Tailoring Yb3+ energy levels by local configuration in the garnet structure: the case of Ca3(NbGa□)5O12 laser single crystals as a model for Yb:YAG

Abstract

Crystallographic information on the cationic interactions that may broaden the Yb³⁺ optical bands in garnets for sustaining shorter femtosecond laser pulses is provided. It is shown that the energy of the Yb^{3+ 2}F_7/2(0) ↔ ²F_5/2(0′) optical absorption (0 ↔ 0′ OA), measured at a cryogenic temperature (T = 6 K) in Ca₃(NbGa□)₅O₁₂ (CNGG)-type disordered single-crystal garnets, is sensitive to the electric charge of the cations incorporated in the 24d tetrahedral site, while it is much less sensitive to substitutions made on the 16a octahedral site. CNGG single-crystals nominally doped with 8 at% of Yb and further modified with other optically silent cations were grown by the Czochralski method. Li⁺, Mg²⁺, Ge⁴⁺, and Ti⁴⁺ were successfully incorporated into the crystals. Further, Yb-doped CNGG crystals grown in the composition limits for congruent melting were studied to explore the effect of a change in the cationic vacancy density. The lattice position of the three latter cations is unequivocally determined by the combined analyses of X-ray absorption spectroscopy (XAS) and single-crystal X-ray diffraction (scXRD) refinements. Li⁺ and Ge⁴⁺ exclusively fill the tetrahedral garnet site, with a strong reduction in the cationic vacancy density monitored in the Li case by positron annihilation lifetime spectroscopy (PALS). Ti⁴⁺ sits exclusively in the octahedral garnet site with no effect on the crystal cationic vacancy density, while Mg²⁺ is distributed in the three garnet sites with preference for the tetrahedral one and a moderate reduction in the crystal vacancy density. The Yb³⁺ 0 ↔ 0′ OA bands observed at λ = 973 nm, 971.9 nm, 971.3 nm, 971.1 nm and 969.6 nm are correlated with the presence of a vacancy (electric charge Q = 0), Li⁺, Mg²⁺, Ga³⁺ and Ge⁴⁺ in the tetrahedral garnet sites nearest to Yb³, respectively. However, the incorporation of Ti⁴⁺ in the octahedral site substituting mainly Ga³⁺ has no observable effect on the Yb³⁺ 0 ↔ 0′ OA. The elimination of tetrahedral cationic vacancies slightly reduces the Yb³⁺ 300 K luminescence bandwidth in the Li⁺- and Mg²⁺-modified CNGG crystals, while the new energy levels associated with the tetrahedral Ge⁴⁺ compensate for this effect and slightly increase it.