Infrared-to-visible upconversion luminescence of Er³⁺ and Yb³⁺ co-doped germanate glass

Frequency upconversion into green and red luminescence in Er³⁺ and Yb³⁺ co-doped germanate glass pumped with a 978 nm diode laser was presented. With increasing the excitation density, the green upconversion emission intensity was enhanced relative to the red one. Based on the emission spectra and the excitation density dependence of the upconversion emission intensity, we attributed the change of the upconversion emission spectra to the enhancement of three-photon excitation process to produce green luminescence with increasing the excitation density. The effects of this three-photon process on the 1.54 μm emission were also discussed.