Upconversion luminescence of Tm3+ sensitized by Yb3+ ions in monoclinic KGd(WO4)2 single crystals

We observed a significant blue emission at 476 nm in Tm3+ and Yb3+-codoped KGd(WO4)2 monoclinic single crystals by pumping at 940 nm. The single crystals were grown using the top-seeded-solution-growth slow-cooling method (TSSG). We also report thulium emissions at 650 nm, 802 nm and 1.48 μm. The emission spectra are characterized as a function of the Tm3+ and Yb3+ ions concentration and temperature from 6 K to room temperature. The energy upconversion excitation mechanism for thulium emitting levels is assigned to a cooperative energy transfer process by the ytterbium-sensitizer ions.