Solution-Based Techniques for Biomimetics and Bioreplication - Engineered Biomimicry - page 366

14.3 BIOREPLICA PHOTONIC
CRYSTALS

some portions of white light—those with wave-
lengths having certain proportionalities to the
length scale of the structure's periodicity—are
directionally reflected, producing the angle-
dependent sparkling colors seen in many insects,
birds, and marine animals.
Let us consider the one-dimensional case of a
structural color: a dielectric composite with a
periodic variation of the refractive index in a
single dimension, as shown in Figure 14.9 [44] .
An incident wave (in the forward direction) will
be reflected at each low-to-high refractive index
interface. In a simplified picture, for waves with
wavelengths half the periodic length of the one-
dimensional lattice ( Figure 14.9 a), the reflecting
waves are all in phase and will reinforce each
other ( Figure 14.9 b). When they're combined
with the forward-traveling wave, the result is a
standing wave: i.e. waves with this wavelength
do not propagate through the composite and are
instead reflected ( Figure 14.9 c). This composite
is said to have a photonic band gap with its width
dependent on the refractive index contrast of the
two dielectrics and their filling fractions.
On the other hand, for waves with wave-
lengths outside the band gap ( Figure 14.9 d), the
reflected waves are out of phase and cancel each
other ( Figure 14.9 e), resulting in a net forward
traveling wave (albeit with slightly lowered
intensity), as shown in Figure 14.9 f. Note, how-
ever, that this effect is only valid for a given
frequency range of incident light normal to the
periodic structure. For off-normal directions, the
periodic length changes and waves of a different
range, or color, are reflected.
Such one-dimensional periodic structures
have long been used as optical components such
as mirrors, filters, and optical cavities, but their
two and three-dimensional analogs promise
to open the door to entirely new optical con-
cepts based on photonic band structures [45] .
This idea of a photonic crystal was independently
proposed by Yablonovitch [46] and John [47] in
1987 with the goal to control radiative properties

In this section, we provide a specific example of
solution-based bioreplication: Using silica and
titania sol-gel chemistry, the three-dimensional
photonic crystal structure of colored weevil
scales is replicated [26-28] . This combination of
structure engineering in biology—still beyond
our synthetic engineering abilities—with sol-gel
synthesis can result in entirely new optical mate-
rials with fascinating properties and new oppor-
tunities in energy and information technology
applications [43] .

14.3.1 Photonic Crystals
Upon closely examining the green color of spin-
ach leaves or the red color of strawberries, you
will find that the origin of this coloration is
based on molecules. When illuminated with
white light, electronic transitions in these mol-
ecules, also called pigments , absorb selected fre-
quency ranges of the visible part of the
electromagnetic spectrum while diffusely
reflecting the rest. An excellent example of such
a pigmented color is the molecule chlorophyll.
It absorbs large portions of the blue and red
spectral regimes, rendering the flora around us
in a familiar green.
Now look closely at the green weevil Lampro-
cyphus augustus or the colorful wings of many
butterflies and you will discover that the origin
of this coloration stems from interesting biopol-
ymeric structures with a high degree of periodic
ordering in one, two, or three dimensions ( Fig-
ures 14.3 and 14.8 ) [7, 26] . The mechanism
behind these structural colors is very different.
Unlike pigments, structural colors are based on
diffraction and specular reflection rather than
absorption and diffuse reflection [9] . Most of the
light incident upon these periodically organized
biopolymeric structures can travel through
without being significantly affected. However,