Photonics deals with photons as a medium for transmitting information. Typically, photonic circuits either rely on passive devices with pre-designed optical functions, or use active components where application of external fields changes the optical properties of the materials (e.g. in electro-optical devices). Our projects in fluidic optics explore alternatives to application of external fields - in these projects we demonstrate the generation and reconfiguration of photonic devices in real time by manipulating flowing liquids.

Fluid Optical Waveguides

We take advantage of laminar flow in microscopic channels (i.e. microfluidic systems) and of diffusion. In the simplest demonstration, we sandwich a fluid of higher index of refraction between two streams of liquid with lower index of refraction (Figuire 1). In microchannels, the liquids flowing through the channel will not mix except by molecular diffusion; thus, the flow is laminar and the two liquids flowing side-by-side form an optically smooth interface [1,2]. This system acts as a waveguide (we call it a "liquid-liquid" or L2 waveguide).

Fluid optical waveguides are fabricated easily and rapidly in organic polymers using the convenient techniques for rapid prototyping developed in our group. The L2 waveguides are dynamic their structure and function depend on a continuous flow of the core and cladding liquids. They can be reconfigured, renewed (if damaged), and continuously adapted in ways that are not possible with solid-state waveguides. Manipulation of the rate of flow and the composition of the liquids (thus the optical properties) tunes the characteristics of these optical systems in real time. Currently, we are studying the design and operation of fluid analogs of several common optical elements: single- and multi-mode waveguides, optical switches, and evanescent couplers [3].

Generation of Light in Microchannels

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 [4]. When laminar streams of fluorescent organic dyes are separated by a low index fluid and illuminated by an incandescent light source (Figure 2), 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.

Microfluidic Dye Laser

We used microfluidic technology to design a miniaturized waveguide dye laser, in which the laser cavity contained a liquid core-liquid cladding waveguide (Figure 3). 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 (Figure 4). 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.

Select Publications

1. Stroock, A. D.; Dertinger, S. K. W.; Ajdari, A.; Mezic, I.; Stone, H. A.; Whitesides, G. M. Science 2002, 295, 647-651.

2. Ma, H.; Jen, A. K. Y.; Dalton, L. R. Adv. Mater. 2002, 14, 1339-1365.

3. Wolfe, D. B.; Conroy, R. S.; Garstecki, P.; Mayers, B. T.; Fischbach, M. A.; Paul, K. E.; Prentiss, M.; Whitesides, G. M. Proc. Nat. Acad. Sci. USA 2004, 101, 12434-12438.

4. Vezenov, D. V.; Mayers, B. T.; Wolfe, D. B.; Whitesides, G. M. Appl. Phys. Lett. 2005, 86, 041104.

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