Sensitized high-order ultraviolet upconversion emissions of Gd3+ by Er3+ in NaYF4 microcrystals

Abstract High-order ultraviolet (UV) upconversion (UC) emissions of Gd 3+ and Er 3+ ions were observed in NaYF 4 :Yb 3+ /Gd 3+ /Er 3+ microcrystals under 980 nm excitation. These UC emissions came from six- and five-photon UC processes at low pump power range, which were confirmed by the pumping power dependences of UC fluorescence intensities. In these high-order UC processes, energy transfer (ET) processes of Er 3+  → Gd 3+ played crucial roles in populating the excited states of Gd 3+ ions. Experiments on concentration variation and dynamic analysis revealed the ET processes between Er 3+ and Gd 3+ in detail. Some of possible population routes for populating excited Gd 3+ ions were proposed based on spectral and dynamic analysis.