Investigation of the structure, optical properties and Cr4+ conversion level of Yb3+ and Cr3+ codoped YAG transparent ceramics

In this paper, the Cr4+ conversion level and optical characteristic of YAG transparent ceramics doped with various Cr3+ (0-0.25 at.%), Yb3+ (0-10 at.%) and Ca2+ (0-1.5 wt%) concentrations are investigated by morphology, XPS, EPR and spectral spectroscopy. It is determined that Yb ions doping will not only induce double R lines formation and asymmetry structure, but also decrease the conversion efficiency of tetrahedrallycoordinated Cr4+ ions in the Yb,Cr:YAG ceramics; these are both due to the emergence of Yb2+/Ca2+-F+ composite defect centers during vacuum sintering. The underlying mechanisms by which the cation doping influence the luminescence properties and affect the conversion efficiency of Cr4+ ion are discussed in detail using Huang Rhys factor and theoretical calculation. It is also indicated that excessive CaO additives can improve micro structural homogeneity in Yb,Cr:YAG transparent ceramics; while barely increase the conversion efficiency of Cr4+ ions. A high-quality Yb,Cr:YAG transparent ceramic with CaO as a sintering aid is presented, with an optical transmittance of 84.5% at a wavelength of 1.2 mu m, which is close to the single crystal transmittance value. This materials shows promise for passively self-Q-switched solid-state laser applications.