Fabrication and characterisation of a CdS-doped silica-on-silicon planar waveguide

A planar silica-titania waveguide doped with CdS nanocrystallites has been fabricated on a silicon substrate. A passive nanoporous host layer and a pure silica buffer layer, of thickness 2.2 and 7.5 μm, respectively, were deposited by the sol-gel technique. Crystallites were then precipitated in the porous matrix by soaking in a CdF₂ solution followed by reaction in H₂S gas, and a sputtered layer of silica was used to seal the structure. The absorption spectrum of an equivalent film indicates crystallites with radii up to ~26 Å, while energy dispersion spectroscopy indicates a dopant concentration of ~0.5 volume%. The third order optical nonlinearity (Kerr effect) was measured using an m-line technique, yielding a value of ~5×10^-9 cm²/kW