Absorption spectra, defect site distribution and upconversion excitation spectra of CaF2/SrF2/BaF2:Yb3+:Er3+ nanoparticles

We report studies of the Yb3+ site distribution in Er3+ co-doped upconverting alkaline earth fluoride nanoparticles using high-resolution absorption measurements. It is found that Yb3(+) single ion cubic (O-h) sites and preferentially formed clusters are the dominant centres for moderate dopant concentrations. At higher concentrations, and for larger particle sizes, a minority C-4v(F-) centre is also observed. Furthermore, the behaviour of Yb3+ cluster centres, whose primary absorption band is centred at 10205 cm(-1) (in near resonance with 980 nm laser diodes) was studied by tuning the metal cation in the host lattice from Ca2+ -> Sr2+ -> Ba2+. For co-doped materials, with heterogeneous cluster centres, Er3+ upconversion fluorescence was observed upon selective excitation of Yb3+, resulting in a strong visible emission from H-2(11/2), S-4(3/2) and F-4(9/2) multiplets. By monitoring the Er3+ fluorescence, whilst scanning a laser through the wavelength dependent Yb3+ absorption, we obtain an excitation spectrum for these heterogeneous cluster centres. Spectral features associated with excited state absorption as well as significant deviation from the linear absorption spectra are observed. (c) 2020 Elsevier B.V. All rights reserved.