To build a better infrared sensor, a team of scientists turned
to the wings of a butterfly for inspiration.
The group, led by Radislav Potyrailo, a principal scientist at
General Electric, coated a butterfly wing with carbon nanotubes.
The result was an infrared sensor that was more sensitive and had
higher resolution than current models.
What makes it work so well? The answer is a combination of the
carbon nanotubes and the iridescence of the wing.
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"The great thing about nanotubes is that they are black in
visible light and don't disturb the iridescence," said Potyrailo,
who created the sensor with colleagues at State University of New
York in Albany.
A butterfly wing looks iridescent because it is covered with
tiny structures shaped something like Christmas trees. The little
"branches" are called lamellae. When light hits the structures,
they reflect it. But the lamellae are near the size of a wavelength
of light, only about 100-200 nanometers long. So they scatter some
of the light even as they reflect it.
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In addition, the lamellae are organized into layers. Some of the
light that is scattered and reflected by the lamellae has to go
through more layers, so it is refracted more. As these light waves
bounce back towards the eye, they interfere with each other. Some
interfere destructively, and cancel each other out, while others
interfere constructively, becoming more intense. The combination of
these effects creates iridescence.
Potyrailo and his team found something else, though: the
iridescence is changed by infrared light, which means that when
heat (which is infrared radiation) hits the wing, a human can see
it happening (or at least the effect it has on the colors we
perceive).
Current infrared sensors need complex electronics to make the
infrared radiation visible on a display. Doing away with that would
simplify building them tremendously -- and that is what this
mechanism can help to do.
But butterfly wings are designed to reflect visible light, not
absorb infrared light. That's where the carbon nanotubes come in.
Suspending millions of carbon nanotubes in a toluene solution, the
team "painted" the wings with them, and exposed them to infrared.
The result was a very good absorption of infrared light and more
efficient changes in iridescence.
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"Nanotubes, especially as they are single walled, are vey
efficient absorbers of infrared light," Portyrailo explained. "They
also redistribute the energy absorbed into whatever surface they
are on."
They found that in future they can make nanostructures that
would absorb the infrared light over a range that is significantly
wider than current imagers.
The carbon nanotubes also made the butterfly wing sensor more
sensitive to temperature changes. Current systems can see
temperature changes of 0.06 to 0.3 degrees Fahrenheit. The
butterfly wings picked up changes of 0.03 to 0.12 degrees
Fahrenheit. The carbon nanotube-enhanced sensor also did it faster
– as much as 40 times per second – and could respond to changes in
the infrared signal in as little as 0.025 seconds.
Each "pixel" is also much smaller than on most digital sensors.
Typical pixel sizes on infrared imaging systems are anywhere from
17 micrometers to 30 micrometers – pretty small, but not as small
as the lamellae, which are only 150 nanometers long on average, and
spaced 770 nanometers apart. That makes them 22 times smaller than
the best infrared imagers available. Along with better resolution
that cold boost the number of "colors" (really, infrared
wavelengths) one could build into an imager of a given size.
Potyrailo doesn't plan to harvest butterflies to build sensors,
though. "We have artificial materials that are better," he said.
And there is a lot of work yet to be done on actually constructing
something to bring to market.
"We're definitely far from a commercial product," he said. "This
is only a tiny step. But we're happy that it gives us
inspiration."