Using carefully tailored fluid flow, researchers at the Colorado School of Mines are creating all-fluid optical waveguides (contained on a solid substrate) composed of two liquids with mismatched viscosity. The waveguides are potentially useful in dynamically switched microfluidic-based analytical systems for biomedical research. The more-viscous core, a 67%-by-weight sucrose/water solution, has a refractive index of 1.457, while the pure-water cladding has an index of 1.33. Two flows of cladding squeeze the central core to provide a taper that ends in a core width of about 15 µm.
The core/cladding boundary is smooth due to laminar flow. Over the length of the waveguide, the boundary becomes graded because of diffusion-a phenomenon that can be tailored based on flow such that step-index, graded-index, multimode, or single-mode waveguides are possible. The researchers demonstrated fluorescence detection and emission collection in a fluid waveguide that carried fluorescent colloids in its core, which was pumped with 632 nm light (the waveguide flowed up against and was diverted by a transparent wall, through which the emission was detected). Contact David Marr at [email protected].