Biomedical Engineering Reference
In-Depth Information
In principle, TMM, FDTD, and PWE are applicable to arbitrary photonic
structures. For some special cases, e.g., photonic structures consisting of spheres
or spherical cylinders, the scattering matrix method based on Korringa-Kohn-
Rostoker techniques is much more efficient [
36
].
8.5
Photonic Structures Occurring in the Biological World
Color-producing photonic structures are widely spread in the biological world [
1
-
9
].
With the advancement of characterization tools, more and more interesting natural
photonic structures have been revealed, including thin films, multilayers, diffraction
gratings, photonic crystals, amorphous photonic structures, and even composite
structures. These photonic structures show a great diversity of construction and
topology, leading to optical effects that may implement vital biological functions.
8.5.1
Single Thin Film
A single thin film is the simplest structure that can produce structural coloration via
thin-film interference. Perceived structural colors are due to the constructive inter-
ference of reflected light from the upper and lower surfaces. Thin-film interference
will produce a series of harmonic reflection peaks with equal intensity and frequency
interval if the refractive index of a thin film is wavelength independent. However,
the intensity of the reflection peaks is rather low. For example, for a thin film with a
refractive index of 1.5 its maximal reflectance is less than 20% at normal incidence.
In contrast, a multilayer can achieve a very high peak reflectance, ideally as high as
100% in principle.
The wings of most insects are mainly composed of two integument layers
apposed closely to form a thin membrane. It was found more than 50 years ago that
the transparent wings of some houseflies can produce structural colors via thin-film
interference [
37
]. However, the biological significance of the structural colors as
well as color patterns caused by reflecting insect wings has been largely disregarded.
This might be due to the fact that these colors are simply produced by thin-film
interference like soap bubbles, and imaginarily supposed to flash randomly over the
wing surfaces.
In a recent paper [
38
], the structural-color patterns in the transparent wings of
small Hymenoptera and Diptera were studied. These extremely thin wings reflect
vivid structural-color patterns caused by the wing membranes. The visibility of these
color patterns depends strongly on backgrounds, as shown in Fig.
8.7
. In a white
background, the reflected structural colors and resulting color patterns are faint or
even invisible. This is due to the transparency of the wings. Background light can
transmit the wings which will overpower the interference colors. In contrast, these
colors and color patterns can be easily perceived by any animal with color vision in a
