Frequency upconversion in rare-earth doped fluoroindate glasses

We present recent results on frequency upconversion (UPC) obtained in fluoroindate glasses (FIG) doped with Ho³⁺, Tm³⁺ and Nd³⁺ ions and codoped with Pr³⁺/Nd³⁺ and Yb³⁺/Tb³⁺ ions. The results for the Ho³⁺-doped samples show strong evidence of energy transfer (ET) between Ho³⁺ ions resonantly excited at 640 nm. The origin of the blue-green upconverted fluorescence observed was identified and the dynamics of the signals revealed the pathways involved in the UPC process. In the case of Tm³⁺-doped FIG, the samples were resonantly excited at 650 nm and the main mechanism that contributes for the red-to-blue upconversion is excited-state absorption (ESA). The FIG samples codoped with Pr³⁺/Nd³⁺ were excited at 588 nm in resonance with transitions starting from the ground state of the Nd³⁺ and the Pr³⁺ ions. It was observed that the presence of Nd³⁺ ions enhanced the Pr³⁺ emission at 480 nm by two orders of magnitude. Multiphonon (MP)-assisted upconversion is also discussed for Nd³⁺-doped FIG pumped at 866 nm. Emission at 750 nm with a peculiar linear dependence with the laser intensity was observed and explained. A rate-equation model that includes MP absorption via thermally coupled electronic excited states of Nd³⁺ was developed and describes well the experimental results. The role played by effective phonon modes is clearly demonstrated. MP-assisted UPC process was also studied in Yb³⁺/Tb³⁺-codoped FIG samples excited at 1064 nm, which is off-resonance with electronic transitions starting from the ground state. It was determined that the mechanism leading to Tb³⁺ emission in the blue is due to ET from a pair of excited Yb³⁺ ions followed by ESA in the Tb³⁺ ions.