Strong Visible Upconversion in Rare Earth Ion-Doped NaYF4 Crystals

NaYF4:Er3+, Yb3+ crystals were prepared by simple synthetic method. Under 980 nm laser excitation, 408 nm, 539 nm and 655 nm upconversion signals were recorded. Laser power and signal intensities of the upconverted emissions were obtained to understand the upconversion mechanisms.