Tunable upconversion emission in NaLuF(4)–glass-ceramic fibers doped with Er(3+) and Yb(3+)

Novel glass-ceramic optical fibers containing NaLuF(4) nanocrystals doped with 0.5ErF(3) and 2YbF(3) (mol%) have been prepared by the rod-in-tube method and controlled crystallization. NaLuF(4) nanocrystals with a size around 20 nm are obtained after heat treatment at 600 °C. Intense upconverted green and red emissions due to ((2)H(11/2), (4)S(3/2)) → (4)I(15/2) and (4)F(9/2) → (4)I(15/2) transitions, respectively, together with a blue emission due to (2)H(9/2) → (4)I(15/2) transition have been observed under excitation at 980 nm. The intensity of the green and red upconversion bands shows a nearly linear dependence on the excitation power which can be explained by saturation effects in the intermediate energy states and proves that a sensitized energy transfer upconversion process is responsible for the population of the emitting levels of Er(3+) ions. The upconversion emission color changes from yellow to green by increasing the excitation power density which allows to manipulate the color output of the Er(3+) emission in the glass-ceramic fibers. The tunable emission color is easily detected with the naked eye. This interesting characteristic makes these glass-ceramic fibers promising materials for photonic applications.