Temperature sensor based on the UV upconversion luminescence of Gd ³⁺ in Yb³⁺-Tm³⁺-Gd³⁺ codoped NaLuF₄ microcrystals

Optical temperature sensing characteristics based on the ultraviolet (UV) upconversion luminescence (UCL) of Gd³⁺ ions are reported here for the first time. Under 980 nm excitation, the temperature dependence five-photon UV UCL from the ⁶P_J and ⁶I_J levels of Gd³⁺ ions in NaLuF₄:Yb³⁺, Tm³⁺, Gd³⁺ microcrystals were investigated systematically. The fluorescence intensity ratios (FIR) of two pairs of thermally coupled levels ( ⁶P_5/2, ⁶P_7/2 and ⁶I _9/2, ⁶I_7/2) were studied as a function of temperature in a range of 298-523 K. The maximum sensor sensitivities were found to be about 0.0004 K^-1 (333 K) and 0.0029 K^-1 (298 K) by exploiting the UC emissions from the ⁶P_J and ⁶I_J levels, respectively. This suggests that the Gd ³⁺-based UCL materials are promising prototypes for application as multi-mode probes for use in bio-separation, MRI imaging, optical thermometers, etc.