Visible upconversion emission in Er3+-doped GeO2-PbO-PbF2 glass

The upconversion fluorescence emission of Er3+ -doped 60GeO(2)-20PbO-20PbF(2) glass was experimentally investigated under the pump of 976-nm laser diode. The results reveal the existence of intense emission bands centred around 524, 545, and 657 nm at room temperature. The green emission at 524 and 545 nm is due to the S-4(3/2) + 2H(11/2) --> I-4(15/2) transition and the red emission of 657 nm originates from the F-4(9/2) --> I-4(15/2) transition of Er3+. The quadratic dependence of the green and red emissions on excitation power indicates that a two-photon absorption process occurs under the 976-nm excitation. The excited- state absorption from I-4(11/2) and the cross relaxation between two Er3+ ions in the I-4(11/2) state contribute to the green emission. The red emission at 657nm is attributed to the excited-state absorption and cross relaxation processes in the I-4(13/2) level as well as the S-4(3/2) level nonradiative transition of Er3+.