Thermal-enhanced upconversion luminescence and optical temperature sensing properties based on the Stark sublevels of Er3+ ion in CaGdGaO4:Yb3+,Er3+

Accurate optical thermometry with thermally enhanced fluorescence is urgently desired for non-invasive and high sensitivity temperature determination in high temperature operating environment. Here we explore thermally enhanced upconversion luminescence (UCL) and thermal sensing properties of Er³⁺/Yb³⁺ doped CaGdGaO₄ (CGGO) phosphors under near-infrared excitation. The UCL spectra exhibit multiple sharp peaks from ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels of Er³⁺ ions, arising from the electronic transitions between crystal-field Stark sublevels. The emission color of samples is tuned from green to red successfully by introducing Yb³⁺ dopants and adjusting different excitation wavelengths. The UCL intensity is greatly enhanced with the increase of temperature, which benefits from the thermally enhanced phonon-assisted effect. By using the fluorescence intensity ratio technique, temperature sensing performances are evaluated based on thermally coupled energy levels (TCELs) involving Stark sublevels of Er³⁺ over a wide temperature range of 303–573 K. The maximum relative sensitivity of ²H_11/2(1) and ⁴S_3/2(2) levels reaches up to 1.55 % K⁻¹ at 303 K, which is higher than that of the traditional TCELs of ²H_11/2/⁴S_3/2. The optical thermometer CGGO:Er³⁺,Yb³⁺ shows a high repeatability of 99 % and minimum temperature resolution of 0.28 K. Our findings demonstrate that CGGO:Yb³⁺,Er³⁺ phosphors with thermal enhancement UCL can be used as candidates for non-contact and high sensitivity optical thermometry.