Page 155 - Optofluidics Fundamentals, Devices, and Applications

130 Cha pte r S i x

crystals exactly respond to changes in the refractive indices. Especially,
the detection of biomolecules could be possible using this scheme—
without the need to label them. Although colloidal crystals do not
show appreciable reflectance intensity for index-matching liquids,
hybrid colloidal crystals composed of particles with different indices
remove the blind index region. Unlike reflection-based applications,
colloidal crystals can be used as lasing resonators. If the fluids contain
active materials, such as dye molecules or quantum dots, the emission
spectra change with respect to that corresponding to the bulk
emission. This is a result of the modulated electromagnetic density of
states in the photonic crystals. Because of the high density of states
and the low group velocity at the band edge and at defect modes, it is
possible to obtain an amplified stimulated emission. Especially,
tuning the bandgap properties using fluids enables the control of the
emission wavelength of the laser, which is important for a wide range
of micro-TAS or lab-on-a-chip applications.
Unlike colloidal-crystal integrated systems, optofluidic devices can
be used to continuously generate discrete colloidal crystals. Especially, if
the emulsion droplets are used as templates for crystallization, photonic
balls with optical isotropy can be generated. To achieve this, an optofluidic
device composed of a microfluidic emulsion generator and a UV-
exposure unit is prepared. In the emulsion-generator part of the device,
photocurable emulsion droplets containing concentrated repulsive
colloids are generated. The colloidal particles in the droplet spontaneously
arrange into an onion-ring-like crystal structure, from the outermost
layer, by minimizing the total repulsive energy. The droplets are then
photopolymerized by passing them through the UV irradiated area.
Finally, the photonic balls are generated. In addition, photonic Janus
balls with two different colors on their own hemispherical domains can
be generated with the paired inner capillaries.
Furthermore, double-emulsion droplets, which are droplets in
droplets, could also be generated with the microfluidic device. Here,
the middle phase of the emulsion was a photocurable resin and the
inner droplets contained an aqueous suspension of PS particles with a
high surface charge density. Using an optofluidic scheme similar to
that described in the previous example, photonic balls in the liquid
state were generated by encapsulating the suspension within a polymer
shell. The prepared photonic balls exhibited long-term stability, even in
the presence of high concentrations of ionic impurities, because the
shell did not permit the penetration of impurities.

References
1. W. B. Russel, D. A. Saville, and W. R. Schowalter, Colloidal Dispersion, Cambridge
University Press, (1999), New York.
2. S.-M. Yang, S.-H. Kim, J.-M.Lim, and G.-R. Yi, “Synthesis and assembly of struc-
tured colloidal particles,” Journal of Materials Chemistry, 18, (2008), 2177–2190.