Is There Segregation of Rare Earth Ions in Garnet Optical Ceramics?

Abstract

Research on advanced optical materials for a large variety of applications is always increasing. As an example, we can note high progress in solid-state laser sources like laser-diode (LD) – pumped solid-state lasers (DPSSL) including developments of new materials and high-power laser diode led to high-power and tuneable solid-state lasers. A wide variety of materials has been studied to develop more efficient and high power microchip lasers [1]. In end-pumping schemes, in particular, materials with a short absorption length for the LD pump beam are strongly anticipated for highly efficient operations because of the excellent match between the mode and pump beam profiles. High Nd³⁺ concentrations were so considered such as NdP₅O14, LiNdP₄O₁₂ (LNP), and NdAl₃(BO₃)O₄. However, crystal growths of these compositions are not so easy. Cubic crystals are much more researched. When looking at the literature for actual applications, we see immediately the importance of cubic garnet crystals for which dodecahedral (Y³⁺), octahedral (Al³⁺) and tetrahedral (Al³⁺) sites are considered as a reservoir for many activators like: Ce³⁺, Nd³⁺, Er³⁺, Tm³⁺, Ho³⁺, Yb³⁺ rare earth ions in dodecahedral symmetry sites and transition metal ions like Cr³⁺ in the octahedral symmetry sites or Cr⁴⁺ in the tetrahedral symmetry sites. Among garnet crystals, Y₃Al₅O₁₂ (YAG) host is the most used, commercially produced by the Czochralski method. However, in the case of the most used Nd³⁺: YAG laser crystal, the Nd³⁺ concentration that affects the performance in laser applications, is strongly limited to 0.2–1.4 Nd³⁺ at. % as a result of the segregation distribution coefficient [1].