IT鈥橲 A STRANGE place to find one of the world鈥檚 finest optical engineers. But
there he is, entombed inside a drinks coaster, as dazzling in death as when he
fluttered through the Amazon rainforest. The male Morpho butterfly鈥檚
amazing tricks with light make him popular with manufacturers of novelty
coasters, trays and a whole line of other tacky ornaments. They are also the
reason why generations of physicists have taken a keen interest in
entomology鈥� intrigued by the way the butterfly produces such an intense,
electric blue colour. And they explain why today鈥檚 materials scientists have
taken such a shine to butterflies. They would like to borrow a few of their
optical innovations to create anticounterfeiting devices that would thwart the
forgers of bank notes or provide a cheap, paintless way to camouflage military
vehicles.
The Morpho鈥檚 iridescent blue is one of nature鈥檚 most eyecatching
colours. Henry Bates, the 19th-century English naturalist, reckoned he could
spot a Morpho almost half a kilometre away. More recent observers have
seen their blue flashes from small planes flying over the forest canopy. Yet
despite more than a century of study, physicists are still trying to discover
why the butterfly is so dazzling. The metallic blue of the male Morpho鈥檚
wing is a 鈥渟tructural colour鈥�. It owes its brightness and glitter not to
pigments but to the way the millions of tiny scales covering the surface of its
wings interfere with light. Just how sophisticated an optical apparatus
Morpho has is only now becoming apparent.
Far from the Amazon, in a gloomy basement laboratory at the University of
Exeter, Roy Sambles and members of his Thin-Film Photonics Group are piecing
together the story, one scale at a time. 鈥淵ou can鈥檛 understand all the
properties of Morpho鈥檚 colour until you study single scales,鈥� says
physicist Pete Vukusic. Sambles and Vukusic and their colleague Chris Lawrence
from the Defence Evaluation and Research Agency (DERA) in Farnborough have found
that these butterflies have evolved an array of special effects to make
themselves as visible as possible.
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A Morpho male is an exhibitionist. He wants to be seen, particularly
by other Morpho males. The bright colour is designed to intimidate any
rivals that might flutter into his territory. The more visible he is, the larger
the patch of forest he can defend.
The Morpho鈥檚 brilliance is the result of the microscopic
architecture of the scales that cover its wings. The wings of most butterflies
and moths are 鈥渢iled鈥� with overlapping scales, thin plates of chitin that slot
neatly into sockets on the wing. Each scale, typically measuring 75 by 200
micrometres and manufactured by a single epidermal cell, is generally etched on
the upper surface with a pattern of fine ridges a few micrometres apart. In the
Morpho butterflies, each of these ridges itself has an elaborate
structure which turns it into a mirror-like surface called a multilayer
reflector.
In cross section, a single ridge resembles a miniature Christmas tree, with
thin branches of transparent chitin sticking out to either side, short branches
at the top and longer ones near the bottom. The surfaces of each branch
interfere with light in the same way as a thin film of oil on water,
transmitting some colours and reflecting others depending on the thickness of
the layer [see 鈥淭o dazzle or not to dazzle鈥漖.
The more branches there are, the brighter the reflected blue. Some species of
Morpho have exploited this phenomenon, making themselves more dazzling
by adding extra branches to their Christmas trees鈥攁s Sambles and Vukusic
discovered when they measured the light reflected by single scales. They took
scales from two species of Morpho, the intensely blue M.
rhetenor, and the slightly pearly blue M. didius. By shining a
beam of laser light at a single scale fixed to the tip of a needle and
collecting the reflected light, they found that M. rhetenor鈥檚 scale
reflects 70 per cent of the blue light reaching it, an astonishingly high figure
for any natural material. The scale from M. didius reflected 40 per
cent of the blue light falling on it, still more than twice as much as any
natural pigment. The difference between the two species is easily explained: the
brighter butterfly has between 10 and 12 branches on each of its Christmas
trees, while the less dazzling species has between six and eight.
Although the efficiency of the Morpho鈥檚 multilayers was something of
a surprise, the team鈥檚 painstaking measurements of light reflected from
individual scales turned up something even more remarkable. Somehow, the
butterfly has found a way of reflecting its bright, iridescent colour over an
extremely wide angle, a trick designed to maximise its visibility in the
rainforest.
A characteristic feature of iridescence is that the colour you see depends on
both the angle at which light hits the reflecting surface and the angle you view
it from. When light hits a stack of perfectly parallel thin films, the reflected
rays form a very narrow cone. If Morpho reflected light in this way,
only a butterfly in exactly the right spot in the forest would get the message
to keep out. Morpho鈥檚 colour does shift slightly with viewing angle,
from bright blue to violet and eventually ultraviolet鈥攚hich human eyes
can鈥檛 see but which butterflies can. However, Morpho鈥檚 blue reflects
across an angle of around 100掳, a huge spread for an iridescent colour.
Flashes of blue
鈥淭he butterfly wants to be as visible as possible over a very large viewing
angle to increase its chances of being seen by other butterflies,鈥� says Vukusic.
And it seems that these insects have gone to extraordinary lengths to achieve
this. So far, the team has found three different ways in which the butterflies
increase the visibility of their blue flashes.
Although the branches of each tree inside a scale are arranged neatly, they
are not perfectly parallel; some tilt slightly upwards, some slightly downwards.
Because the tilts are irregular, the outgoing blue rays leave at all sorts of
angles, effectively splashing the reflected colour around.
