Absorption Spectra, Defect Site Distribution and Upconversion Excitation Spectra of CaF${}_2$/SrF${}_2$/BaF${}_2$:Yb${}^{3+}$:Er${}^{3+}$ Nanoparticles

Abstract

We report studies of the Yb${}^{3+}$ site distribution in Er${}^{3+}$ co-doped upconverting alkaline earth fluoride nanoparticles using high-resolution absorption measurements. Similar to bulk crystals, Yb${}^{3+}$ single ion cubic (O${}_h$), 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 Yb${}^{3+}$ cluster centres, whose primary absorption band is located around 10205 cm${}^{-1}$ (in near resonance with 980 nm laser diodes) was studied by tuning the metal cation in the host lattice from Ca${}^{2+}\to$Sr${}^{2+}\to$Ba${}^{2+}$. For co-doped materials, with heterogeneous cluster centres, Er${}^{3+}$ upconversion fluorescence was observed upon selective excitation of Yb${}^{3+}$, resulting in strong visible emission from ${}^4$H${}_{11/2}$, ${}^4$S${}_{3/2}$ and ${}^4$F${}_{9/2}$ multiplets. By monitoring the Er${}^{3+}$ ${}^4$S${}_{3/2}{\to }^4$I${}_{15/2}$ (540 nm) fluorescence, whilst scanning a laser through the wavelength dependent Yb3+ absorption, we obtain an excitation spectrum for these heterogeneous cluster centres. It is demonstrated that the group of Yb${}^{3+}$ ions residing near an Er${}^{3+}$ ion represent a small subset of the overall Yb${}^{3+}$ ensemble and therefore a linear absorption measurement is not necessarily a good indication of the optimum laser pump wavelength.