Page 163 - Optofluidics Fundamentals, Devices, and Applications

138 Cha pte r Se v e n

This enables compactness in devices that rely upon interference. The
utilization of a fluid phase as an optical element enables further versa-
tility; the optical properties of fluids, as a whole, are very broad and
limited only by chemistry and composition. As such, optofluidic tun-
ing enables a much broader range of optical properties in a given pho-
tonic structure than solid-state fabrication methods. Bringing fluids
into contact with optical fields also enables a variety of sensing geom-
etries. Optofluidically designed structures allow small, well, con-
trolled volumes of reagents or analytes to interact efficiently with the
optical field in the photonic layer effectively adding optical interroga-
tion methods to the “lab-on-a-chip” analysis methods [42]. All these
attributes of optofluidics combine to enable unique modulation geom-
etries, device functionalities, and sensing platforms.
Potentially foreshadowing this modern development, the first
observation of guided light was in a stream of water from a fountain
by Jacques Babinet in 1840 [43]. From there, optofluidics again saw
use in the development in some of the first purpose, built optical
waveguides [4] whose cores were filled with fluid to provide the
necessary refractive index contrast to guide light. From there, opto-
fluidic development was relatively quiet until the emergence of
microfluidics [40–42] and “lab-on-a-chip” technologies [43] that
entail the control of fluids, typically chemical reagents or analytes,
inside micron-scale flow channels. The length scales of microfluidic
structures are similar to those of microphotonic structures, paving
the way for the integration of both kinds of devices [44]. Optofluidic
devices can be broadly classified by their underlying photonic tech-
nology that fall into either planar photonics or optical fibers. Both
these varieties of optofluidics were developed essentially concur-
rently. Planar optofluidic devices use planar photonic structures
such as integrated planar waveguides or photonic crystals [3,45–50]
as their photonic layers. While highly compact and functional, these
planar optofluidic devices require significant investment in design,
experimentation, fabrication time, and cost.
7-1-4 Fiber-Based Optofluidics
In contrast to planar optofluidics, fiber-based optofluidics use optical
fibers of various designs for photonic transport around the optoflu-
idic environment, for microfluidic transport, or both. As such, there
are two broad subcategories of fiber-based optofluidic devices. First
is the “all-fiber” device, where both the microfluidic transport net-
work and the photonic transport layers are provided exclusively by
optical fibers. Second is the “semi-planar fiber” device, which uses
optical fibers as the photonic transport layer, but relies on a more
integrated, planar microfluidic environment. This device operates on
a photonic and microfluidic level using optical fibers alone (typically
MOFs). The reasons for doing this, aside from those outlined above,
involve the quality of the microfluidic environment in silica