Structural Colors - Engineered Biomimicry - page 286

a biaxial dielectric continuum that is nonhomo-
geneous in the thickness direction.
STFs with periodic nonhomogeneity are rou-
tinely made these days, some comprising two-
dimensional nematic nanowires [119] , others
comprising three-dimensional helical nanowires
[120] . These periodic STFs are one-dimensional
photonic crystals that display the Bragg phe-
nomenon selectively for light of a certain polari-
zation state [79] . For example, a chiral STF is an
array of parallel helical nanowires, as shown in
Figure 11.22 , and displays the circular Bragg
phenomenon--best described as the high reflec-
tance, within a narrow spectral regime, of circu-
larly polarized light of the same handedness as
the chiral STF of sufficient thickness, whereas
circularly polarized light of the opposite hand-
edness is reflected very little. In other words, a
chiral STF is a circular Bragg filter.
Tremendous control of the circular Bragg
phenomenon is possible because the helical
morphology is nanoengineered during deposi-
tion. Structural color can be preserved even
when a chiral STF is crushed down to sub-mil-
limeter platelets, as shown in Figure 11.23 . This
fine powder could be suspended in an appropri-
ate cosmetic fluid (lotion) or perfumed sub-
stance to form exhilarating body gels.

FIGURE 11.23 Optical image of platelets taken from a
3-period-thick chiral STF of magnesium fluoride deposited
on a 30-nm-thick aluminum film. The pitch of the chiral STF
is 450 nm, and the image was taken at 100x magnification.

11.5.5 Environmentally Responsive
Structural Color

All approaches employing engineering bio-
mimicry discussed so far in this section repli-
cate the passive color generated by a periodic
structure. Most recently, changes in structural
color displayed by some fish (e.g., blue damsel-
fish or neon tetra fish), octopuses, squids, and
beetles in response to changes in the environ-
ment have inspired research on bioinspired and
biomimetic materials--such as colloidal crys-
tals--with dynamically tunable structural color
[121, 122] . The fabrication of photonic materials
with tunable structural color exploits the three
mechanisms of reversible active structural color
in nature: change in refractive index, change
in spacing of a periodic structure, and change
in direction of illumination. For example, the
porous exocuticle of some beetles changes its
refractive index by absorbing water [62] . The
spacing of the periodic structure in cephalo-
pods is tuned by change in the protein platelets'

FIGURE 11.22 SEM of a chiral STF made of chalcoge-

nide glass [120] .