Effect of Li+ on frequency upconversion and intrinsic optical bistability of Ho3+/Yb3+ co-doped gadolinium tungstate phosphor

This paper reports frequency upconversion and intrinsic optical bistability in the Ho3+/Yb3+ co-doped gadolinium tungstate phosphor. The structural analyses confirm the formation of two monoclinic crystalline phases iso-structural to Gd2WO6 and Gd-2(WO4)(3) with the space groups I2/a and C2/c, respectively. The phase fraction of the two monoclinic phases varies with Li+ concentrations. The Ho3+/Yb3+ co-doped phosphors show intense upconversion (UC) emission on excitation with 976 rim. Addition of Li+ ion enhances the UC emission intensity of Ho3+/Yb3+ co-doped phosphors significantly. The mechanisms involved in UC emissions have been discussed using pump power dependence analysis and energy level diagram. A hysteresis loop has been observed for Ho3+/Yb3+ co-doped phosphors in increasing and decreasing pump power versus emission intensity curves at room temperature. The polarity of the hysteresis loop is found to be reversed on addition of Li+ in the Ho3+/Yb3+ co-doped phosphors. These phosphors can be a potential candidate for optical switching devices.