Unveiling Spatial Correlations in Biophotonic Architecture
of Transparent Insect Wings
Pramod Kumar
, Danish Shamoon
, Dhirendra P. Singh
, Sudip Mandal
and Kamal P. Singh
Femtosecond Laser Laboratory, Department of Physical Sciences,
Indian Institute of Science Education and Research (IISER)
Mohali, Knowledge City, Sector 81, S.A.S. Nagar, Mohali-140306, Punjab, India
Department of Biological Sciences, Indian Institute of Science Education and Research (IISER)
Mohali, Knowledge City, Sector 81, S.A.S. Nagar, Mohali-140306, Punjab, India
Keywords: Natural Photonic Structures, Optical Diffraction and Interference, Fourier Optics, Nanophtonics and
Photonics Crystal.
Abstract: We probe the natural complex structures in the transparent insect wings by a simple, non-invasive, real time
optical technique using both monochromatic and broadband femtosecond lasers. A stable, reproducible and
novel diffraction pattern is observed unveiling long range spatial correlations and structural-symmetry at
various length scales for a large variety of wings. While matching the sensitivity of SEM for such
microstructures, it is highly efficient for extracting long range structural organization with potentially broad
Natural Photonics structures (Paris et al., 2012);
(Mika et al., 2012); (Parker, 2009); (Xu et al., 2007)
in the transparent insect wings have attracted much
attention in recent years not only because of their
potential for various biomimetics technological
applications but also as ideal test bed to learn
principles of coherent manipulation of light by
nature (Wiederhecker et al., 2009); (Mathias et. al.,
2010); (Shevtsovaa et al., 2011). Compared with the
equivalent man made optical devices, the
biophotonic structures often possess large
complexity and even render better performance in
some cases (Bar-cohen, 2011). One of the main
organizing principles of complex patterns in
transparent insect wings is their symmetry and long
range correlations at various length scales from
nanometer to micrometer structures. Hence the study
of symmetry in natural structural arrangements is
crucial to explore novel optical effects (Shevtsovaa
et al., 2011); (Pouya et al., 2011); (Trzeciak et al.,
2012). Variations in both dimensionality and degree
of periodicity contributes greatly to the over-
whelming variety of common optical phenomena
reported like reflection, refraction, interference,
fluorescence, iridescence, and so forth (Jordan et al.,
Insect wings are a multifunctional material
system having various distributions in size and
shape, spatial heterogeneity in its structural
arrangements, and orientation of the photonic
architectures. Despite this quasi-disorder wings are
known to manipulate light in a coherent way. Many
studies and techniques like SEM/TEM have
postulated various explanations for insect wings
complexity with high resolution locally. However, a
systematic and efficient approach to explore long
range structural correlations over the entire length
scale of the wing is desired. Because of the absence
of any obvious periodicity, such systems are in
general difficult to handle in theory and a super-cell
is usually needed in numerical simulations
(Mihailescu and Costescu, 2012); (Kenji Yamamoto
et al., 2012). Here we optically probe long range
correlations and spatial symmetry of the structural
organization of the photonic architecture of wing.
Due to the sensitivity of diffraction, our technique
matches the local resolution of SEM for such
microstructures, yet is highly efficient to extract in
situ structural organization which would be very
tedious otherwise. Understanding these symmetry
principles are crucial for biomimetics photonic
structures as well as functional significance of the
Kumar P., Shamoon D., P. Singh D., Mandal S. and P. Singh K..
Unveiling Spatial Correlations in Biophotonic Architecture of Transparent Insect Wings.
DOI: 10.5220/0004339001060110
In Proceedings of the International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2013), pages 106-110
ISBN: 978-989-8565-44-0
2013 SCITEPRESS (Science and Technology Publications, Lda.)
photonic design.
The experimental setup primarily consists of a wing
sample holder mounted on a precision xy translation
stage, a collimated laser beam, and a screen (see Fig.
1 for a real picture). We have used both a
monochromatic cw laser and a broadband
femtosecond laser in near IR range. The choice of
these wavelengths is dictated by the observed
transparency of the wing material for these spectral
regions. The typical 1/e
beam waist of laser passing
through the wing is ~2mm and the diffracted light is
collected on a white screen placed about few meters
Figure 1: (a) Femtosecond laser system; (b) SEM image of
the insect wing surface; (c) Actual picture of the
experimental set-up for the diffraction measurement.
The snapshots of the diffraction pattern on the
screen is recorded by a digital camera and the
obtained images are analyzed using Matlab
programming. It is worth mentioning that with this
simple setup, no preparation of the wing sample is
required and in fact it can be used for in viva non-
destructive imaging with the insect alive. By
scanning the beam spot across the wing various
regions can be probed in a single-shot manner.
A typical diffraction pattern of the wing in
transmission is shown for a femtosecond broadband
laser cantered at 800nm (top panel in Fig. 2) and
with a green 532nm cw laser (bottom panel in Fig.
2). A stable diffraction pattern is clearly visible for
both the lasers that exhibit a bright central spot and
up to two distinct maxima on either side (see
intensity cuts of the corresponding profiles in Fig.
2). The dimensions (1D, 2D, 3D) of periodicity
affect the light spectrum which could change the
surface colour or forms anti-reflective transparent
surface. That is why, we used broadband light pulses
to reveal the correlations in structural symmetry at
various length scales. Due to complex multi-scale
architecture and their local orientational symmetry,
the light diffract in such way that the global
structures reflect a single homogeneous photonic
surface. A SEM imaging of the wing surface (Fig. 3)
reveals that it consists of a large quasi-periodic
arrangement of micro-hooks on the surface of
transparent wing. Clearly, SEM provides local
details of the structural arrangement of the shape and
short range correlation. However, due to their quasi-
periodicity (Liu et al., 2011) these micro-hooks
create stable diffraction pattern that is the central
observation of this work. Notably, for a mm size
beam spot, the pattern is a result of interference
created by several thousands of these hooks. In
principle, one can extract all possible structural
information, including long range distribution, mean
spacing between these, and their shape distribution.
