The objective of the optofluidics work in Whitesides group is to explore and develop photonic components based in part, or entirely on liquid based systems. Current research directions involve liquid-core/liquid-cladding (L2) waveguides, where two liquids flow laminarly in a microchannel and form an optically smooth interface. These fluid optical waveguides will be fabricated in organic polymers using rapid prototyping techniques developed in the Whitesides group. The L2 waveguides will be dynamic – their structure and function depend on a continuous flow of the core and cladding liquids.  Manipulation of the rates of flow and the composition of the liquids (thus the optical properties) will tune the characteristics of these photonic systems in real time.

Fluorescent Microfluidic Light Sources

We have also demonstrated that fluid waveguides can generate light in microchannels, thus simplifying the coupling of light from external sources to these fluidic devices . When laminar streams of fluorescent organic dyes are separated by a low index fluid and illuminated by an incandescent light source, they each produce fluorescence of specific color that can be collected and propagated by a fluid waveguide. One can tune the wavelength (color), position, shape and intensity of these microfluidic light sources by making adjustments of the rate of flow or composition of individual streams. Such simple fluidic light sources could be important, for example, for microanalysis "on-chip" in integrated biophotonic microsystems.

We used microfluidic technology to design a miniaturized waveguide dye laser, in which the laser cavity contained a liquid core-liquid cladding waveguide. The key feature of the laser is a long optical path length along the waveguide axis that allows us to achieve high gain in one pass and thus lower the threshold for lasing. By adding thin gold coatings on the surfaces of the T-junctions, we built the laser mirrors into flouresent L2 waveguide light source. Rhodamine 640 perchlorate dissolved in methanol served as the core stream, and pure methanol worked as the cladding stream. Optical pumping of the microlaser with a 532-nm frequency-doubled Nd:YAG laser at 50 Hz results in the bandwidth decrease by an order of magnitude at laser threshold. The fluid waveguide laser is readily tunable by continuously varying the composition of the mixed solvent (methanol-dimethylsulfoxide) while using the same concentration of the dye. The ability to easily change wavelength is critical for applications in spectroscopy and for various types of optical detection requiring different wavelengths.

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