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The cuticle of many beetles exhibit optical properties such as the
selective reflection of the light, and the high optical rotation
of transmitted light, due to the helical structure of chitin
molecules. The rule of polarization selectivity is also respected,
apart from Plusiotis Resplendens: for this gold beetle, the
reflected light consists of both left and right circular polarizations;
a half-wave plate is included in the multi-layer structure
of the cuticle and functions for the wavelengths between 520
and 640 nm. Indeed, by placing two similar filters either
side of a λ/2 plate, the right-handed polarized light
that is passed through the first filter is converted into
left-handed polarized light by the λ/2 plate and reflected
from the second filter[*]. | In
this way a simple notch filter is produced. Man-made materials
have been produced by following such a fabrication scheme.
By choosing to stack filters, it is also well known that a
couple of cholesteric cells with the same mean reflection
wavelength but opposite helicity senses may be associated
to increase the reflected light intensity.
The right- (left-) handed transmitted component through the
upper layer is then reflected from the layer below, passes
back through the upper layer, and contributes to the reflected
left- (right-) handed component from the upper layer; the
total reflectance may thus approach unity. From purely practical
and technological considerations, one of the goals of the
elaboration process we present was exactly to give up the
idea of stacking layers to add up their individual optical
properties. Indeed, the above solutions cannot be reached
with fluid media like usual low molar mass lcs or anisotropic
gels (due to the diffusion between soft layers) and optical
filtering with a multilayer-system is sensitive to optical
defects and losses at the interfaces (between organic layers
or, still worse, sandwich-glass cells); additionally, it is
more interesting to envisage the subsequent modification of
the reflection properties by applying an electric field to
a single glass cell. The increase of the reflectance was solely
reached by playing with the thermal history of a single mixture
layer during the UV light induced fabrication of the gel.
[*] S. Caveney, Proc.Soc.Lond.B, 178, 205 (1971).
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