Sensitized thulium blue and red upconversion emission and energy-transfer processes in Nd3+/Yb3+/Tm3+-codoped PbGeO3-based glass excited around 800 nm

This paper reports on the generation of bright blue and red upconversion luminescence through the sensitization of thulium ions in Nd³⁺/Yb³⁺/Tm³⁺ triply-doped 70%XoPbGeO₃:15%PbF₂:15%CdF₂-glass excited by a diode laser source around 800 nm. The upconversion fluorescence spectra of Tm³⁺ ions under 800 nm excitation is presented, for a sample codoped with Yb and a Nd³⁺/Yb³⁺/Tm³⁺ triply-doped sample. It is observed that the presence of Nd³⁺ in the system, produces a tenfold enhancement in the blue emission associated to the ¹G₄ - ³H₆ transition of Tm³⁺ ions, as compared to samples doped with Tm³⁺/Yb³⁺. The dependence of the upconversion emission intensity as a function of the excitation power was examined and a quadratic power law behavior was obtained indicating the participation of two pump photons in the upconversion excitation mechanism. The upconversion excitation process of the Tm³⁺ ion ¹G₄ emitting level was achieved by an initial energy-transfer process from the 800 nm excited Nd³⁺ to Yb³⁺. The excited Yb³⁺ then, transfer its energy to a Tm³⁺ ion at the ³H₄ level excited by a pump photon at 800 nm.