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

Abstract 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 (Oh) sites and preferentially formed clusters are the dominant centres for moderate dopant concentrations. At higher concentrations, and for larger particle sizes, a minority C4v(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 2H11/2, 4S3/2 and 4F9/2 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.