Infrared to ultraviolet upconversion fluorescence of Gd³⁺ in β-NaYF₄ microcrystals induced by 1560 nm excitation

Under 1560 nm excitation, ultraviolet (UV) upconversion (UC) emissions of Gd³⁺ and Er³⁺ ions were observed in β-NaYF ₄:Yb³⁺/Gd³⁺/Er³⁺ microcrystals, which were synthesized through a facile EDTA-assisted hydrothermal method. Experimental analysis exhibited that these UV emissions came from high-order UC processes. In the Yb³⁺-Gd³⁺-Er³⁺ codoped system, ground state absorption (GSA) and excited state absorption (ESA) of Er³⁺, ESA of Gd³⁺, energy transfers (ETs) from Yb ³⁺ to Er³⁺ and from Er³⁺ to Gd³⁺ ions worked simultaneously in populating these high-energy states of Er ³⁺ and Gd³⁺ ions.