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

applicability.

1 INTRODUCTION

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.,

2012

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

106

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.

2 EXPERIMENTAL SET-UP

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

away.

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.

3 RESULTS AND DISCUSSION

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

UnveilingSpatialCorrelationsinBiophotonicArchitectureofTransparentInsectWings

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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

zero

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

by:

dxdy

yxA

ikr

),()','('















(1)

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





(2)

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.

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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

symmetry.

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.

4 CONCLUSIONS

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

ACKNOWLEDGEMENTS

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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