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

The cationic charge increase on the tetrahedral garnet site shifts the ytterbium level energy towards higher values.

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 cations incorporated in the 24d tetrahedral site while very little sensitive to the substitutions made on the 16a octahedral one.

Li⁺ and Ge⁴⁺ fill exclusively tetrahedral garnet site with a strong reduction of 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 of the crystal vacancy density.

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 in the nearest to Yb³⁺ tetrahedral garnet sites of a vacancy (electric charge Q=0), Li⁺, Mg²⁺, Ga³⁺ and Ge⁴⁺, respectively. However, the incorporation of Ti⁴⁺ in the octahedral site substituting mainly Ga³⁺ has not observable effect on the Yb³⁺ 0↔0´ OA.