Spectroscopic properties of KGd(WO₄)₂d single crystals doped with Er³⁺, Ho³⁺, Tm³⁺ and Yb³⁺ ions: Luminescence and micro-Raman investigations

KGd(WO₄)₂ single crystals doped with Ho ³⁺/Yb³⁺/Er³⁺ and Ho³⁺/Tm ³⁺/Yb³⁺/Er³⁺ ions were grown by means of the Top Seeded Solution Growth (TSSG) method. It was shown that these systems exhibit multicolor up-conversion luminescence in the visible and near-infrared spectral ranges under 980 nm laser excitation. In the investigated crystals the Yb³⁺ ions were used as the energy sensitizer ion and the upconverted luminescence originates from transitions between the energy levels of other co-doped rare earth ions such as Er³⁺, Ho³⁺ or Tm ³⁺, which play the role of the energy activator ions. The vibrational properties of the crystals were investigated using micro-Raman spectroscopy and the role of vibrational energy in the effective energy transfer process between the rare earth ions was discussed. The proposed combinations of activator ions may be suitable for gaining novel materials with effective green, red and near-infrared emission.