Page 141 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Colloidal Photonic Crystals 119

originate from the photonic bandgaps, whose frequencies (or
wavelengths) are determined by the effective refractive index or the
lattice constant for a given crystal structure. In the case of close-
packed crystals, which are prepared by evaporation or centrifugal-
force-induced crystallization, the lattice constant cannot be changed
because it is determined solely by the size of the colloidal building
blocks. However, the refractive index can be easily controlled by
introducing a fluid medium through the colloidal crystals. Kamp
et al. reported optical chromatography results of alkanes that exhib-
ited only small differences in their refractive indices [19]. Through a
glass capillary packed with colloidal crystals—similar to a chemical
chromatography column—they introduced liquid alkanes, such as
octane, nonane, and decane. The position of the reflectance peak
shifted to longer wavelengths upon reflecting the index changes of
the interstices. Even though the refractive indices of the alkanes
were distributed within a very small range (i.e., between 1.398 and
1.411), they could all be identified by the shift of the reflectance
peak. Figure 6-4a shows the optical chromatography results of the
alkanes.
Incorporation of colloidal crystals into a microfluidic chip leads
to an enhanced performance compared to that of the crystals in a
simple capillary. By using the microfluidics technology, it is possible
to manipulate the flow characteristics of the fluid medium. In
addition, a flow of single or multiple components can be introduced
into the crystals. As shown previously, Lee et al. fabricated sectioned
colloidal-crystal columns in a centrifugal microfluidic chip [20]. Using
their technique, it is possible to create colloidal crystals composed of
two different materials. Because the reflectance intensity of colloidal
photonic crystals strongly depends on the refractive index contrast
for crystals with a finite number of layers, it is difficult to obtain
appreciable reflectance spectra for colloid/fluid combinations with
very similar refractive indices. In Fig. 6-4b, the reflectance spectrum
of a silica colloidal crystal is represented by solid lines. The spectra of
the silica colloidal crystals containing various infiltrated fluids, such
as water, hexadecane, decaline, ethanol, and isopropanol (IPA), are
also shown. Among them, the silica colloidal crystal containing
ethanol and IPA only show unclear reflectance signals due to their
small refractive index contrast. Hybrid colloidal crystals composed of
two materials with different refractive indices could solve this
problem. Using PS colloids, which have a higher refractive index than
silica, it is possible to compensate the blind region of the silica
colloidal crystals. The reflectance spectra of PS colloidal crystals
(dotted lines) are characterized by a significant signal, even for
ethanol and IPA. Therefore, for a hybrid colloidal crystal composed of
silica and PS crystal blocks, each complements the blind regions of
the other.