Low-threshold, highly efficient Gd³⁺, Lu³⁺ co-doped KY(WO₄)₂:Yb³⁺ planar waveguide lasers

High-quality crystalline $KY(WO_4)_2:Yb^{3+}$ layers codoped with large concentrations of optically inert Gd3+ and Lu3+ ions were grown by liquid phase epitaxy. Continuous-wave laser operation in a planar waveguide configuration was observed at 1025 nm. For 23% output coupling, a maximum output power of 195 mW was obtained; a slope efficiency of 82.3% was derived, which to the best of our knowledge is the highest value yet reported for a planar waveguide laser to date. For 1.7% output coupling, a laser threshold as low as 18 mW of absorbed pump power was achieved, which is largely due to the enhanced refractive index contrast of ~$7.5\times 10^{-3}$ between the active layer and the undoped $KY(WO_4)_2$ substrate attained by the co-doping, leading to well-confined pump and laser modes