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Glowing Butterflies Shine With Natural LEDs

John Roach
for National Geographic News
November 17, 2005
 
For 30 million years African swallowtail butterflies have dazzled their mates with glowing splashes of color on their wings (see photo). And the process they use to control the flow of light in their wings is strikingly similar to a technology that humans only recently developed, physicists report.

From the lasers used to read information on CDs and DVDs to the data carried across oceans along optical fibers, the control of light is essential to modern living.

One way scientists control light is through the use of light emitting diodes (LEDs). These electronic devices are made out of semiconductor material—a type of solid substance that can conduct electricity—that lights up when a current passes through them.

LEDs are found in traffic lights, computer screens, car brake lights, and many other gadgets that flash color without the use of a conventional light bulb.

But physicists had long deemed traditional LEDs inefficient, because most of the light they created was unable to escape the semiconductor material. All the generated light rays that went sideways or downward, for example, were essentially lost.

Improved LEDs

After years of research, physicists recently overcame these limitations with "high-efficient LEDs." The new breed of LEDs uses specialized mirrors and tiny structures called photonic crystals to generate more usable light from the semiconductor materials.

Pete Vukusic and Ian Hooper, physicists at the University of Exeter in the U.K., report that nature perfected this method in the African swallowtail butterfly long ago.

The butterfly has natural versions of these specialized mirrors and photonic crystals, which brighten the fluorescent blue and green splotches on their wings.

"The analogy is to the way light is extracted from both systems, which is really exciting, it is amazing," Vukusic said. He and Hooper report the discovery in tomorrow's issue of the journal Science.

Alexei Erchak is the founder of Luminus Devices in Woburn, Massachusetts. While a graduate student at the Massachusetts Institute of Technology in Cambridge, he developed the first prototype high-efficient LED in 2001.

Erchak said he is "amazed" at the similarity between the structure in the butterfly wing and his own LEDs.

"We must have been doing something right," he said. "Knowing that nature can confirm the inventions we came up with certainly gives a good feeling."

Mirrors and Crystals

High-efficient LEDs reflect and direct light outward, including light that would be lost with traditional LEDs.

The first trick is the use of specialized mirrors called distributed Bragg reflectors (DBRs), which are multilayered mirrors that bounce back light of a specific wavelength or color.

This multilayer mirror is placed underneath the LED, meaning that any light of the desired wavelength directed downward is reflected upward.

The second trick is the use of photonic crystals, which prevent light of the desired wavelength from going sideways or getting "trapped."

The result is better control over the flow of light and thus a more efficient LED, Vukusic said.

Butterfly Wings

Wondering how the swallowtail butterfly controls the flow of light, study authors Vukusic and Hooper examined a wing under powerful microscopes.

They found that each wing contains hundreds of thousands of tiny scales. Each scale has a slab on it where fluorescent pigment is stored.

Fluorescence is the process by which high-energy ultraviolet radiation is absorbed and reemitted as a lower-energy visible radiation. In the case of the butterflies, sunlight is reemitted as blue or green light.

Underneath the pigment slab are layers of reflective surfaces—natural versions of DBR mirrors. The butterfly's "mirrors" are tuned to reflect blue-green light.

Like the photonic crystals in high-efficient LEDs, the pigment slabs are structured so that blue-green light does not radiate sideways from the slab. As in high-efficient LEDs, the light is directed up and out.

The combined effect of the reflectors and the structure of the slab, explained Vukusic, is a much brighter blue-green fluorescence than could be achieved with pigment alone.

The brighter wings allow the butterflies to better signal to each other, he added.

"Nature has had to come up with very elegant and ingenious design protocols in order to achieve a significant control over the flow of light," Vukusic said. "Those are design protocols which may point the way to newer, better, more efficient systems."

Erchak, the high-efficient-LED pioneer, quipped that the finding "means that butterflies are smarter than MIT students."

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