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We report Zeeman infra-red spectroscopy of electronic-nuclear levels of $^5$I$_8 \rightarrow ^5$I$_7$ transitions of Ho$^{3+}$ in the C$_{\rm 4v}$(F$^-$) centre in CaF$_2$ with the magnetic field along the $\langle 111\rangle$ direction of the crystal. Transitions to the lowest $^5$I$_7$ state, an isolated electronic doublet, and the next group of states, a pseudo-quadruplet consisting of a doublet and two nearby singlets, exhibit strongly non-linear Zeeman splittings and intensity variations. Simulated spectra based upon a crystal-field analysis give an excellent approximation to the data, illustrating the strong predictive ability of the parametrised crystal-field approach. Anti-crossings in the hyperfine splittings, the basis of quantum information storage in rare-earth doped insulating dielectrics, are also predicted.

High-resolution Zeeman spectroscopy of electronic-nuclear hyperfine levels of ${^5\mathrm{I}_8}\rightarrow{^5\mathrm{I}_7}$ transitions in Ho$^{3+}$:Y$_2$SiO$_5$ is reported. Crystal-field parameters determined for the two $C_1$ symmetry sites in Er$^{3+}$:Y$_2$SiO$_5$ are successfully used to model the Zeeman-hyperfine data, including the prediction of avoided crossings between hyperfine levels under the influence of an external magnetic field. The two six- and seven-coordinate substitutional sites may be distinguished by comparing the spectra with crystal-field calculations.

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$^{2+}{\rightarrow}$Sr$^{2+}{\rightarrow}$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$_{3/2}{\rightarrow}^4$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.

Zeeman spectroscopy is used to demonstrate that phenomenological crystal-field parameters determined for the two $C_1$ point-group sites in Er$^{3+}$:Y$_2$SiO$_5$ may be transferred to other ions. The two crystallographic six- and seven-coordinate substitutional sites may be distinguished by comparing the spectra with crystal-field calculations.

We present a comprehensive spectroscopic study of C$_{2v}$ point-group symmetry sites in Sm$^{3+}$:CaF$_2$/SrF$_2$ codoped with either NaF or LiF. Data includes electron paramagnetic resonance measurements of Zeeman and hyperfine interactions for the ground state, as well as site-selective excitation and fluorescence spectroscopy up to the $^{4}$G$_{5/2}$ multiplet. Inclusion of the EPR data allowed us to determine unique crystal-field parameters. The parameters provide information about the geometry of the sites and the nature of the interactions between the Sm$^{3+}$ dopant and the alkaline earth co-dopant.

We show that crystal-field calculations for $C_1$ point-group symmetry are possible, and that such calculations can be performed with sufficient accuracy to have substantial utility for rare-earth based quantum information applications. In particular, we perform crystal-field fitting for a C$_1$-symmetry site in $^{167}$Er$^{3+}$:Y$_2$SiO$_5$. The calculation simultaneously includes site-selective spectroscopic data up to 20,000 cm$^{-1}$, rotational Zeeman data, and ground- and excited-state hyperfine structure determined from high-resolution Raman-heterodyne spectroscopy on the 1.5 $\mu$m telecom transition. We achieve an agreement of better than 50 MHz for assigned hyperfine transitions. The success of this analysis opens the possibility of systematically evaluating the coherence properties, as well as transition energies and intensities, of any rare-earth ion doped into Y$_2$SiO$_5$ ., Comment: 6 pages, plus 5 pages in supplementary information, 4 figures total

We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF$_2$:Yb$^{2+}$. The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation., Comment: Minor corrections. arXiv admin note: text overlap with arXiv:1107.4888v2