This paper focuses on the kinetics of Cr4+ formation in Cr,Ca:YAG ceramics prepared by solid-state reaction sintering. The kinetics of Cr4+ formation was studied by annealing of Cr,Ca:YAG ceramics in ambient air under different temperatures at different times, resulting in the transformation of Cr3+ to Cr4+. The activation energy (Ea) of Cr3+ oxidation determined by the Jander model was 2.7 ± 0.2 eV, which is in good correlation with the activation energy of innergrain oxygen diffusion in the YAG lattice. It is concluded that Cr3+ to Cr4+ transformation in YAG ceramics is limited by oxygen diffusion through the grain body. It was established that in Cr,Ca:YAG ceramics, the intralattice cation exchange, in which the Cr4+ ions exchange positions with the Al3+ ions, switching from “A” to “D” sites, is faster than Cr3+ to Cr4+ oxidation. In the temperature range of 900–1300 °C, the reaction enthalpy of Al3+/Cr4+ ion exchange between octahedral “A” and tetrahedral “D” lattice sites is close to zero, and this exchange ratio is thermodynamically driven by entropy.

Transparent cobalt‐doped magnesium aluminate spinel (Co:MgAl2O4) ceramics with a submicrometer grain size were prepared by spark plasma sintering. For the first time, the nonlinear absorption of Co:MgAl2O4 transparent ceramics was experimentally demonstrated. Both ground state absorption (σGSA) and excited state absorption (σESA) were estimated using the solid‐state slow saturable absorber model based on absorption saturation measurements performed at 1.535 μm. σGSA and σESA for 0.03 at.% Co:MgAl2O4 were found to be 4.1 × 10−19 cm2 and 4.0 × 10−20 cm2, respectively. In the case of 0.06 at.% Co:MgAl2O4 ceramics, σGSA = 2.6 × 10−19 cm2 and σESA= 5.3 × 10−20 cm2 were determined.

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Solid state lasers with a planar Nd,Lu,Ga:YAG/YAG waveguide structure have been created using liquid phase epitaxy. The method used to produce such epitaxial structures ensures obtaining Nd3+ ions concentration in the range from 1 at.% to 3 at.% in waveguiding thin films. Spectroscopic characterization of active films was performed. The Nd,Lu,Ga:YAG waveguide for each neodymium ions concentration exhibited lasing operation at room temperature at a wavelength of 1064 nm with impulse laser diodes pumping at 808 nm and 885 nm. The maximum average output power Pav = 2.6 mW and 72.2% slope efficiency was obtained for 1 at.% Nd3+ waveguide pumped at 885 nm.