Probing energy transfer mechanism via the upconversion spectra of Tm3+/Yb3+:LiNbO3 by tri-doping with Ba2+ in different site occupations

Metal doping has been an efficient method to enhance upconversion (UC) luminescence, however, the relationship between fluorescence kinetics and site occupation of luminescence center has barely been revealed. To solve this issue, we have designed a tri-doping system of various Ba2+ doped Tm3+/Yb3+:LiNbO3 to investigate the energy transfer (ET) rate and the site occupation quantitatively, where 4.8 and 7.9 folds enhancement of blue and red emission has been achieved, respectively. Owing to the sensitive 4f-4f transition between lanthanide ions, the ion environment could be presented vividly by the zigzag-shaped UC intensity variation for different Ba2+ doping concentrations. Then, the corresponding ET rate equations are established by steady-state rate equations which draw the site occupation process of lanthanide ions simultaneously. The calculated results for Föster ET rate, local optical power density, and pump power absorption illustrate that Ba2+ could adjust the ET rate by transforming the site of lanthanide ions in crystal lattice. More importantly, this method could shed light on the mechanism of UC enhancement by modulating ET rate via ion doping.