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.