Strong dependence of upconversion luminescence on doping concentration in holmium and ytterbium co-doped Y2O3 phosphor

Under the excitation of 980 nm diode laser, intense green emission (F-5(4)+S-5(2)-I-5(8)) of Ho3+ was observed in Ho3+ and Yb3+ co-doped cubic Y2O3. The doping concentration and laser power dependence of the upconverted emission were studied. The decay curves of F-5(4)+S-5(2) emission of Ho3+ under the excitation of 355 nm pulse laser were measured to investigate the energy transfer process between Ho3+ and Yb3+. The results indicated that two-photon process was responsible for the upconversion (UC) emission. The Ho3+ concentration of 0.04 mol.% and the Yb3+ concentration of 5 mol.% were determined to be the best value for the strongest Ho3+ emission under the excitation of 980 nm light. The cross-relaxation between two neighboring Ho3+ ions and the back energy transfer from Ho3+ to Yb3+ were important factors for determining the optimal doping concentration. This material was a promising candidate for the application in biomedical fluorescent labels for the intense green emission upon excitation of near-infrared (NIR) light.