Down-conversion in ytterbium-doped NaGd(MoO4)2 crystals

A series of NaGd(MoO4)2 single crystals doped with various Yb3+ concentrations (from 0.5 to 30 at %) is grown. The optical absorption spectra of the crystals are measured within the range from 800 to 1200 nm and used to calculate the actual concentrations of Yb in the samples. The luminescence of crystals is studied under UV excitation. It is found that there exist two types of active centers that are not directly related to Yb3+ ions and exhibit broadband luminescence in the visible spectral region. In addition, UV excitation of crystal gives rise to Yb3+ luminescence in the region of 1 μm owing to nonradiative excited-state energy transfer from, probably, donor centers of the host to ytterbium. The character of the dependence of the UV-excited luminescence of Yb on its concentration in the samples, in comparison with the corresponding dependence measured under IR excitation, points to the occurrence of cooperative down-conversion in crystals with high ytterbium concentrations, which means that the excited-state energy of donors is simultaneously transferred to two Yb3+ ions. In other words, excitations are multiplied; i.e., the quantum yield of this down-conversion exceeds unity.