Figure 2: Top left corner: photograph of the diffraction
pattern using 7fs, 2nJ@78MHz laser incident beam, the
inset in figure of top left corner is the corresponding beam
profile; top right corner: ID intensity profile of the
diffraction pattern; bottom left corner: photograph of the
diffraction pattern using 532 nm green laser beam, the
inset in figure of bottom left corner is the corresponding
beam profile; bottom right corner: intensity profile of the
diffraction pattern.
To analyse the diffraction pattern formation, and
hence the structural correlation of photonic structure,
on various length scales we need methods that allow
two things (i) clear identification of individual
surface structures, and (ii) determination of the
distribution function of size, shape and the
separation of the structures to quantify the structural
uniformity, i.e., the degree of organization. The first
is of course accomplished by direct imaging,
whereas for the second the optical diffraction
technique is best suited.
To analyse the data one has
to distinguish between regular surface structures
resulting in diffraction pattern and continuous
surface causing a diffuse intensity in the experiment.
There are two extreme cases of structural order:
the first one is the random arrangement of the
nanostructures that is characterised by complete
absence of any long range order. These structures
show a fractal-like behaviour on the short length
scale and is smooth on the long length scale. The
other extreme case is a perfectly periodic structured
surface that exhibit complete long range order. In
this case, the 2D power spectrum consists of a
periodic arrangement of sharp peaks as they are
known for diffraction grating. From the orientation
and symmetry of these peaks the real space
orientation and the symmetry of underlying structure
can be deduced.
The location of the first order peak in the Fourier
space yields real space lateral periodicity of the
pattern in the corresponding direction. For a non-
identical order, the peaks broaden and the number of
detectable higher order peaks decrease. The full
width at half maximum of the first order peak is a
quantitative measure of the width of the distribution
function of the structural separations. In other words,
narrower the peaks and larger the number of high-
orders, higher is the uniformity of the surface
pattern. If the separation between these structures
are smaller, i.e., of the order of its size, this produces
a more divergent pattern where the distance between
and the first order peak is larger. The width of
these peaks encodes information about their
distribution of shapes and sizes of the individual
hook elements. This therefore provides a measure to
quantify the organization of such structure. The
intricate arrangement of hooks on the surface
architectures control how the photons of light
interact with them. To prove that this technique,
besides being simple and non-invasive, offers
sensitivity matching that of SEM, we have taken
SEM images of the wing on various scales.
The two-dimensional diffraction pattern (Kenji
Yamamoto et al., 2012) by an aperture A(x, y) is the
sum of wave produced by the light source at every
points of the aperture. The diffracted wave front
observed at a distance z from the aperture is given
Since we will only observe the intensity of the
signal, so it can be shown that the far field
diffraction pattern is the Fourier transform of A(x,
y). Then the equation (1) can be written as:
)},({),(' yxAFTA
A fast Fourier transform of these images generates a
computational far-field diffraction pattern that
resembles very closely to the observed pattern (see
Fig. 3).
Figure 3: (left column) SEM images of the wing surface at
200µm, 100µm, 10µm, 1µm length scales ; (right column)
the right column is the FFT pattern computed for the
corresponding images.
The diffraction pattern on various length scales
reveal how the higher order peaks and their
distribution reflect the structural symmetry and
correlation between these hooks. This disorder
arrangement will results in broadening of diffraction
pattern. As illustrated in Fig 3, the diffraction by a
single hook is compared with the corresponding
diffraction from an array of hooks at various length
scales on the wing surface.
Furthermore, we have observed the rotation of
the diffraction pattern when one translates the beam
spot across the wing as shown in Fig. 4. This
unusual behavior reflects both the local and global
patterns in the variation of the distribution
Figure 4: SEM image of the wing, the inset on the top of
the image are the rotated diffraction pattern profile at
various regions on the wing surface.
It is possible that by varying the spot size from few
microns to several mm, one can obtain the spatial
correlation in a high throughput, single-shot manner.
If such information is attempted by SEM imaging it
would be very tedious and inefficient process. The
spatial correlation of the light and high sensitivity of
the diffraction pattern offer unique advantage over
other methods. It is therefore a potentially attractive
optical technique to unravel the natural design of the
photonic architecture of the insect wing and probe
their functional relationship that ultimately dictates
the motive of symmetry and correlation.
In summary, the proposed optical technique may
prove to be a powerful alternative to gain a better
understanding about the systematic of photonic
architecture such as long range spatial correlations
and symmetry in the insect wings. Our finding
directly demonstrates how the transmitted diffraction
pattern from the wing is correlated with the internal
structural symmetry. The rotations of diffraction
pattern were obtained when the laser beam scanned
various regions on the wing surface as shown in Fig.
4. These rotations of the pattern give the signature of
the spatial symmetry in structural arrangements in
the local to global length scale. These optical tools
could be crucial to understand design principles of
natural photonic crystals with potential applications
for mimicking artificial structures (Mathias et al.,
2010); (Bar-cohen, 2011) that may lead to novel
optical devices
All authors thank to the DST and IISER Mohali,
India for supporting this research through grant and
research fellowship for Dr. Pramod Kumar. The
invaluable help of Babita Basoya and Gopal Verma
are grateful acknowledged.
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