Fabrication of monosized silica spheres with customized diameter by sol-gel route

Analogous to the electronic bandgap in semiconductors, photonic crystals exhibit a certain frequency range where light cannot propagate in any direction in the structure [ 1,2]. This frequency range is known as the photonic bandgap. Photonic crystals are formed by a periodic variation in dielectric constant on a S(.;ale comparable to thc wavelength of interest. A simple and a cheap method to obtain three-dimensional (30) photonic crystals, with specific bandgap properties, is based on the self-assembling of micro-spheres. In fact recently significant attention has been paid to various routes and in particular by sol-gel to obtain micro-spheres using catalysts both basic [3] and acid [4] and to fabricate 30 photonic crystals for optical applications. Though the currently available colloidal crystals do not have a full photonic bandgap because of their low index contrast, the self-assemb\ed colloidal systc1n provide an ideal template for the creation of porous structures with a full photonic band gap by infiltrating materiai of high refractive index into the template and then removing the template [5]. In this paper we present the details of the procedure used to synthesize silica spheres, with particular attention to the conditions that permit to tailor their dimension.