Upconversion photoluminescence in GeO2-PbO glass codoped with Nd3+ and Yb3+

Frequency upconversion (UC) photoluminescence (PL) in GeO2-PbO glass codoped with trivalent ions of neodymium (Nd3+) and ytterbium (Yb3+) is reported. A diode laser operating at 977.7 nm, in resonance with the ytterbium transition 2F7/2 → 2F5/2, was the excitation source. Four PL spectral lines, corresponding to the Nd3+ transitions 4G9/2 → 4I9/2 (at 500 nm), 4G7/2 → 4I9/2 (at 550 nm), [4G5/2; 2G7/2] → 4I11/2 (at 595 nm) and 4G7/2 → 4I13/2 (at 660 nm), were observed and characterized. The quadratic dependence of the PL intensities versus the laser power indicates that two laser photons participate in the UC process. The temperature dependence of the PL emissions from 300 to 390 K was also investigated to identify the contribution of phonons for the UC process. The dependence of the UC intensity with the Yb3+ concentration and the time behavior of the UC signal indicated the presence of two energy transfer (ET) pathways involving Nd3+-Yb3+ pairs and Yb3+-Nd3+-Yb3+ triads. Rate-equations for the population densities of the rare-earth energy levels were used to describe the dynamics of the UC emission and to determine the ET rates.