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

High-order ultraviolet (UV) upconversion (UC) emissions of Gd³⁺ and Er³⁺ ions were observed in NaYF₄:Yb ³⁺/Gd³⁺/Er³⁺ 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 Er³⁺ → Gd³⁺ played crucial roles in populating the excited states of Gd³⁺ ions. Experiments on concentration variation and dynamic analysis revealed the ET processes between Er³⁺ and Gd³⁺ in detail. Some of possible population routes for populating excited Gd³⁺ ions were proposed based on spectral and dynamic analysis.