Tm${}^{3+}$/Yb${}^{3+}$ co-doped SrF${}_2$ 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:SrF${}_2$ fluoride powders were fabricated employing a combustion synthesis (CS) technique. When Tm/Yb: SrF${}_2$ samples were excited by near-infrared radiation (~ 980 nm), four UC emission bands at ~ 477 nm (${}^1$G${}_4$ →${}^3$H${}_6$), ~ 649 nm (${}^1$G${}_4$ →${}^3$F${}_4$), ~ 700 nm (${}^3$F${}_{2,3}$ →${}^3$H${}_6$) and ~ 771 nm (${}^3$H${}_4$→${}^3$H${}_6$) 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 S${}_{NTr}$ of ~ 2.2%K${}^{-1}$ for the non-thermally coupled energy levels [(${}^3$H${}_4$→${}^3$H${}_6$)/ (${}^1$G${}_4$→${}^3$H${}_6$)] and a maximum relative sensitivity S${}_r$ of ~ 1.7%K${}^{-1}$ for the thermally coupled levels [(${}^3$F${}_{2,3}$→${}^3$H${}_6$)/(${}^3$H${}_4$→${}^3$H${}_6$)]. 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: SrF${}_2$ UC system has a promising prospect as optical temperature-sensing material.