Near-infrared absorption thermometry exploiting an anomalously large spectral line-shift for the Yb 3+ cubic centre in CaF 2 :Yb 3+ /Er 3+ nanoparticles

Near-infrared absorption spectra of Yb 3+ in Er 3+ co-doped CaF 2 upconverting nanoparticles have been measured. The Yb 3+ ion 2 F 7/2 → 2 F 5/2 inter-multiplet optical absorption spectra show negatively charged preferential clusters and non-locally charge compensated cubic centers as the dominant centres. The temperature dependent absorption spectra measured between 10 K and 413 K for Yb 3+ cubic centres exhibit an anomalously large blue-shift of 16.90 ± 0.02 cm −1. The CaF 2 :Yb 3+ /Er 3+ nanoparticles show an absolute thermal sensitivity S a of 0.044 ± 0.001 cm −1 ⋅K −1 and the maximum relative thermal sensitivity S r of 2.290 ± 0.010%·K −1 at 413 K region based on the large cubic centre absorption line-shift and a value of S r of 0.0723 ± 0.001%·K −1 at 10 K (0.0562 ± 0.010%·K −1 at 413 K) from the line-width. The observed absolute absorption thermal sensitivity is 10 times higher (and the relative absorption thermal sensitivity is 2 times higher) than that obtained using the fluorescence intensity ratio of the thermally coupled 2 H 11/2, 4 S 3/2 energy levels of Er 3+ in co-doped nanoparticles.