Biomedical Engineering Reference
In-Depth Information
structural colors
[25, 41-43]
: fundamental opti-
cal processes, natural photonic structures pro-
ducing structural colors, and applications of
structural color.
The brilliance of structural colors and the
huge potential for replication and engineering
applications, as well as the big impact of pio-
neering Japanese research in this area, inspired
the L'Oréal Art & Science Foundation in Japan
to create the L'Oréal Art & Science of Color
Prizes in 1997. Major monetary prizes are
awarded to artists and scientists for research on
color, the veritable link between art and science
[44]
. The 7th L'Oréal Art & Science of Color
Gold Prize was awarded to Akira Saito, Shinya
Yoshioka, and Shuichi Kinoshita (Osaka Univer-
sity) and Keiichiro Watanabe, Takayuki Hoshino,
and Shinji Matsui (University of Hyogo) for
their comprehensive research on the reproduc-
tion of
Morpho
blues using semiconductor
lithography (the group from Osaka University)
and the fabrication of individual nanoscale
units with an accuracy of 10 nm (the group from
University of Hyogo).
With the development of specialized tech-
niques to make nanostructures, a new research
area has opened up in the present decade: rep-
lication of biological templates such as the
wings of butterflies and cicadas. Nanofabrica-
tion techniques used for bioreplication include
atomic layer deposition
[45]
, nanocasting
[46]
,
nanoimprinting
[47]
, and physical vapor depo-
sition
[48]
. (See also Chapters
14-16.)
structured surfaces
[11, 25, 49, 50]
. In general,
these optical phenomena can be classified into
two groups of mechanisms of interaction of
light with matter: (1) scattering from electri-
cally small particles, and (2) Bragg phenom-
ena, exhibited by structures with periodic
morphology.
11.3.1 Structural Colors Due to
Scattering from Electrically
Small Particles
Scattering from irregular structures is known
to produce color in the biological world. Mason
[51]
reported that the bluish color of some jel-
lyfish (
Cyanea lamarcki
) is that of a suspension
of electrically small colloidal particles. Such a
color is called a
Tyndall blue
, which varies from
deep blue to pale white, depending on the par-
ticle size. The body colors of the dragonflies
Mesothemis simplicicollis
and
Libellula pulchella
arise for a similar reason
[6]
. Tyndall scatter-
ing is also the cause of
chatoyancy,
or the
cat's
eye
effect displayed by many minerals--such as
the tiger's eye shown in
Figure 11.4
--but even
more spectacularly by the gemstone chrysoberyl
(beryllium aluminum oxide). Chatoyancy, arises
from the fibrous inclusions or cavities within the
stone, and the luminous streak of reflected light
is always perpendicular to the direction of the
fibers.
Tyndall blue arises from the scattering of light
from three-dimensional particles that are small
(at least a tenth in maximum linear dimension)
with respect to the minimum wavelength of the
incident light and with a refractive index close
to unity. Since the intensity of the light scattered
by an electrically small particle depends directly
on the fourth power of the frequency
[52]
, light
of shorter free-space wavelength (or higher fre-
quency) is scattered more strongly than light of
longer free-space wavelength (or lower fre-
quency). Scattering by electrically small parti-
cles is called
Rayleigh scattering
. The Rayleigh
11.3 PHYSICAL MECHANISMS FOR
STRUCTURAL COLOR
The following optical phenomena have been
identified as the causes of structural colors
in nature: thin-film interference, multilayer
interference, diffraction-grating effect, scat-
tering from irregular assemblies of small
particles, and collaborative effect in irregular
