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

Monoclinic crystals of KY(WO4)2 (KYW) doped with different rare-earth ions are among the highly promising materials for building compact solid-state lasers. We report the liquid phase epitaxy (LPE) growth of 3-5 µm thick KYW:Gd3+, Lu3+, Yb3+ layers for Yb3+ concentrations of 1.2, 1.7, and 2.4 mol% and 30 to 40-µm-thick KYW:Gd3+, Yb3+ (20 mol%) layers. The concentration of the dopants Yb3+, Lu3+, and Gd3+ in the grown film were determined by laser ablation inductively coupled plasma - mass spectrometry (LA-ICP-MS). The growth conditions were optimized, leading to crack-free layers for all investigated Yb3+ concentrations. X-ray investigations have confirmed the high crystallinity of the films. Based on the Gd3+, Lu3+ co-doped thin films, planar waveguide lasers operating on the Yb3+ transition at 1025 nm were demonstrated. Due to the co-doping, resulting in high refractive-index difference between film and substrate, very thin waveguides with strong light confinement were obtained, thus allowing for a pump threshold of laser operation as low as 18 mW. The highest slope efficiency versus absorbed pump power and output power were 82.3% and 195 mW, respectively.