Page 193 - Optofluidics Fundamentals, Devices, and Applications

168 Cha pte r Se v e n

there are only few good hot spots per unit volume or surface, and
their position is unpredictable. In conventional beam-optics-based
SERS sensors only a few of those hot spots can be exploited, the ran-
dom fluctuation of which makes it difficult to obtain reproducible
quantitative SERS measurements. In a PCF geometry, light can inter-
act over long lengths with the metallic rough surface [170] or with
nanoparticle suspensions in the PCF holes [88,172], maximizing the
likelihood of encountering hot spots and making SERS measurements
more reliable.
From the various methods described here, it is clear that the spec-
trum of applications achievable with microfluidic PCFs can be greatly
extended by coating their holes, be it with metals, dielectrics or even
metamaterials [173] to alter the optical properties, fluorphores and
chemically active materials to add specific chemical sensitivity
[89,163], and surface treatments to improve wetting properties [74,161].
Coating techniques, be they at the drawing or at postprocessing
stages, are thus bound to be explored much further in the future—
experimentally as well as theoretically—adding an entirely new
dimension to microfluidic PCF research.
Another postprocessing technique that has emerged over the last
two years and will no doubt be explored further in the future is that
of enabling side access to PCFs, to optimize filling times. Indeed, to
fully exploit PCF sensitivities, long interaction lengths are desirable;
however, filling more than a few centimeters of PCF with fluids can
be a very lengthy process when only capillary forces or small pres-
sure differences are used. By adding regularly spaced small holes
between the PCF holes and the outside, filling times can be reduced
dramatically. Techniques explored for realizing side access to PCFs
include focused ion beam etching [174], inflating techniques [84], and
also manufacturing fibers with continuous lateral access—in other
words, replacing PCF holes by trenches, at the preform stage, be it for
hollow- or solid-core PCFs [175,176].

7-7 Summary
Optofluidics has emerged as a versatile design principle for enabling
highly functional, compact, and micron-scale devices that bring
together aspects of photonics, microfluidics, and various other disci-
plines. While typically applied to planar photonic structures, micro-
structured fibers provide an ideal optofluidic platform. A large body of
research already exists in the design and fabrication of high-quality,
low-loss, microstructured optical fibers. These fibers provide a high-
quality optical environment for waveguiding, and the fiber microstruc-
ture provides a natural location for the microfluidic bodies used in
optofluidic tuning. Optofluidic microstructured optical fibers thus pro-
vide a wealth of functional and compact device platforms for a