Cubic NaYF4 single crystals co-doped with similar to 1.90 mol%Tm3+ and various Ho3+ concentrations were grown by Bridgman method. The energy transfer from Tm3+ to Ho3+ and the optimum fluorescence emission around 2.04 mu m of Ho3+ ion were investigated based on the measured absorption spectra, emission spectra, emission cross section and decay curves under excitation of 800 nm LD. The emission intensity at 2.04 mu m increased with the increase of Ho3+ concentration from 0.96 mol% to 3.89 mol% when the concentration of Tm3+ was held constantly at similar to 1.90 mol%. Moreover, the maximum emission cross section reached 1.06 x 10(-20) cm(2) and the maximum fluorescence lifetime was 2323 ms for Tm3+(1.90 mol%)/Ho3+ (3.89 mol%) co-doped one. According to the measured lifetime of Tm3+ single-doped and Tm3+/Ho3+ co-doped samples, the maximum energy transfer efficiency of Tm3+:F-3(4) level was 95.0%. Analysis on the fluorescence dynamics indicated that electric dipole-dipole is dominant for the energy transfer from Tm3+ to Ho3+. (C) 2015 Elsevier Ltd. All rights reserved.