Energy transfer and frequency upconversion in Ho3+/Yb3+ co-doped bismuth-germanate glasses

We report an intense room-temperature green and red upconversion in Ho3+/Yb3+ co-doped xBi2O3–(90 − x)GeO2–10Na2O glasses upon excitation with a convenient 980 nm laser diode. Effect of Bi2O3 on upconversion properties of Ho3+/Yb3+ co-doped bismuth-germanate glasses have been investigated based on IR spectra. We find that the increase of Bi2O3 decreasing the phonon energy of host glasses. The upconversion intensity of green and red emissions in Ho3+/Yb3+ co-doped glasses is enhanced largely with increasing Bi2O3 content. Three intense emission bands were observed in Ho3+/Yb3+ co-doped BGN glasses centered at 549, 644 and 750 nm correspond to Ho3+: 5S2(5F4) → 5I8, 5F5 → 5I8 and 5S2(5F4) → 5I7 transitions, respectively. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms had been evaluated by a proper rate equation model.