Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 214

Fig. 8.4 Fabrication of
arrays of polymer nanopillars
with iridescent colors: ( a )
nanoimprint lithography and
shear patterning and ( b )the
collapsed array of nanopillars

a b

polymer

silicon nitride mold

retroreflection from the edges of the multilayer cavity leads to polarization rotation
of the incident electromagnetic field. This retroreflection is at the origin of color
mixing. The optical characteristics of the Papilio blumei wings have been replicated
by a multilayer structure consisting of hexagonally arranged concavities, with a
diameter of 4:5m and a height of 2:3m, fabricated by replicating spherical
polystyrene colloids with 5-m diameter by gold or platinum electroplating of
the interstices in the colloid array. A multilayer consisting of alternating alumina
and titania layers, with average widths of 82 and 57 nm, respectively, is then
deposited directly on the template after colloid removal or forms a planar structure
on top of the template covered by molten polystyrene colloids. Although the color
creation mechanism is different in the two cases, both structures mimic well the
butterfly wings.
Besides angle-dependent, iridescent colors, (almost) angle-independent struc-
tural colors can be found in nature, for instance, in bird feathers, which originate
from light scattering from isotropic structures. One of the methods to mimic struc-
tural colors involves mixing two self-assembled colloidal polymer nanoparticles
with different sizes [see ( Forster et al. 2010 ) and the references therein]. For
example, a mixture of such spherical polystyrene nanoparticles with mean diameters
of 226 and 271 nm shows reduced angle dependence, the perceived color being
determined by the thickness of the colloidal film and/or by the concentration of an
added broadband absorber [carbon black in ( Forster et al. 2010 )]. These synthetic
mixtures can be considered as biomimetic since they consist of high refractive
index spheres in air, while the bird feathers of Lepidothrix coronata , for example,
incorporate spheres of air in a background with a high index of refraction.
Many insects show varying hues as the incidence angle of light changes
from normal to grazing as well as metallic colors that originate from specular
light reflection. For example, the cuticles of Chrysochroa vittata , also known as
the woodboring beetles, display red iridescence at normal incidence and green
iridescence at illumination close to the grazing angle (see Fig. 8.5 ). This color
change can be explained modeling the stacks of about 20 chitin layers with a
thickness of 194 nm separated by 10-nm-thick and irregular air gaps that form the
cuticle as part of a periodic infinite structure/photonic bandgap ( Vigneron et al.
2006 ).
If the refractive index contrast between the layers that make up the photonic
bandgap is not too high (1.56 in chitin and 1 in air in the woodboring beetle), the
dominant colors in the reflection spectrum (those at which the reflection coefficient