Page 191 - Optofluidics Fundamentals, Devices, and Applications

166 Cha pte r Se v e n

thus enable entirely new (and relatively simple) ways of experimen-
tally studying 2-D discrete nonlinear dynamics, which offer a wealth
of unusual physical phenomena, in large part identical to those of
cold atoms in optical lattices [150], which are experimentally much
more challenging.
While we have only discussed properties emerging from micro-
fluidics using liquids, the infusion and flow of gases within fibers is
emerging as a field in its own. Hollow-core PCFs filled with gases
have been used for sensing [150,151] and as gas cells for frequency
references [152], but also as a platform to exploit optical nonlinearity
of gases far more efficiently than can be done in gas-filled capillaries
or free-space tight focusing geometries. This offers particularly
intriguing prospects, such as the generation of frequency combs using
cascaded Raman effects in a hydrogen-filled PCF [153], or in-fiber
electromagnetically induced transparency [154]. One natural exten-
sion of this work is to use optical forces to transport, trap, or acceler-
ate atomic gases, single atoms, particles [155,156], or even Bose-Ein-
stein condensates within the fiber. In a related context, optical forces
have been used to transport microbeads along hollow-core PCFs
[152], and such optically displaced microspheres have in turn been
used to write reconfigurable LPGs in fluid-filled PCFs [157].

7-6-2 Sensing
One of the most obvious—and in many aspects most promising—
applications of microfluidics in PCFs, and one that has been discussed
throughout this chapter, is sensing, and in particular biochemical
sensing. Given their tremendous potential impact on medical diag-
nostics, environmental monitoring, and threat detection, biosensors
are a very active field of research. Detection by optical techniques is
by no means the only approach in the field, but it may be the one
likely to achieve the highest sensitivities [134], as demonstrated by
surface plasmon resonance (SPR) sensors [158], surface-enhanced
Raman scattering (SERS) [159] or microcavity resonance sensors,
which can detect single molecules [160]. It is in the context of this
strong competition in terms of sensing techniques that the potential
of microfluidic PCF-based sensors should be discussed.
While no one has yet demonstrated PCF sensors with sensitivities
comparable to those of SPR, SERS, or microcavity resonantors, PCFs
have a number of benefits that make them a platform worthwhile of
further exploration. The main advantage is certainly that PCFs can be
mass produced much more readily than microresonators or SPR sen-
sors. In the simple geometry of “dip-sensors,” a short piece of PCF
with its own microfluidic channels is simply dipped in the solution to
be analyzed, with light being injected and analyzed through the
opposite end. Information on the content of the analytes is then gath-
ered for example from absorption lines, florescence spectra [161],