Tm3+/Yb3+co-doped SrF2 up-conversion phosphors for non-invasive optical thermometry: ratiometric approach using thermal and non-thermal coupled fluorescent emission bands

To explore novel up-conversion (UC) system for optical temperature sensing, a series of Tm/Yb:SrF2 fluoride powders were fabricated employing a combustion synthesis (CS) technique. When Tm/Yb: SrF2 samples were excited by near-infrared radiation (~ 980 nm), four UC emission bands at ~ 477 nm (1G4 → 3H6), ~ 649 nm (1G4 → 3F4), ~ 700 nm (3F2,3 → 3H6) and ~ 771 nm (3H4 → 3H6) were observed. Non-contact thermal sensing performances based on temperature-dependent fluorescence intensity ratio (FIR) technique of thermally and non-thermally coupled energy levels were estimated in the range from 298 to 573 K. The co-doped phosphor showed, at 298 K, a maximum relative sensitivity SNTr of ~ 2.2%K−1 for the non-thermally coupled energy levels [(3H4 → 3H6)/ (1G4 → 3H6)] and a maximum relative sensitivity Sr of ~ 1.7%K−1 for the thermally coupled levels [(3F2,3 → 3H6)/(3H4 → 3H6)]. These outcomes show a good temperature-sensing performance when the non-thermally coupled levels with different temperature dependences were selected as the thermometric parameters. Hence, the Tm/Yb: SrF2 UC system has a promising prospect as optical temperature-sensing material.