Upconversion fluorescence and its thermometric sensitivity of Er3+:Yb3+ co-doped SrF2 powders prepared by combustion synthesis

Upconversion fluorescence of co-doped Er³⁺:Yb³⁺:SrF₂ powders prepared by combustion synthesis was investigated under near-infrared ( λ = 980 nm) continuous wave laser excitation. Surface morphology of the samples and structures of the Er³⁺:Yb³⁺:SrF₂ powders were studied with scanning electronic microscopy, energy dispersive x-ray, and x-ray powder diffraction. The spectrum of the fluorescence contains bands centered at ~410, ~522, ~545 and ~660 nm, corresponding respectively to transitions from upper levels ²H_9/2, ²H_11/2, ⁴S_3/2 and ⁴F_9/2 to the ground state ⁴I_15/2, which can be identified as 4 f-4 f transitions from Er³⁺ excited states. In addition, the fluorescence is found sensitive to the temperature, which suggests that an optical temperature sensor would be feasible. The maximum sensitivity of the proposed sensor was found 0.00396 K^-1.