Polarization study and crystal-field analysis of the laser-selective excitation spectra of ${\mathrm{Ho}}^{3+}$ ions in ${\mathrm{CaF}}_2$ and ${\mathrm{SrF}}_2$ crystals

The polarized fluorescence of a total of 12 ${\mathrm{Ho}}^{3+}$ ion centers present in ${\mathrm{CaF}}_2$- and ${\mathrm{SrF}}_2$-type crystals and having a variety of ${\mathrm{Ho}}^{3+}$ ion symmetries has been analyzed to give possible model assignments. In contrast to the earlier results for ${\mathrm{SrF}}_2$:${\mathrm{Er}}^{3+}$, which has a dominant trigonal symmetry center, the principal center in ${\mathrm{SrF}}_2$:${\mathrm{Ho}}^{3+}$ is determined to have tetragonal symmetry. Crystal-field analyses are reported for both the tetragonal and trigonal symmetry centers, together with fluorescence-lifetime data for these centers.