Photonic Structures for Coloration in the Biological World - Bioinspiration: From Nano to Micro Scales - page 299

Fig. 8.18 Beetle C. gloriosa seen under unpolarized light or with a left-handed circular polarizer
( a ) and with a right-handed circular polarizer ( b ). In ( a ) the bright green color with silvery
stripes can be seen. In ( b ) the green color is mostly lost. ( c ) Schematic representation of twisted
arrangement of a chiral multilayer structure (Reproduced from [ 92 ])
reflect circularly polarized light [ 10 ]. The ability to reflect circularly polarized light
in certain beetles has attracted considerable attention for nearly a century [ 82 - 92 ].
Ellipsometric measurements confirmed that scarab beetles can reflect predominately
the left-handed circular polarization at visible wavelengths. In some cases, however,
there is a tendency to reflect the elliptical or right-handed circular polarization at red
and near infrared wavelengths [ 90 ].
The jeweled beetle Chrysina gloriosa possesses a brilliant metallic green color
and can reflect selectively the left-handed circularly polarized light [ 92 ], as shown in
Fig. 8.18 . The exocuticle of the beetle consists of many layers of microfibrils aligned
parallel with each other, causing a preference for light polarized along the direction
of the fibers. Each layer is rotated slightly relative to the layer above, forming a
helicoidal stack twisting in the left handedness. This alignment of the layers enables
the reflection of the left-handed circularly polarized light, a phenomenon similar to
that in cholesteric liquid crystals.
This circularly polarized reflection was also found in some plant species [ 71 - 73 ].
The multilayers found in the outer epidermal cell walls of some iridescent plants
produce more elaborate structures, alternating with arcs of cellulose microfibrils.
The angle of the cellulose microfibrils changes gradually through the alternating
layers up to a total of 180 ı rotation. The resulting helicoidal structure is remarkably
similar to the helical stack of chitin microfibrils, and may be an example of
convergent evolution [ 73 ].
Most of the multilayers in the biological world take the form of flat layers
to produce structural coloration. In flat multilayers, the resulting structural colors
are usually very conspicuous and strongly iridescent. However, there exists also a