Optical transitions and upconversion mechanisms in Er3+-doped heavy metal oxyfluoride germanate glass

Er3+-doped heavy metal oxyfluoride germanate glass suitable for developing potential optical amplifiers and upconversion lasers has been fabricated and characterized. Under 975 nm excitation, intense and broad 1533 nm infrared fluorescence and visible upconversion fluorescence were observed. For 1533 nm emission band, the full width at half maximum (FWHM) is 57 nm, the fluorescence lifetime is 5.7 ms, and the quantum efficiency is ∼100%. Efficient 529, 551, and 657 nm upconversion emissions correspond to the transitions 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2, respectively. Raman spectrum indicates the important role of fluoride ions in the formation of glass network. The dependence of upconversion intensities on excitation power involves two-photon absorption processes for the three emission bands. The possible upconversion mechanisms are discussed based on the energy matching conditions and the quadratic dependence on excitation power