Upconversion of IR Radiation in a KGd(WO 4 ) 2 Crystal on Erbium and Thulium Ions in Trace Concentrations: Change of Excitation Schemes and Energy Transfer Through the Crystal Lattice

Data on the observation of simultaneous upconversion on trace erbium and thulium ions in a KGd(WO4)2 crystal excited by IR radiation with the wavelength near 806 nm are presented. The change of excitation schemes with an increase in pumping level and participation of the crystal matrix in the energy transfer from erbium to thulium ions are discussed.