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.
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
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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