Biomedical Engineering Reference
In-Depth Information
Fig. 8.21
(
a
) Indonesian male butterfly
P. palinurus
.(
b
) Close-up optical image of the surface
of a
P. palinurus
iridescent scale, showing the dual-color nature. (
c
) Cross-sectional TEM image
showing one concavity on a
P. palinurus
iridescent scale. The inset is the SEM image of the surface
of an iridescent scale (Reproduced from [
97
])
In the wing scales of the Indonesian male butterfly
Papilio palinurus
[
97
], there
also exits a sculpted multilayer, as shown in Fig.
8.21
. The bright green coloration
of the wing scales is caused by this ingeniously designed sculpted multilayer,
which enables the reflection of yellow light at normal incidence from the base of
each concavity and blue light through a double reflection from the opposite and
perpendicularly inclined sides of each pit, producing a blue annulus with a yellow
center. In the beetle
C. obscuripennis
shown in Fig.
8.19
, however, the inclined sides
produce a dull blue, giving a minor contribution to color mixing. The production of
a blue color at the inclined sides in the
P. palinurus
butterfly lies in the double
reflection by a pair of orthogonal surfaces inclined about 45
ı
to the scale surface.
At normal incidence, reflected light from one inclined surface is directed across
to its adjacent inclined side, where it is reflected back to the incident direction. In
the beetle
C. obscuripennis
, the inclined angle is, however, much smaller than 45
ı
.
Moreover, the pits in the beetle
C. obscuripennis
is hexagonal rather than square in
the butterfly
P. palinurus
. As a result, at normal incidence double reflection cannot
cause light emergence back to the incidence direction. Nevertheless, at oblique
incidence we may still observe a dull structural color produced by the inclined sides
(see Fig.
8.19
b).
In the Madagascan sunset moth
Chrysiridia rhipheus
, curved multilayers were
found showing interesting features [
20
,
98
,
99
]. The wings of
C. rhipheus
exhibit
a striking iridescent pattern from the arranged scales, as shown in Fig.
8.22
.The
scales consist of a stack of three to six thin layers and are highly curved from the
root to the distal end. Owing to this strong curvature, a valley-like deep grooved
structure is present between adjacent two rows of the regularly arranged scales. The
combination of the grooved structure and the multilayer gives rise to two different
colors: one is the direct reflection from the top flat part of the scales, and the other
is the dual reflection between adjacent scales. This color mixing is also strongly
polarization-sensitive even under unpolarized illumination, because the reflectivity
is naturally different for
s
and
p
polarizations.
