Biomedical Engineering Reference
In-Depth Information
distribution of the visual colors [
174
]. And also, they are sensitive to the polarized
light. Therefore, the knowledge on the structural origination of the two reflection
peaks and polarization property of butterfly wings would have broad biological
implications [
175
,
176
]. Furthermore, with the understanding of the correlation
between the optical properties and the corresponding structures, researchers could
be able to find a way to mimic natural structural colors with designated properties.
Evidently, the ability to mimic the structural color with its spectacular function will
broaden the biomimicry field in the design of structural color materials targeted for
ultra and smart performance.
Kolle et al. adopted the combined techniques of colloidal crystal self-assembly,
sputtering, and ALD to fabricate photonic structures that mimic the color mixing
and polarization effect found on the colored wing scales of
P. blumei
[
172
]. They
demonstrated the replication of the periodically shaped multilayer structure of the
Papilio
butterfly scale in five steps (Fig.
7.16
c). Polystyrene colloids with a diameter
of 5
m were assembled on a gold-coated silicon substrate to create regularly
arranged concavities. A layer of platinum or gold with thickness 2.5
m was then
electrochemically grown into the interstitial space between the colloids, creating
a negative replica [
177
-
179
]. Ultrasonication of the sample in dimethylformamide
or acetone removed the colloids, resulting in a template of hexagonally arranged
metal concavities. An 20-nm-thick carbon film was sputtered onto the gold surface.
Finally, a conformal multilayer of thin quarter-wave titania and alumina films
was grown by ALD [
180
]. The carbon layer between the gold (or platinum) and
the multilayer stack adsorbs light passing through the multilayer stack, reducing
specular reflections and unwanted destructive interferences that would otherwise
severely limit the optical performance.
During the fabrication process, the diameter and the height of the concavities
for the artificial mimic were well controlled by the size of colloidal spheres and
thickness of platinum or gold layer. The number of the alternating titania and
alumina layers, as well as their thicknesses, could be carefully controlled during
the ALD process. By choosing proper thicknesses of the titania and alumina layers,
the stop-band center wavelength for the artificial multilayer structure could locate
550 nm, matching the reflectance band of the natural
P. blumei
structure closely. Due
to the concave structure, the center of the concavities exhibit yellow-green reflection
( 550 nm), while the reflection light from the four segments of the concavities
blue-shifts to blue color. The observation of the artificial mimic between crossed
polarizers leads to a similar effect as described for the
P. blumei
structure. Only
light incident onto four segments of the concavity edges is detected. The local
surface normal of 45
ı
gives rise to a double reflection at the opposing cavity
walls, causing a polarization rotation. The artificial mimic therefore displays the
same optical characteristics as the natural
P. blumei
wing scale structure.
Nevertheless, this approach described above needs to fabricate complex nanos-
tructures, wherein the two reflections come from different parts of the structure.
This technique is costly and difficult to obtain different properties by tuning the
structure. In this regard, the key challenges in future work to biomimic natural
structural color turn out to be how to design and fabricate photonic crystals with the
