Sol-gel-processed SiO₂/TiO₂/poly(vinylpyrrolidone) composite materials for optical waveguides

SiO₂/TiO₂/poly(vinylpyrrolidone) (PVP) composite materials processed by the sol-gel technique were studied for optical waveguide applications. Change of refractive index through the range 1.39-1.65 at 633 nm (optical wavelength) was achieved by varying the ratio between the SiO₂ and TiO₂ concentrations. To produce low optical propagation loss in the waveguide, it was necessary to avoid the precipitation of TiO₂ particles caused by the fast hydrolysis reaction of titanium alkoxide. By using titanium ethylhexoxide as a titanium alkoxide together with acetic acid and 2-propanol as the solvent, the precipitation of TiO₂ was avoided in the TiO₂ concentration range of up to 30 wt %, resulting in low optical propagation losses in the slab waveguide configuration. In the higher TiO₂ concentration range, an additional measure, peptization was necessary. With this method, we could produce optical waveguides possessing optical propagation losses of 0.62 dB/cm or lower at 633 nm. Low optical propagation loss in all the TiO₂ concentrations, from 0% TiO₂/50% SiO₂/50% PVP to 50% TiO₂/0% SiO₂/50% PVP was achieved. The reactions of the sol-gel precursors were also studied using UV-visible and FT-IR spectroscopy.