Lifetime thermometry in the third biological window using KMnF3:Yb/Er nanoparticles

We report on Yb3+/Er3+ co-doped KMnF3 nanoparticles used as lifetime-based luminescence thermometers within the NIR-III biological window. The nanoparticles were synthesized using a hydrothermal method. Their structure and morphology were confirmed by X-ray diffraction and transmission electron microscopy. Basic optical characterisation was performed using excitation into the strong Yb3+ 2F7/2→2F5/2 inter-multiplet absorption transitions. Er3+ fluorescence was observed from the 2H11/2, 4S3/2 and 4F9/2 and 4I13/2 multiplets due to both upconversion and downshifting processes. The Er3+ 4I13/2 fluorescence transient was measured at 1525 nm (4I13/2) while exciting at 523 nm; for sample temperatures between 10 K and 400 K. These measurements demonstrate a maximum relative thermal sensitivity Sr of 1.32 %⋅K−1 and a minimum thermal uncertainty of 0.009 K at 150 K in the NIR-III region.