Growth of Yb³⁺, Lu³⁺, Gd³⁺ co-doped KY(WO₄)₂ thin layers

Rare-earth-ion-doped $KY(WO_4)_2 (KYW)$ is an important candidate for solid-state lasers. Its high refractive indices of the order of 2.0 make it attractive also for applications as integrated optical devices. Liquid phase epitaxy was employed for growing mono-crystalline KYW thin films co-doped with $Yb^{3+}$, $Lu^{3+}$, and $Gd^{3+}$. Concentrations of the optically inert co-dopants $Lu^{3+}$ and $Gd^{3+}$, which decrease or increase the KYW lattice parameters, respectively, were optimized for lattice matching with the KYW substrate. $Lu^{3+}$ was partially replaced by $Yb^{3+}$ to achieve active optical doping from a few up to 20%. Optimized growth conditions provided crack-free layers.