Optical temperature sensing is a promising method to achieve the contactless temperature measurement and large-scale imaging. The current status of optical thermometry of rare-earth ions doped phosphors is reviewed in detail.

NaYbF₄:Tm³⁺@SiO₂ core-shell micro-particles were synthesized by a hydrothermal method and subsequent ultrasonic coating process. Optical temperature sensing has been observed in NaYbF₄: Tm³⁺@SiO₂ core-shell micro-particles with a 980 nm infrared laser as excitation source. The fluorescence intensity ratios, optical temperature sensitivity, and temperature dependent population re-distribution ability from the thermally coupled ¹D₂ /¹G₄ and ³F₂ /³H₄ levels of the Tm³⁺ ion have been analyzed as a function of temperature in the range of 100~700 K in order to check its availability as a optical temperature sensor. A better behavior as a low-temperature sensor has been obtained with a minimum sensitivity of 5.4 × 10⁻⁴ K⁻¹ at 430 K. It exhibits temperature induced population re-distribution from ¹D₂ /¹G₄ thermally coupled levels at higher temperature range.

Optical thermometry has been widely studied to achieve an inaccessible temperature measurement in submicron scale and it has been reported that the temperature sensitivity depends mainly on host types. In this work, we propose a new method to improve the optical temperature sensitivity of Yb³⁺-Er³⁺ co-doped CaWO₄ phosphors by doping with Li⁺, Sr²⁺, and Mg²⁺ ions and by controlling excitation powers of 980 nm laser. It is found that the thermometric parameters such as upconversion emission intensity, intensity ratio of green-to-red emission, fluorescence color, emission intensity ratios of thermally coupled levels (²H_11/2/⁴S_3/2), and relative and absolute temperature sensitivity can be effectively controlled by doping with Li⁺, Sr²⁺, and Mg²⁺ ions in the Yb³⁺-Er³⁺ co-doped CaWO₄ system. Moreover, the relative sensitivity S_R and the absolute sensitivity S_A are proved to be dependent on the pump power of 980 nm laser. The sensitivities of S_R and S_A in Yb³⁺-Er³⁺ co-doped CaWO₄ increase about 31.5% and 12%, respectively, by doping with 1 mol% Sr²⁺.

The knowledge of the pump power for which the population of thermally coupled energy levels (TCL) changes with power increase is of valuable importance for optical temperature sensors. In this paper, novel Er³⁺ doped transparent Sr_0.69La_0.31F_2.31 glass ceramics was fabricated successfully, and its structure is studied by XRD, TEM and HRTEM analyses. The ²H_11/2/⁴S_3/2, ⁴F_9/2(1)/⁴F_9/2(2), and ⁴I_9/2(1)/⁴I_9/2(2) levels of Er³⁺ are proved as TCL by analyzing the temperature dependent fluorescence intensity ratios. The spectrum split, thermal quenching ratio, population stability, and temperature sensitivity from three TCL are observed to be dependent on the pump power. A new fitting method has been developed to establish the relation between fluorescence intensity ratios and temperature. It is found that the combined use of ²H_11/2/⁴S_3/2 and ⁴F_9/2(1)/⁴F_9/2(2) as thermally coupled energy levels will get a more precise temperature reading from 62.7 K to 800 K with the help of low excitation power at 66.8 mW/mm².