Biomedical Engineering Reference
In-Depth Information
In addition to birds, amorphous photonic structures are also present in beetles
[
148
,
149
,
151
,
152
]. For example, the scales of the longhorn beetle
Anoplophora
graafi
contain a random-close-packing photonic structure of chitin nanoparticles
[
148
], while the scales of the longhorn beetle
Sphingnotus mirabilis
possess a dis-
ordered bi-continuous photonic structure of chitin [
149
]. Both photonic structures
have only short-range order and are responsible for the non-iridescent structural
coloration of the scales.
AsshowninFig.
8.37
, the beetle
A. graafi
has a dull metallic blue or green
color on its elytra marked with brilliant greenish white lateral stripes. Under optical
microscopy, these stripes are composed of differently colored scales which are seed-
like, about 50
m long and 20
m wide. Each scale has a distinct non-iridescent
color and the scale color can cover almost the whole visible range. The perceived
greenish white is thus a mixed color resulting from differently colored scales in a
pointillistic way. Cross-sectional SEM images revealed that the interior of scales is
an array of chitin nanoparticles in a form of random close-packing (RCP), confirmed
by comparing with the generated RCP structure. The 2D radial distribution function
indicates that the RCP photonic structure in the scale interior possesses only short-
range order.
To get insight into the coloration mechanism of RCP photonic structures, the
PDOS of model RCP photonic structures were calculated, as shown in Fig.
8.38
.
Two prominent dips (photonic pseudogaps) appear in the calculated PDOS. Com-
pared with the PDOS, the calculated reflection peaks and photonic pseudogaps
show a one-to-one correspondence. This indicates that photonic pseudogaps are
the ultimate cause of the non-iridescent structural coloration of RCP photonic
structures.
8.6
Outlook
Photonic structures occurring in the biological world show striking diversity,
complexity, and delicacy. They have received increasing research interest from a
variety of fields ranging from physics, biology, and chemistry to material science
due to their fundamental and application-oriented significance. The studies of
natural photonic structures and resultant structural colors may not only provide
insights into our understanding of their biological and physical aspects but also
offer valuable inspirations. Despite considerable advances made especially in recent
years, there exist still many important questions and challenging problems to be
answered. Interesting topics related to photonic structures in the biological world
include their evolution, structural formation, function, structural characterization,
material determination, mechanism, and bio-inspired design and fabrication.
The evolution of natural photonic structures, a fundamental problem, is poorly
researched. Although some progress has been made in recent years, there remain
a number of interesting and challenging problems [
2
,
58
,
59
,
145
,
153
-
156
]suchas
the evolution mechanism, how natural photonic structures develop and form during
individual growth, and the underling genetic mechanisms that determine phenotype.
