For a better understanding of the outstanding real-time photorefractive medium ${\mathrm{LiNbO}}_3$:(Bi,Mg) and excitonic processes in similar systems in general, stoichiometric ${\mathrm{LiNbO}}_3$ (SLN) crystals grown by two different methods and doped by Bi, Fe, or (Bi,Mg) have been investigated using low-intensity near-UV irradiation and absorption spectroscopy. Tracking down the incorporation of dopants and impurities including Fe and H by chemical and IR-UV spectroscopic analyses we assign the various spectroscopic features to specific defect complex types. Absorption bands near 4 eV overlapping the UV band edge and polarized along Nb–O bonds are ascribed to the generation of self-trapped excitons (STEs) pinned to dipolar defects consisting of $\mathrm{B}{\mathrm{i}}^{3+}$ or $\mathrm{F}{\mathrm{e}}^{3+}$ ions and their charge compensators, becoming exceptionally long-lived in the presence of lone pairs on ${\mathrm{Bi}}^{3+}$ ions. Near-UV illumination induces persistent additional absorption bands at ≈2.7 and ≈3.4 eV in Bi-single-doped crystals which can be eliminated by a short annealing above $\approx 150{}^{\circ }\mathrm{C}$ or fade within a few months in the dark. These and similar photochromic bands in SLN:(Bi,Mg) are attributed to further excitations of pinned STEs, resulting in charge release. Shortening of the buildup time of the photochromic bands in (Bi,Mg)-codoped crystals is ascribed to the absence of ${\mathrm{Nb}}_{\mathrm{Li}}$ antisite complexes serving as exciton sinks in under-threshold LN. Due to its markedly different properties an additional stable narrow band at 3.24 eV always present in SLN:(Bi,Mg) is attributed to fast reversible charge transfer processes from ${\mathrm{O}}^{2–}$ to $\mathrm{B}{\mathrm{i}}^{5+}$ ions on the Nb site. Processes on widely differing timescales are predicted for one- and two-photon excitations involving different excited states, relevant for further optimization of the system.

Structural changes of Er- or Yb-ion doped LiNbO₃ (LN) nanocrystals were studied in relation to the high-energy ball milling process. The evolution of the size of the particles and the formation of different phases were followed by dynamic light scattering and X-ray diffraction measurements, while the electronic transitions of rare-earth (RE) ions were investigated by absorption spectroscopy in the infrared spectral range. During the milling process, RE ions left the crystal lattice and an RE₂O₃ phase appeared to an increasing extent next to the LN. The change in the absorption spectra and the phases formed during the grinding process were found to be very similar for both investigated RE ions and were independent of their original concentration in the starting crystal samples. The extent of the RE loss was found to be 90% after 100 min of wet grinding.

The energy levels of Dy³⁺ ions have been determined in lithium yttrium borate (Li₆Y(BO₃)₃) single crystals in a wide spectral range between 3000 and 40,000 cm⁻¹ using optical absorption and luminescence spectroscopy, which also allow for an analysis of the ground state. The crystal field splittings of the ⁶H_15/2 ground state and all excited states up to the ⁴F_7/2 manifold were obtained at a low temperature, based on luminescence (T = 4.2–78 K) and absorption (T = 8–100 K) measurements, respectively. The numbers of experimentally observed Stark sublevels are in agreement with those expected theoretically for Dy³⁺ ions occupying a single low symmetry (C₁) site.

The results of a series of pump–probe spectral hole-burning experiments are presented on Yb³⁺- or Er³⁺-doped Li₆Y(BO₃)₃ (LYB) single crystals in the temperature range of 2–14 K and 9–28 K, respectively. The spectral hole has a complex structure for Yb³⁺ with superposed narrow and broad bands, while a single absorption hole has been observed for Er³⁺. Population relaxation times (T₁) at about 850 ± 60 μs and 1010 ± 50 μs and dipole relaxation times (T₂) with values of 1100 ± 120 ns and 14.2 ± 0.3 ns have been obtained for the two components measured for the Yb³⁺:²F_7/2—²F_5/2 transition. T₁ = 402 ± 8 μs and T₂ = 11.9 ± 0.2 ns values have been found for the Er³⁺:⁴I_15/2—⁴I_11/2 excitation. The spectral diffusion rate at about 1 and 5 MHz/ms has been determined for the narrow and broad spectral line in Yb³⁺-doped crystal, respectively. The temperature dependence of the spectral hole halfwidth has also been investigated.