Total internal reflection-based optofluidic waveguides fabricated in aerogels

Liquid-core optofluidic waveguides based on total internal reflection of light were built in water-filled cylindrical microchannels fabricated in hydrophobic silica aerogels. Silica aerogels with densities ranging from 0.15 to 0.39 g/cm³ were produced by aging of alcogels in tetraethylorthosilicate solution for various time periods, followed by supercritical extraction of the solvent from the alcogel network. Subsequently, the resulting hydrophilic aerogel samples were made hydrophobic by hexamethyldisilazane vapor treatment. The synthesized samples retained their low refractive index (below ~1.09) and, hence, they could serve as suitable optical cladding materials for aqueous waveguide cores (refractive index n _core = 1.33). Hydrophobic silica aerogel samples produced by the above technique also had low absorption coefficients in the visible part of the spectrum. Fabrication of microchannels in aerogel blocks by manual drilling preserving nanoporous and monolithic structure of aerogels was demonstrated for the first time. Long channels (up to ~7.5 cm) with varying geometries such as straight and inclined L-shaped channels could be fabricated. Multimode optofluidic waveguides prepared by filling the channels in the drilled aerogel monoliths with water yielded high numerical aperture values (~0.8). Efficient guiding of light by total internal reflection in the water-filled channels in aerogels was visually revealed and characterized by monitoring the channel output. The presented technique is expected to open up further possibilities for creating three-dimensional networks of liquid channels in aerogels for optofluidic applications.