Short-range order around Er³⁺ in silica waveguides containing aluminium, titanium and hafnium

Er³⁺-doped silica waveguides, co-doped with aluminium, titanium, and hafnium oxides, were prepared using the sol-gel method and dip-coating processing. Here, we present a characterisation of the local environment around Er³⁺ ions, as determined from the Er L₃-edge extended X-ray absorption fine structure (EXAFS) measurements performed at ESRF (France). The first coordination shell is composed of 5-6 oxygen atoms at distances ∼2.32-2.35 Å, slightly varying as a function of the modifier oxide. Er³⁺ nearest neighbors distance does not show a significant compositional dependence. On the contrary, outer shells analysis shows significant differences: Al₂O₃ doping (less than 2% mol) induces an ordering around Er and increases the distance of the second shell, probably due to the substitution of Si by Al atoms; for TiO₂ doping (7-15% mol), it is most probable that the second shell is composed from Si atoms as the case of pure SiO₂; for HfO₂ doping (from 10 to 50 mol%) there is a very clear evidence of Er-Hf coordination already at the lowest Hf content, but still in amorphous environment.