Infrared-to-ultraviolet up-conversion luminescence of YF3:Yb3+, Tm3+ microsheets

YF3:Yb3+,Tm3+ microsheets were synthesized by a simple hydrothermal method. The sheets structure was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Strong ultraviolet and blue up-conversion emissions were observed at 978 nm. The intensity dependence of up-conversion emission on pump power was measured, which shows that the populations of the states 1I6, 1D2 and 1G4 may come from five-photon, four-photon and three-photon energy transfer up-conversion processes, respectively.