Page 183 - Optofluidics Fundamentals, Devices, and Applications

158 Cha pte r Se v e n

The compact, single-beam interferometer provides natural exposi-
tion of the advantages of optofluidic tuning. The very high refractive
index contrast between the water and its meniscus provides a 2π phase
shift in a path length of only 10 μm or 7.7 wavelengths at the reso-
nance considered above. Creating a vertical feature on the micron
scale is rendered a routine task using optofluidics. Even though the
meniscus is curved naturally in a silica capillary, established surface
chemistry can provide almost arbitrary control over the meniscus
shape. The mobility of the fluid automatically supplies the optofluidic
interferometer with additional functionality. This intensity modula-
tion of the device resonance is exactly the kind of enhancement enabled
by optofluidics that is impossible using solid-state materials.

7-5 Fluidic Photonic Bandgap Fiber
In Sec. 7-3, we showed how photonic bandgap structures may be
used as wavelength-selective reflectors. Such structures may then
also be used for light confinement (indeed, photonic crystals were
first proposed for this purpose), either in a cavity or in a waveguiding
geometry. In photonic bandgap fibers (PBGFs), light propagating
perpendicular to the plane of periodicity is confined in a 2-D defect
core [77]. Unlike the in-plane propagation experiment we described
earlier, however, the wavelengths transmitted through PGBFs lie in
the bandgap, and the transmission spectrum consists of a passband
(or a series of passbands) rather than a notch. Also, we generally
observe bandgap guidance in fibers only when the core index is lower
than the effective cladding index (bandgap-guided modes can still
exist if this is not the case, but index-guided modes dominate [78, 79]);
unlike the case of transverse propagation, whether we observe band-
gap effects depends on whether the refractive index of the fluid is
greater or less than that of the fiber background material.
Here we focus on PBGFs consisting of silica-core PCFs filled with
a high-index fluid, shown for low-index fluid in Fig. 7-11. We note
that fluidic PBGFs may also be made by filling hollow-core PCFs
with low-index fluids [80–82]. These are technologically significant
because low-index fluids include water and most organic solvents,
and since the mode is confined in the fluid, rather than just its eva-
nescent tail, one can make very efficient chemicals or biosensors
(e.g., using Raman, fluorescence, or absorption spectroscopy) with
long interaction lengths. Of course, one may use selective filling
techniques [57,65,83–85] to achieve index guidance in a low-index
fluid core [83–90], and the increased transmission bandwidth of such
a design is often desirable when the primary application is efficient
liquid/light interaction. However, for many of the device designs
we will describe, the inherently resonant nature of bandgap guidance
combined with the enhanced tunability of liquid phase materials