Biomedical Engineering Reference
In-Depth Information
Pitta maxima
, and revealed a spongy structure of keratin in the barbs. Convention-
ally, two kinds of electron microscopy are frequently used in the characterizations
of natural photonic structures: one is scanning electron microscopy (SEM) and
the other is transmission electron microscopy (TEM). With the powerful image
resolution of SEM and TEM, a wide variety of natural photonic structures have
been identified, including single thin films, multilayers, diffraction gratings, two-
dimensional (2D) and 3D periodical photonic structures, amorphous photonic
structures with only short-range order, or their composites [
1
-
9
].
As for the physical mechanisms of structural coloration, early understandings
were almost all attributed to the interference of thin films or multilayers due
to the lack of structural information. With the development of both structural
characterizations and computation algorithms, interesting mechanisms have been
uncovered. Structural coloration can thus be understood by interference, diffraction,
scattering, or their combination, depending on detailed structural configurations.
Despite the great progress in understanding of the mechanisms, there remain still
many important questions to be answered in a quantitative way owing to the
diversity and complexity of natural photonic structures.
8.3
Mechanisms for Structural Coloration
Different from pigmentary coloration, structural coloration is purely of structural
origin, produced by the interaction of natural light with photonic structures via
optical phenomena. These optical effects include interference, diffraction, scatter-
ing, or their combination.
8.3.1
Interference
The simplest way of structural coloration is thin-film interference [
23
], as shown
schematically in Fig.
8.1
a. A soap bubble in sunlight is a known example. As a
light beam is incident upon a thin film, it will be reflected and transmitted
(refracted) at the upper surface. The transmitted beam at the upper surface will once
again be reflected and transmitted when reaching the lower surface. The reflected
beam at the lower surface will also encounter reflection and transmission at the
upper surface. Due to the different optical path, the reflected light beams at the upper
surface possess different phases, eventually leading to interference. The optical path
difference between the reflected light beams depends on the refracted angle
,and
the thickness
d
and refractive index
n
of the film, given by
2nd
cos
. In calculating
the optical path difference, one should consider an abrupt phase change of
when
a light beam gets reflected from a low-refractive-index medium to a high-refractive-
index medium. This will give rise to an extra contribution
=2
to the optical path
difference.
