Biomedical Engineering Reference
In-Depth Information
Fig. 8.36
(
a
) TEM image of a collagen array from dark blue caruncle tissue of
Neodrepanis
hypoxantha
.(
b
) 2D Fourier power spectrum of the tissue. Scale bar: (
a
) 200 nm (Reproduced
from [
150
])
Recently, the optical response of amorphous photonic structures was studied
numerically by the authors with rigorous methods to solve Maxwell's equations
[
148
,
149
]. From the calculated photon density of states (PDOS) for amorphous
photonic structures, there exist angle-independent (isotropic)
photonic pseudo-
gaps
due to coherent light scattering from short-range order. Unlike photonic
bandgaps in photonic crystals within which the PDOS is zero, photonic pseudogaps
show nonzero dips in the PDOS. Our results indicate unambiguously that photonic
pseudogaps are the ultimate physical origin for the non-iridescent structural col-
oration of amorphous photonic structures. Non-iridescence can be understood by
the fact that light is scattered evenly in all directions since there is no preferred
orientation in amorphous photonic structures due to the lack of long-orange order.
In feather barbs, the amorphous photonic structures responsible for the blue
coloration are mostly a keratin network in the spongy medullary layer [
143
-
147
],
with keratin bars and air channels in tortuous and twisting forms, as shown in
Fig.
8.35
a. These nanostructures possess only short-range order and are responsible
for the iridescent structural coloration. Another kind of nanostructure was found in
the blue back plumage of the Eastern Bluebird (
S. sialis
) where the air cavities are
nearly spherical and close-packed [
146
], as shown in Fig.
8.35
b. The nanostructure
of spherical air cavities also exhibits short-range order.
Amorphous photonic structures were also found in the structurally colored skin
of some birds [
121
,
150
]. The dermis of structurally colored skin consists of a thick
(100-500
m) layer of collagen. TEM characterizations revealed that most of the
color producing dermal collagen layers consist of a quasi-ordered array of parallel
collagen fibers as shown in Fig.
8.36
. 2D Fourier analyses showed that the array of
collagen fibers has short-range order, indicating that the collagen fibers are arranged
in the form of 2D amorphous photonic structures. The dermal colors are determined
by the nanostructure of the collagen arrays via coherent light scattering.
