Wide-range non-contact fluorescence intensity ratio thermometer based on Yb3+/Nd3+ co-doped La2O3 microcrystals operating from 290 to 1230 K

Abstract

Non-contact ratiometric thermometry has applications ranging from in situ physiological measurements to industrial process monitoring. The technique is based on the optical detection of the fluorescence intensity ratio (FIR) of two thermally-coupled levels (TCLs). Here, we report a Yb³⁺/Nd³⁺ co-doped La₂O₃ microcrystal ratiometric thermometer based on upconverted emission in the near-infrared (NIR) after excitation with a 980 nm laser diode, which operates effectively over the wide temperature range from 290–1230 K. The thermometer uses the TCLs of Nd³⁺:⁴F_7/2 (emission peak 762 nm) and Nd³⁺:⁴F_5/2 (emission peak 825 nm). The chosen levels combine desirable characteristics to act as a sensitive temperature sensor over a wide range of elevated temperatures: namely a suitable energy gap (ΔE = 950 cm⁻¹); and weak thermal quenching effects (maximum photoluminescence at 803 and 853 K respectively for two Nd³⁺ emission peaks). This leads to a high relative sensitivity (S_R) of 1334/T², and low temperature uncertainty ΔT_min of 0.1 K (<400 K), 1 K (400–853 K) and 3 K (853–1233 K). In addition to these characteristics, the excellent repeatability of FIR of the two Nd³⁺ emission peaks makes Yb³⁺/Nd³⁺ co-doped La₂O₃ microcrystals a promising non-contact NIR ratiometric thermometer for temperatures up to 1230 K.