion (UC) luminescence of rare-earth ions have numerous benefits; however, their operational temperature range has remained limited. In this study, NaLuF₄:Yb³⁺/Ho³⁺ samples were prepared by the hydrothermal method. The samples exhibited exceptional UC luminescence properties at low temperatures. The intensity of the green emission (with peak wavelengths of 540 and 546 nm) gradually decreased with increasing temperature, and the green emissions showed a unique change at low temperatures. In addition, we studied the dependence of the UC luminescence intensity on the excitation power and the variation in the decay lifetime with temperature. The experiments revealed excellent luminous performance and significantly enhanced sensitivity at low temperatures; the maximum absolute sensitivity S and relative sensitivity S of the 540 and 546 nm thermally coupled energy levels were 1.02% and 0.55% K^-1, respectively. The potential temperature sensing properties of Yb³⁺/Ho³⁺-co-doped NaLuF₄ makes it suitable for temperature sensing applications at temperatures as low as 30 K. This study offers a novel approach for the advancement of temperature sensing technology at low temperatures.