M. didius has a second optical trick that extends the viewing angle still
more. This species has a layer of simple, transparent 鈥済lass鈥� scales partially
overlapping the elaborately sculpted 鈥済round鈥� scales. Almost all the white light
hitting a glass scale passes through, but it is diffracted by the tiny ridges on
the surface and, like light glittering off the faces of a cut diamond, it sprays
out and strikes the ground scale beneath from many angles. The blue light
reflected by the ground scales鈥攁lready spread out by the tilted branch
effect鈥攑asses back through the glass scale and is diffracted yet again
before it emerges. Finally, in both M. rhetenor and M. didius,
the fine ridges on the surface of the ground scales help to spread the reflected
light that bit more.
Morphos, of course, are not the only iridescent butterflies. The team are
also investigating a range of other species, from the green Papilios of
Southeast Asia to the European peacock with its glittering purple eye spots.
Every species they look at seems to display its bright colours in a different
way. 鈥淓ach family seems to have very different structures inside its scales. But
even within a subfamily there are differences,鈥� says Vukusic. The physicists
suspect that butterflies have other nifty ways of manipulating light
too鈥攖hey may even tinker around with the polarisation of light.
While the optics of these diverse structures are fascinating to physicists,
biologists are equally interested in how butterflies manipulate light. 鈥淚f you
learn more about the optics, you learn about what the system is doing for the
animal,鈥� says Helen Ghiradella, an entomologist at the State University of New
York at Albany. 鈥淭hese animals do things that engineers have tried and not
succeeded. They are fine-tuning their reflectivity and we haven鈥檛 got a reason
for it. But it makes us start thinking.鈥� If they can control visible light, then
what might they be doing with light from other parts of the spectrum? 鈥淲e just
don鈥檛 know how capable these organisms are,鈥� says Ghiradella. Nor do they know
how a single epidermal cell makes a structure of such enormous complexity with
such precision.
Materials scientists are not so interested in how the butterfly does it but
in whether they can copy the structures to produce the same optical effects.
DERA, for instance, is interested in mimicking the structure of butterfly scales
to provide 鈥渜uick change鈥� camouflage for military vehicles. Repainting a plane
or a fleet of trucks to suit a new area of operation is slow and costly. If you
could cover them with thin plastic sheets incorporating a multilayered structure
that generates a particular colour effect, it would be much easier to switch
shades. It might even be possible to make active camouflage, altering the colour
and sheen simply by adjusting the spacing between the layers. An amphibious
vehicle, for instance, might be made to shimmer like a mirror when on the water,
but switch to a dull green when it moves on land. 鈥淲e can tweak a lot of
factors, the thickness of the thin film, the separation of the layers and the
angles of the branches,鈥� says Lawrence.
Structures that reflect brilliant colours over long distances and a wide
angle could improve the visibility of safety clothing, road signs and the
screens of laptop computers. They could even end up on the catwalk, as fashion
designers discover they can have a new range of fabrics that shimmer to order or
change colour as the wearer moves.
But the biggest interest in the butterflies鈥� optical skills comes from banks
and credit card companies seeking better ways to beat counterfeiters. The
growing trend towards plastic banknotes, pioneered in Australia, makes it
practical to include tiny optical devices that change colour when tilted or
turned through a specific angle. These could be made still more complex by
building in polarisation effects, for example, which would only become visible
to a sensor tuned to detect light with a specific polarisation.
鈥淏utterflies use cunning methods to produce colours,鈥� says Lawrence. 鈥淚t鈥檚
very difficult to copy these structures unless you have an electron microscope
and know what鈥檚 going on at the microscale level.鈥� And the more complex the
structure, the harder it is to copy. 鈥淚 see great potential for this sort of
structure,鈥� says Gary Power of Securency, the company that manufactures
Australian banknotes.
It seems that every difference in structure, no matter how slight, gives
material scientists a potential new tool. In some cases, one part of a complex
structure may turn out to be useful; in others the combination of optical
effects might be what鈥檚 wanted. You might want the reflectivity or pure colour
of one structure but the angle spread of another, for instance.
So far the butterflies are still several steps ahead of humans. 鈥淪ome people
look at the complexity and say it鈥檚 too difficult to mimic,鈥� says Lawrence. 鈥淏ut
this presumes that you need to make an exact copy.鈥� Often, you can achieve the
effect you want by picking out particular features of the structure. 鈥淭he
butterflies are pointing out useful techniques rather than giving us absolute
blueprints,鈥� he says. 鈥淭hey offer us a tool kit to work with.鈥�
THE THICKNESS of a layer鈥攚hether a film of oil or a sliver of
chitin鈥攄ictates which wavelengths of light it reflects most strongly. As
light passes through a transparent film, some is reflected at the top surface;
the rest travels on until it reaches the lower surface when a little more is
reflected. Since this light has travelled further, it may have a different phase
to the light reflected from the top surface.
This phase difference depends on the colour of the reflected light, and on
the thickness of the transparent film. If two reflected rays of green light, for
instance, are in phase, they combine and produce a brighter green by the process
of constructive interference. If they are out of phase, they cancel each other
out and green light isn鈥檛 reflected. The result: some colours will be reflected
brightly, and others will not.
In the Morpho鈥檚 case, the layers of chitin are about 70 nanometres thick and
are separated by an air space around 100 nanometres wide鈥攄imensions that
reflect mostly blue with a little ultraviolet. 鈥淏lue light is just the right
wavelength to bounce off each branch of the Christmas tree, and each one
reinforces the next,鈥� says Vukusic. 鈥淥ther colours pass through the layers and
their energy is absorbed by the wing beneath.鈥�
