Improved ultraviolet upconversion emissions of Ho³⁺ in hexagonal NaYF₄ microcrystals under 980 nm excitation

Under 980 nm excitation, enhanced ultraviolet (UV) upconversion (UC) emissions at 242.4 nm, 276.1 nm, 289.7 nm, 296.4 nm, 303.6 nm, 357.7 nm and 387.8 nm of Ho³⁺ ions were observed in β-NaYF ₄:20%Yb³⁺, 1.5%Ho³⁺ microcrystals (MC) which were synthesized through a hydrothermal method. The results indicated that these UV emissions came from five- and four-photon UC processes. Dynamical analysis on Ho³⁺ excited states suggests that, for excited Ho³⁺ ions, the higher the energy level is, the shorter the lifetime is.