Near-Infrared Light Excited Highly Pure Green Upconversion Photoluminescence and Intrinsic Optical Bistability Sensing in a Ho3+/Yb3+ Co-Doped ZnGa2O4 Phosphor through Li+ Doping

We report on highly pure green upconversion (UC) photoluminescence in a Ho3+/Yb3+/Li+ co-doped ZnGa2O4 phosphor prepared by the solid-state reaction method for the first time. The crystallite size, dislocation density, and microstrain values show the crystalline nature of the phosphors. The scanning electron micrographs show shapes and sizes of the phosphor, and they change in the presence of Li+ ions. The energy-dispersive X-ray spectroscopic analysis reveals the presence of Ga, Zn, Ho, Yb, and O elements in the phosphor. Fourier transform infrared studies show vibrational bands because of Zn-O and Ga-O groups. The diffuse reflectance spectra contain large number of absorption bands of Ho3+ and Yb3+ ions. The optical band gap of the phosphor sample slightly decreases through Li+ doping. The Ho3+/Yb3+ co-doped phosphor shows intense green along with weak blue, red, and near-infrared UC emissions excited at 980 nm. The intensity of green UC emission at 537 nm is a dominant one. The emission intensity of the Ho3+-doped phosphor is enhanced upto 372 times via Yb3+ doping, which further enhanced upto 966 times via Li+ doping. It is because of the increase in the local crystal structure and particle size. The spectral color purity (S-gr) is achieved as 0.98 in the phosphor. The lifetimes of the F-5(4) state of Ho3+ ions are increased in the presence of Li+ ions. Interestingly, the variation of pump power gives rise to intrinsic optical bistability sensing in the Ho3+/Yb3+/Li+ co-doped ZnGa2O4 phosphor, and it gives efficient sensing for the green emission. Thus, the Ho3+/Yb3+/Li+ co-doped ZnGa2O4 phosphor may be used in green-emitting sources, UC-based devices, optical memory devices, and optically bistable devices.