The effects of TiO 2 on the structure and spectroscopic properties of silica - hafnia based sol - gel waveguides

Refereed paper The effects of titania additions on Er 3+ doped silica– hafnia based optical planar waveguides, prepared by sol-gel processing have been studied. Structural changes, including crystallisation induced by the presence of TiO 2 , were assessed by x-ray diffraction, waveguide Raman spectroscopy and Fourier transform infrared spectroscopy. The Er 3+ photoluminescence spectra at ~1·5 µm, the corresponding lifetimes and the Er 3+ relative luminescence yields have been determined as a function of the matrix composition, the densification heat treatments and the Er 3+ ion concentration. The Er 3+ 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 Er 3+ concentrations ranging from 0·15–2%, in a 75% SiO 2 –5% TiO 2 –20% HfO 2 matrix. The presence of TiO 2 appears to promote the formation of hafnia–titania mixed crystals. Although the Er 3+ quenching concentration was similar for all the systems studied, at ~0·6–1·0% Er, even small additions of TiO 2 (~5 mol%) were found to increase the Er 3+ fluorescence lifetimes of silica–hafnia waveguides by as much as 100%, depending on the actual compositions. Er 3+ doped silicate glasses have been the subject of extensive research as optical planar waveguide materials , (1–6) due to the coincidence of the Er 3+ emission band around 1·5 µm with the lowest loss window of silica-based fibre optic telecommunications systems. For this reason, there has been a strong drive to optimise the active waveguide performance and, therefore the processing parameters, together with the composition of the host material, including the Er 3+ ion content. All these aspects must be effectively controlled, in order to achieve long metastable level lifetimes, high radiative quantum efficiency, high absorption and emission cross sections, low optical propagation losses and a strong and stable Er 3+ photoluminescence (PL) signal at 1·5 µm. Rare earth (RE) ions have partially filled 4f shells which are shielded from external fields by the 5s and 5p electrons, which makes their energy levels largely independent of the host material. However, the disordered structure of glass leads to site-to-site variations in the local bonding, which affect the electronic energy levels of the RE ions, resulting in inhomogeneous broadening of spectral features, (7) caused by a full Stark splitting. The temperature factor, on the other hand, results in so called homogeneous broadening, which is …