Ultraviolet irradiation 646 INDEX

Sol-gel techniques are being employed to fabricate components not only for mainstream applications such as photonics, thermal insulation, electronics and microfluidics, but also for more exotic applications such as space dust and radiation collectors [1]. Methods have been developed to tailor the physical properties of sol-gel materials to the requirements of a specific application. For example, porosity and pore size distribution can be controlled by forming micelles in a sol [2-4-] gels can be made hydrophobic by derivatizing the otherwise hydrophilic pore walls with hydrophobic moieties [5] superhydrophilicity can be obtained by ultraviolet irradiation [6, 7] mechanical strength can be increased by cross-linking the oxide nanoparticles that make up the gel [1, 8, 9], and optical properties can be controlled by adding chromophores and nanoparticles to control index of refraction, absorption and luminescence [10-12]. 

Rothschild M, Erlich D J and Shaver D C, Effects of excimer irradiation on the transmission, index of refraction, and density of ultraviolet grade fused silica , Appl. Phys. Lett., 1989, 55(13), 1276-8. 

A second, purely optical method is to incorporate a rod-shaped azo dye as a side group"" or dopant" — in the trans-azo form these compounds favor liquid crystal formation. When irradiated with ultraviolet light at 350 nm the trans-azo configuration isomaizes to the cis-azo which does not exhibit liquid crystal phases and the polymer, in the irradiated region, becomes isotropic and exhibits a different refractive index. A number of variants of this effect have been demonstrated including its use in holograms. Reversal of the isomerization occurs when irradiated with 420 nm light. 

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