Biomedical Engineering Reference
In-Depth Information
Fig. 8.23
Beetles showing diffractive structural colors (
upper panels
) and corresponding nanos-
tructures of diffraction gratings (
lower panels
). (
a
) Beetle
Sphaeridiinae gen.
sp. and SEM
image of the cuticular diffraction grating. Labels indicate the zero-, first-, second-, and third-
order diffractive colors. (
b
) Beetle
A. tumerus
and SEM image of the quasi-ordered, discontinuous
cuticular diffraction grating. (
c
) Beetle
Pallodes
sp. and SEM image of the 2D cuticular diffraction
grating (Reproduced from [
46
])
beetle
Aglyptinus tumerus
(Leiodidae: Leiodinae) is not very ordered, as shown in
Fig.
8.23
b. It consists of weakly parallel, discontinuous ridges, but still can diffract
incident light and produce structural colors.
In addition to 1D gratings, there also exist 2D diffraction gratings. In a 1D
diffraction grating, it cannot diffract light along the direction perpendicular to the
grating. In contrast, 2D diffraction gratings can diffract light in all azimuth angles.
The beetle
Pallodes
sp. displays a spectral “halo” on its elytra around the point
of the specular reflection, which is caused by a 2D cuticular diffraction grating, as
shown in Fig.
8.23
c.
Some marine animals also use diffraction gratings to produce iridescent colors.
One example is the halophores (hairs) on the first antenna of the seed-shrimp
Azygocypridina lowryi
(ostracod) [
110
],asshowninFig.
8.24
. The halophores
possess a diffraction grating with a period of about 600 nm on the surface. The
colors on the halophores are caused by this diffraction grating. Different diffraction
colors are due to the different orientations of the halophores.
In addition to animals, diffraction gratings also exist in plants, e.g., on the petals
of the flowers
Tulipa
sp.,
Hibiscus trionum
,and
Mentzelia lindleyi
[
111
,
112
], as
shown in Fig.
8.25
. In these species, the petal epidermal cells are elongated and flat.
In white epidermis, the epidermal cell surface is smooth. In contrast, the pigmented
(dark) epidermal cell surface is microscopically uneven, marked with microscopic
striations. These striations are nearly periodically arranged, acting as a diffraction
