Biomedical Engineering Reference
In-Depth Information
Fig. 8.26
SEM images of the corneal nipple arrays in the nymphalid
Pseudozizeeria maha
(
a
)and
the lycaenid
Pseudozizeeria maha
(
b
), showing differences in nipple height and shape. Scale bar:
500 nm (Reproduced from [
117
])
corneal nipple arrays on the corneal surface of eyes in certain insects function as an
antireflective layer [
114
-
116
],asshowninFig.
8.26
. Similar structures were also
found on the transparent wings of certain insects [
117
-
119
], which can increase the
transparency of the wings and hence reduce visibility.
8.5.4
Photonic Crystal
Photonic crystals are composite structures with a spatially periodic variation of
refractive index [
27
-
29
]. As discussed above, light interactions with photonic
crystals involve scattering by the constituent units, diffraction by the periodic
arrangement of the units, and interference among scattered or diffracted light. Due
to the introduced periodicity, light propagation in photonic crystals is strongly
modified by the multiple Bragg scattering. As a result, the dispersion relation is
characterized by complicated photonic band structures displaying photonic bands
and even bandgaps (stop bands). For frequency within photonic bands, light
propagation is allowed. For frequency within photonic bandgaps, however, the
propagation of light is forbidden. There may exist a complete photonic bandgap for
frequency within which light propagations are forbidden for all directions. These
interesting features render the control of light propagation by photonic crystals
possible, potentially leading to revolutionary approaches in photonics [
29
].
According to the periodicity in different dimensions, photonic crystals can be
classified into 1D, 2D, and 3D. Periodic multilayers are 1D photonic crystals which
are already discussed in the previous subsection. Here, our discussions will focus
on 2D and 3D photonic crystals in the biological world. As aforementioned, there
is no complete photonic bandgap in natural photonic crystals due to the fact that
the refractive-index contrast among the constituent materials is not big enough.
Only partial photonic bandgaps exist along certain directions. For a photonic crystal
with a partial photonic bandgap along a certain direction, light cannot propagate
