Spectroscopic assessment of titania and hafnia silica-based planar waveguides

Silicate-based glasses remain the most investigated systems for optical planar waveguides, since they offer a reasonable solubility for rare-earth ions, they are transparent in the NIR-Visible region and they are compatible with Integrated Optics (IO) technology. In the last decade, various technologies have been employed for the fabrication of silica-based IO components and a broad variety of silica-based systems have been investigated. Among the different binary systems, SiO2 - TiO2 has been extensively employed, mainly because it offers the possibility of producing planar waveguides with a controlled refractive index, depending on the TiO2 / SiO2 ratio. Recently, it has been shown that SiO2 - HfO2 could be a further viable system for 1.5 ?m applications. In fact, HfO2 is transparent over a wide range of wavelengths, it exhibits high refractive index and allows the preparation of good optical quality waveguides. This paper compares spectroscopic results, in particular photoluminescence and vibrational spectra, in order to assess optical and structural properties of erbium-activated silica-titania and silica-hafnia planar waveguides, prepared by two different techniques: radio-frequency sputtering and sol-gel. Particular attention is devoted to the homogeneity of the material structures obtained in each case.