Biomedical Engineering Reference
In-Depth Information
that there exist three relatively closely spaced partial photonic bandgaps along the
(110), (111), and (200) directions for the butterfly gyroid photonic crystal, which
are the origin of the structural coloration of the wings.
8.5.5
Amorphous Photonic Structure
According to their arrangements, photonic structures can be classified into three
classes: ordered structures with both short- and long-range order, quasi-ordered
or amorphous structures with only short-range order, and random structures with
neither short- nor long-range order. These three classes of photonic structures have
different optical response, leading to structural coloration with distinctive features.
For ordered photonic structures, they can produce iridescent structural coloration.
The iridescence of ordered photonic structures stems from long-range order. On
the other hand, quasi-ordered or amorphous photonic structures can cause angle-
independent structural coloration, i.e., non-iridescent structural coloration. Random
photonic structures that lack both short- and long-range order may only produce
white colors.
6
In addition to ordered categories in the biological world, there also exist quasi-
ordered or amorphous photonic structures that possess only short-range order,
giving rise to non-iridescent structural coloration. The most known example is the
spongy structure of keratin in the feather barbs of many birds. These non-iridescent
colors were hypothesized over 100 years ago to be produced by incoherent
scattering from individual scatters, namely, individual air vacuoles [
1
,
17
,
138
,
139
],
where phase relations among scattered light are random. Two types of incoherent
scattering mechanisms were proposed: one is Rayleigh scattering [
138
]andthe
other is Tyndall or Mie scattering [
1
,
17
,
139
].
In 1934 Raman opposed this incoherent-scattering hypothesis with the thorough
observations of the feathers of the bird
Coracias Indica
which displays a non-
iridescent blue color under natural light [
140
]. He concluded that incoherent
scattering such as a Tyndall effect was definitely insufficient to explain the observa-
tions, and suggested that the diffraction by the air cavities and the interference from
the surfaces of minute films were responsible for the non-iridescent blue feather
barb coloration.
In 1970s Dyck challenged the Rayleigh model for the blue coloration of feather
barbs [
141
,
142
]. He noticed that the reflection spectra of many bird feathers display
discrete peaks, which is inconsistent with the prediction of a continued scattering
efficiency that is inversely proportional to the fourth power of wavelength by the
Rayleigh law. Dyck hypothesized that coherent light scattering by the ordered
matrix of air vacuoles and keratin was the cause of the blue coloration.
6
White colors are always of structural origins.
