The dominant role of excitation diffusion in energy transfer upconversion of Lu2O3: Tm3+, Yb3+

Emission spectra of Tm3+ and Yb3+ codoped Lu2O3 are studied as a function of the dopant concentration upon 980 nm excitation. The upconverted near infrared emission around 811 nm from Tm3+:3H4→3H6 transition is observed due to two step energy transfers from Yb3+ through Tm3+:3F4 as the intermediate level. The optimal dopants concentrations for the strongest upconversion emission are 0.1 mol% Tm3+ and 4 mol% Yb3+. The dependence of energy transfer coefficients for the two step energy transfers on Tm3+ and/or Yb3+ concentration is studied. The obtained results indicate that the excitation diffusions among Yb3+ ions and among Tm3+ ions mainly control the energy transfer rates for the optimal dopants concentrations and thus dominate the energy transfer upconversion process.