Upconversion luminescence and mechanisms of Nd3+/Ho3+/Yb3+ triply doped TeO2-K2O-Nb2O5 glasses

Nd3+/Ho3+/Yb3+ triply doped TeO2-K2O-Nb2O5 glasses were synthesized by the conventional melting-quenching technique. Single green upconversion (UC) luminescence centered at 532 nm, corresponding to the (5F4, 5S2)→5I8 transitions of Ho3+, was observed under 800 nm excitation. However, green and red emission bands centered at 547, 662 and 758 nm, corresponding to the (5F4, 5S2)→5I8, 5F5→5I8 and (5F4, 5S2)→5I7 transitions of Ho3+, respectively, were simultaneously observed under 980 nm excitation. The dependence of UC luminescence intensity of Ho3+ under 800 and 980 nm excitations on Yb3+ concentration was studied. It was found, under 800 nm excitation, the 532 nm emission intensity of Ho3+ begin to reduce when the Yb3+ concentration is higher than 0.8 mol%. However, at the same concentration of Yb3+, the emission quenching was not observed under 980 nm excitation. The UC mechanisms under two pumping sources were discussed in terms of the experiment results. It was found that the UC luminescence of Ho3+ mainly depends on ET under 800 nm excitation; however under 980 nm excitation, it mainly depends on ESA.