Frequency upconversion in rare-earth doped fluoroindate glasses

Abstract

We present recent results on frequency upconversion (UPC) obtained in fluoroindate glasses (FIG) doped with Ho 3+ , Tm 3+ and Nd 3+ ions and codoped with Pr 3+ /Nd 3+ and Yb 3+ /Tb 3+ ions. The results for the Ho 3+ -doped samples show strong evidence of energy transfer (ET) between Ho 3+ 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 Tm 3+ -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 Pr 3+ /Nd 3+ were excited at 588 nm in resonance with transitions starting from the ground state of the Nd 3+ and the Pr 3+ ions. It was observed that the presence of Nd 3+ ions enhanced the Pr 3+ emission at 480 nm by two orders of magnitude. Multiphonon (MP)-assisted upconversion is also discussed for Nd 3+ -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 Nd 3+ 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 Yb 3+ /Tb 3+ -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 Tb 3+ emission in the blue is due to ET from a pair of excited Yb 3+ ions followed by ESA in the Tb 3+ ions.