Sputter-deposited yttria-alumina thin films for optical waveguiding

Yttria-alumina planar waveguides were fabricated by reactive codeposition from yttrium and aluminum targets. These waveguides were verified to be amorphous with yttria contents as high as 23.4% by x-ray diffraction, both conventional and glancing angle, and by transmission electron microscopy. Above 41.8% yttria, the waveguides had mixed crystallinity and contained microcrystallites (0.5-0.9 nm) dispersed throughout an amorphous matrix. These microstructures appeared to be unaffected by the oxygen content in the sputtering gas. The index of refraction was found to increase strongly as a function of yttria content, but remain constant with oxygen content. Although the waveguides had wide transmission windows, the infrared edges were observed to shift with composition toward the values of the end member edges of 11 (alumina) to ≥ 15 μm (yttria). In general, the waveguides had optical losses of 10-20 dB/cm. However, films with compositions roughly equivalent to that of yttrium iron garnet had lower losses (1-5 dB/cm). These losses are the lowest reported to date for yttria-alumina waveguides, and combined with the extended infrared transparencies, make these films very promising hosts for waveguide amplifiers.