Biomedical Engineering Reference
In-Depth Information
As shown in Fig.
7.16
a(v), the transverse cross section of the wing scales consists
of 21 alternative cuticle and air layers, which share almost the same thickness
( 95 nm). The multilayer structure of the concavities (Fig.
7.16
a(iii)) produces the
green reflection light and gives rise to a main reflection peak at 550 nm for normal
incident light. The 2D photonic crystal slab of the ridges tilts 30
ı
to the surface
of scales, interacting with the normal incident light at 60
ı
and producing a small
reflection located at 380 nm. Under 45
ı
incident light, the main reflection peak
produced by concavity blue-shifts to 350 nm; the ridges interact with the incident
light at 15
ı
and give rise to the small reflection peak at 550 nm. Therefore, the
two spectra peaks originating from the concavities and the ridges could be observed
for both normal and 45
ı
incident light. The shifting of peaks suggests the iridescent
property of structural color for
P. ulysses
. These two reflection peaks mix to the blue
color perceived by human eyes.
The bright green-colored wings of
P. blumei
(Fig.
7.16
b, another breed of
Papilio
butterfly) result from a juxtaposition of blue and yellow-green light reflected from
different microscopic regions on the wing scales. Optical microscopy reveals that
these regions are the centers (yellow) (Fig.
7.16
b(ii)) and the edges (blue) of
concavities (Fig.
7.16
b(iii)). Unlike
P. ulysses
, the concavities of
P. blumei
are cap
shaped, of 4-6
m in diameter (Fig.
7.11
b(vi)). The profile is much deeper than
P.
ulysses
. The inclined sides of each concavity tilt 45
ı
with respect to the horizontal
surface, and the opposites of each concavity are perpendicular to each other. The
ridges run through the full length of each scale with a periodicity of 7-8
m. The
transverse cross section of the concavities also consists of 21 alternative cuticle and
air layers with a thickness around 110 nm (Fig.
7.16
b(v)). For the normal incident
light, theoretical calculation predicts that the reflection peak resulted from the flat
portions locates at 600 nm, in agreement with the yellow color observed under
optical microscopy, while the light incident on the edges of concavities with an
angle 45
ı
, producing a reflection peak at 450 nm, in accordance with the blue
color observed under optical microscope. The light incident on one side of the
concavity, reflected from one 45
ı
side, travels across the concavity to the opposite
orthogonal side and then reflects backward in parallel to the original incident
direction. Through this double reflection process, the blue reflected light undergoes
a polarization conversion. As a result, it survives upon the crossed polarizers. Under
45
ı
incident light, the reflection peak arising from the flat portions is 450 nm
(blue color) (Fig.
7.16
b(iii), right), while the light is incident normally on the
inclined sides and gives rise to a reflection peak located at 600 nm (yellow color)
(Fig.
7.16
b(ii), left). Therefore, for normal and 45
ı
incident light, the cap-shaped
concavities produce both yellow and blue colors. These two colors mix up to the
green coloration caught by human eyes. It follows that the two breeds of butterfly
take advantage of the color-mixing strategy. The blue color of
P. ulysses
is mixed by
the green and deep purple colors reflected by concavities and ridges, respectively.
The green color seen from
P. blumei
is a mixture of yellow and blue colors reflected
by the flat portions and inclined sides of concavities, respectively. We notice that
many studies show that the eyes of the butterflies have a duplicated gene, allowing
them to see ultraviolet colors and distinguish the spectral properties and spatial
