Polarized laser-selective excitation and Zeeman infrared absorption ofSm3+centers inCaF2andSrF2crystals

Abstract

Laser-selective excitation and fluorescence and Zeeman infrared-absorption spectroscopy have been employed to study ${\mathrm{Sm}}^{3+}$ centers in doped ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$ crystals. The dominant center present in both host crystals has been determined to consist of a ${C}_{4v}$ symmetry ${\mathrm{Sm}}^{3+}{\ensuremath{-}\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ pair. A weakly fluorescing, nonlocally charge-compensated ${\mathrm{Sm}}^{3+}$ ion center has also been observed. These centers have cubic symmetry. In ${\mathrm{CaF}}_{2}{:\mathrm{S}\mathrm{m}}^{3+},$ absorption features at 17 809 and 17 828 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ yield no visible fluorescence upon resonant excitation. These transitions are associated with anion excess dimer and trimer centers of which the ${\mathrm{Sm}}^{3+}$ optical excitation is entirely quenched through nonradiative cross relaxation energy transfer. Codoping the crystals with ${\mathrm{La}}^{3+},$ ${\mathrm{Ce}}^{3+},$ ${\mathrm{Gd}}^{3+}$ or ${\mathrm{Tb}}^{3+}$ ions creates heterogeneous clusters from which ${\mathrm{Sm}}^{3+}$ fluorescence is observed. However, in the case of codoping with ${\mathrm{Eu}}^{3+},$ efficient and complete ${\mathrm{Sm}}^{3+}{(}^{4}{\mathrm{G}}_{5/2}{)\ensuremath{\rightarrow}\mathrm{Eu}}^{3+}{(}^{5}{\mathrm{D}}_{0})$ energy transfer is observed through multi-phonon-assisted processes. Treating the crystals with oxygen yields ${C}_{3v}$ symmetry centers associated with ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ charge compensation. Crystal- and magnetic-field analyses of the ${O}_{h}$ and ${C}_{4v}$ symmetry centers gives optimized crystal-field parameters which are consistent with those of other rare-earth ions and which well account for the magnetic splitting measured by Zeeman infrared absorption.