Highly efficient lasers and amplifiers in double tungstates

The double tungstates KY(WO4)2, KGd(WO4)2, and KLu(WO4)2 are excellent candidates for solid-state lasers because of the large transition cross-sections of optically active rare-earth ions doped into these hosts. We grow actively doped KY1-x-yGdxLuy(WO4)2 layers onto KY(WO4)2 substrates by liquid-phase epitaxy. Co-doping the layers with optically inert Gd3+ and Lu3+ ions simultaneously allows for lattice matching and enhanced refractive index contrast with respect to the substrate. Low-loss channel waveguides are microstructured into the layers by Ar+-beam etching, resulting in strong pump- and signal-mode confinement. Yb-doped channel waveguide lasers deliver 650 mW output power at 1 µm. Record-high slope efficiency (85%) and record-low quantum defect (0.7%) for dielectric lasers are achieved. In pump-signal experiments, exploiting highly doped KGd0.447Lu0.078Yb0.475(WO4)2 channel waveguides, we demonstrate a giant optical gain of 950 dB/cm, exceeding the gain previously reported in rare-earth-ion-doped materials by two orders of magnitude and comparable to the gain obtained in semiconductor optical amplifiers.