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 beta-NaYF4:20%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.