Biomedical Engineering Reference
In-Depth Information
coloration effects. This has led to the identification
of an enormous diversity in biological photonic
structures: from simple multilayer film compos-
ites to two- and three-dimensional periodic lat-
tices with chiral, honeycomb, and cubic geometries
(and combinations thereof)
[6-11]
.
The biggest structural surprise from a photonic
crystal standpoint came in 2008 with the discovery
of a photonic crystal with the “champion”
diamond-based lattice in the green Brazilian
weevil
L. augustus
(
Figure 14.8
)
[54]
. Soon after,
other weevils (
Eupholus schoenherri
,
Eudiagogus
pulcher
,
Pachyrhynchus moniliferus
) were identified
as creating their blue, green, and red exoskeleton
colorations with diamond-based photonic crystals
[26]
. These structures have the same lattice
geometries as
L. augustus
but different lattice
constants (
Figure 14.10
). It should be emphasized
that these biological structures are still the only
examples (to date) of diamond-based photonic
crystals with lattice constants comparable to
wavelengths in the visible spectral regime.
Diamond-based photonic crystal structures
are located within the cuticle scales of a weevil
[26, 54]
. These scales have a leaf-like shape
(around 100
×
50
×
5-20
μ
m
3
in size) and are
attached to the exoskeleton of the beetles. Each
scale has a solid outer shell and an interior dia-
mond-based photonic crystal framework, both
made out of chitin-based insect cuticle. The exact
structure of this diamond-based lattice was eval-
uated through various structural and optical
characterization tools, combined with modeling
and photonic band structure calculations. The
results showed a lattice composed of ABC stacked
layers of hexagonally arranged air cylinders in a
surrounding matrix of cuticle material. The
refractive index of cuticle (around 1.5) is not high
enough to form a complete photonic band gap in
any of the weevil photonic crystals. However,
calculations revealed that a complete band gap
would open in the green part of the visible spec-
trum if the photonic crystal framework of the
weevil
L. augustus
were made from a dielectric
material with a refractive index of at least 2.1,
number of approaches ranging from self-assem-
bly to lithography, direct-writing techniques,
and even mechanical drilling of arrays of holes
into ceramic blocks
[49-53]
.
Unfortunately, there are limitations in extend-
ing these methods to the visible range, due to
inherent size limitations of patterning and writ-
ing techniques and the lack of robust assembly
methods needed to yield diamond-based crystal
geometries with lattice constants of a few hun-
dred nanometers. As a consequence, a synthetic
three-dimensional photonic crystal with a com-
plete band gap in the visible spectral regime has
proven elusive.
14.3.2 Surprising Weevils
In contrast to our limited abilities in engineering
photonic crystals operating in the visible part of
the electromagnetic spectrum, biological species
have developed sophisticated structures to effi-
ciently interact with visible light. The results of
these interactions include large angular fields of
view, reduced surface reflection, Bragg diffrac-
tion, and multiple scattering
[6-9]
.
The latter two are of particular importance in
the world of insects, which often rely on struc-
tural colors for defense, camouflage, and repro-
duction. For example, it is the periodic variation
of biopolymeric compounds embedded into
wings and exoskeletons that lends many but-
terflies and beetles their iridescent appearance,
which can be used to scare off predators, hide
in plain sight, or attract mates. Furthermore,
photonic structures in these species were opti-
mized to function under various illumination
conditions, from glaring sunlight to the diffuse
and dim lighting on forest floors.
Combined with the pure beauty of biological
iridescence, these diverse applications and the
ability of these materials to function in a wide
range conditions have motivated biologists and
physicists alike. Great efforts have been under-
taken to search for the structural origins of these
