The effects of TiO₂ on the structure and spectroscopic properties of silica-hafnia based sol-gel waveguides

The effects of titania additions on Er³⁺ doped silica–hafnia based optical planar waveguides, prepared by solgel processing have been studied. Structural changes, including crystallisation induced by the presence of TiO₂, were assessed by x-ray diffraction, waveguide Raman spectroscopy and Fourier transform infrared spectroscopy. The Er³⁺ photoluminescence spectra at ∼1·5 μm, the corresponding lifetimes and the Er³⁺ relative luminescence yields have been determined as a function of the matrix composition, the densification heat treatments and the Er³⁺ ion concentration. The Er³⁺ quenching concentration and its lifetime at the limit of zero rare earth concentration have also been determined. Typical values for the fluorescence lifetimes varied between 6·5–1·5 ms, for Er³⁺ concentrations ranging from 0·15–2%, in a 75% SiO₂–5% TiO₂–20% HfO₂ matrix. The presence of TiO₂ appears to promote the formation of hafnia–titania mixed crystals. Although the Er³⁺ quenching concentration was similar for all the systems studied, at ∼0·6–1·0% Er, even small additions of TiO² (∼5 mol%) were found to increase the Er³⁺ fluorescence lifetimes of silica–hafnia waveguides by as much as 100%, depending on the actual compositions.