Conversion of infrared radiation into visible emission in NaY(WO4)2:Yb3+, Ho3+ crystal

The spectroscopic characteristics and fluorescence dynamics for Yb3+/Ho3+:NaY(WO4)2 crystal were investigated. The parameters of oscillator strengths, the spontaneous transition probabilities, the fluorescence branching ratios, the radiative lifetimes and the stimulated emission cross sections have been calculated based on Judd–Ofelt theory and Füchtbauer–Ladenburg method. The energy transfer efficiency from Yb3+ to Ho3+ was 65.85%. The green emission (530–570 nm) corresponding to (5F4, 5S2)→5I8 transition, red emission (640–670 nm) due to 5F5→5I8 transition and NIR emission (740–770 nm) attributed to (5F4, 5S2)→5I7 transition were observed on 974 nm excitation at room temperature. Under low pump power, the intensity of green light emission is weaker than that of the red light, while under high pump power, the case is on the contrary. The upconversion is based on the two-photon process either the energy transfer from Yb3+ ions or by the excited state absorption. The proposed mechanisms of upconversion emissions were provided.