Sol-Gel Derived Titaniajormosil Composite Thin Films For Optical Waveguide Applications
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ABSTRACT We report the preparation of sol-gel waveguide films based on a newly developed recipe to incorporate organic molecules into the inorganic sol-gel glass matrix. The film was derived from a sol that has a higher titanium content in an organically modified silane (ORMOSIL), namely, y-Glycidoxypropyltrimethoxysilane. We have shown that using spin-coating and low temperature baking, a single coating layer can have a thickness of more than 1.5 ptm. When such a single layer film is deposited on a microscope glass slide or a piece of silicon with a buffercladding layer, it is able to support the guiding of optical waves. We have characterized the film using scanning electron microscopy, atomic force microscopy, X-ray diffractometry, thermal gravimetric analysis. differential thermal analysis and Fourier transform infrared spectroscopy and have studied the properties of the waveguide film, including the microstructural properties. the chemical bonding properties, and the optical properties. Based on these experimental results, we found that a heat-treatment at a temperature slightly below 200'C is necessary to attain a dense pore-free film. It has also been noted that a purely inorganic and crack-free silica-titania film can be obtained after baking the titania-ORMOSIL composite film at 500'C or higher. INTRODUCTION Increasing interest in optical integrated circuits has stimulated studies on optical waveguide materials. Many researchers have spent a great deal of effort to realize useful materials for optical xvaveguide applications. Among the different synthesis methods, the sol-gel process is
considered promising because of the very mild reaction conditions, particularly, the low reaction temperature that allow the incorporation of organic molecules in inorganic materials. This has led to a conceptually novel class of materials composed of both inorganic and organic groupings [I]. This method is able to make high optical quality materials and it also has the freedom to impregnate them with a variety of additives to modify their optical characteristics. Many sol-gel processed materials employ TiO 2 . TiO 2 is a high refractive index material, so it is used to increase the refractive index of the system. Titanium isopropoxide is one of the most widely used titanium alkoxides as a TiO 2 source in the sol-gel reaction; however, its high hydrolysis rate results in the precipitation of TiO
2
particles, causing severe scattering loss [2].
Therefore. acetylacetone is often used to reduce the reaction rate through the formation of a complex. This complex has a strong absorption band in the visible region, so even after the sample is baked at 300'C, the residual of the complex still absorbs in the visible wavelength, resulting in a high optical loss. To avoid using acetylacetone and to reduce the reaction rate, acetic acid can be used as a solvent for titanium alkoxide. Acetic acid works not only as a solvent but also as a reagent to produce a relatively stable acetyl compound [3-6]. Doeff et al. has found that acetic
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