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.
"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.
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.
"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 the 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."
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