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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.

The Yb3+ and Tm3+ co-doped 55.98ZrF4-28BaF2-2.5LaF3-4AlF3-7NaF-2.5YbF3-0.02TmF3 amorphous fluoride film was prepared by pulsed laser deposition. The spectroscopic properties and energy transfer analysis of this film were studies in detail. Ultraviolet and visible upconversion emissions were observed under the infrared excitation at 980 nm. In comparison with that of its target, the upconversion emissions of the film in the visible and ultraviolet range were greatly enhanced. The possible energy transfer mechanism of the emissions was given to understand the upconversion process. This kind of thin films has potential applications for the integrated optical waveguide amplifier and ultraviolet laser.

GdF3:0.23Yb3+, 0.005Tm3+, x%Li+ (x = 0-7) NIR to NIR upconversion nanocrystals (UCNPs) were synthesized by a hydrothermal method. Their XRD patterns show that they are all orthorhombic phase despite of different Li+ ion concentrations. The detailed analysis indicates that lithium ions substitute Gd3+ sites at x < 3. As the Li+ content increases, more Li+ ions enter host lattice interstitially. The doped Li+ ions affect the crystal field symmetry around Tm3+ ions, which results in the change of the irradiation transition probabilities between their corresponding transition levels. Compared with GdF3:0.23Yb3+, 0.005Tm3+, the NIR to NIR upconversion emission intensity of GdF3:0.23Yb3+, 0.005Tm3+, 0.03Li+ nanocrystals (excitation at 980 nm, emission at 808 nm) increases 2.2 times.