Biomedical Engineering Reference
In-Depth Information
Fig. 8.22
(
a
) Ventral side of a Madagascan sunset moth
C. rhipheus
.(
b
)SEMimageofa
longitudinal cross-section of the hind wing. (
c
) Cross-sectional TEM image of a cover scale in
the longitudinal direction. The enlarged views at various positions are given in the insets. Scale
bars: (
b
) 300
mand(
c
)2
m (Reproduced from [
98
])
8.5.3
Diffraction Grating
Diffraction gratings can generate structural colors by diffraction. In addition to the
specular reflection (zero-order diffraction), light can be diffracted at non-specular
angles. For natural light, the zero-order diffracted light should be white in color.
For a fixed observing angle that differs from the specular one, diffracted light with
wavelengths that satisfy the grating equation (
8.3
) can be perceived. By varying
the observing angle, color changes can be seen, leading to iridescence. The ability
to disperse spectral light spatially renders diffraction gratings as an indispensable
optical element [
100
]. Diffractive colors and their intensity and spatial distributions
depend on many parameters such as the grating period, grating morphology, and
refractive index of the grating.
The first revealed biological diffraction grating was probably the one found in
1942 by Anderson and Richards [
56
] on the elytra of the scarab beetle
Serica
sericea
. This dark brown beetle possesses line gratings on its elytra with a period
of 800 nm, which can produce brilliant iridescent colors. A taxa list of beetles
with cuticular diffraction gratings was described later by Hinton and Gibbs [
101
-
107
]. Diffraction gratings are widespread in beetles in 1D as well as 2D forms
[
46
,
108
,
109
], as shown in Fig.
8.23
.
The beetle
Sphaeridiinae gen.
sp. (Hydrophilidae) displays bright diffractive
colors caused by an ordered diffraction grating on its elytral surface, as shown
in Fig.
8.23
a. Up to the third-order diffraction can be clearly seen. The zero-
order diffraction is just the specular reflection, showing a bright white appearance.
Higher-order diffraction creates interesting rainbow-like spectral colors. Unlike
that in the beetle
Sphaeridiinae gen.
sp., the cuticular diffraction grating in the
