Biomedical Engineering Reference
In-Depth Information
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Fig. 8.38 ( a ) Calculated PDOS of a model RCP photonic structure ( inset ) as a function of reduced
frequency d= ,where d is the diameter of the nanoparticles and is the wavelength in vacuum.
The PDOS of a homogeneous medium with a refractive index of 1.38 ( dashed red line )isgiven
for comparison. Photonic pseudogaps are indicated by arrows .( b ) Calculated ( solid line )and
measured ( dashed line ) reflection spectra. Measured reflection spectrum for green scales was
obtained by macro-optical spectroscopy. In the calculations, the nanospheres take a refractive index
of 1.56, a typical value for chitin, and their diameter is assumed to be 240 nm, a typical value for
the chitin nanoparticles in green scales (Reproduced from [ 148 ])
and cryptic coloration [ 157 ]. Noncommunicative functions of natural photonic
structures have also been proposed such as thermoregulation [ 158 ], structural
support and strengthening [ 159 , 160 ], friction reduction [ 161 ], and photoprotection
and visual enhancement [ 162 ]. Most of the biological, physical, and physiological
functions are hypothetical, and more research efforts are hence needed.
Structural determinations are essential in understanding structural coloration. For
simple natural photonic structures such as thin films, multilayers, and 2D photonic
crystals, they can be explored by SEM and TEM. As aforementioned, however,
structural determinations are sometimes extremely difficult especially for natural
photonic structures with complex building units or connectivity, e.g., nontrivial 3D
photonic structures, since frequently used tools such as SEM and TEM can only
offer 2D structural information in certain orientations. Combinational, correlational,
and advanced tools are highly desired in order to get correct structures.
Another important and also less known and researched problem is about the con-
stituent materials and their distribution in natural photonic structures, particularly
with refractive index (both real and imaginary components) and dispersion across
the relevant wavelength range. Additionally, surrounding materials are sometimes
also important in determining the optical response of natural photonic structures.
The structural and optical properties of surrounding materials ought to be studied in
detail.
Among the relatively better understood aspects of natural photonic structures are
the mechanisms of their structural color production. Generally, structural coloration
can be understood in terms of interference, diffraction, and scattering. As is
shown, natural photonic structures exhibit astonishing diversity and complexity. For
complicated photonic structures with architectures of different levels, their color
 
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