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Bio-Inspired LEDs 55% More Efficient

Jessica Lyons Hardcastle

The nighttime twinkling of fireflies has inspired scientists to modify an LED so it is more than one and a half times as efficient as the original, according to papers published in the Optical Society’s open-access journal Optics Express.

Researchers from Belgium, France, and Canada studied the internal structure of firefly lanterns, the organs on the bioluminescent insects’ abdomens that flash to attract mates. The scientists identified an unexpected pattern of jagged scales that enhanced the lanterns’ glow, and applied that knowledge to LED design to create an LED overlayer that mimicked the natural structure.

Researchers say the overlayer, which increased LED light extraction by up to 55 percent, could be easily tailored to existing diode designs to help humans light up the night while using less energy.

Fireflies create light through a chemical reaction that takes place in specialized cells called photocytes. The light is emitted through a part of the insect’s exoskeleton called the cuticle. Light traveling through the cuticle more slowly than it travels through air, and the mismatch means a proportion of the light is reflected back into the lantern, dimming the glow.

The unique surface geometry of some fireflies’ cuticles, however, can help minimize internal reflections, meaning more light escapes to reach the eyes of potential firefly suitors.

Using scanning electron microscopes, the researchers identified larger, misfit scales (pictured), on the fireflies’ cuticles. When the researchers used computer simulations to model how the structures affected light transmission, they found that the sharp edges of the jagged, misfit scales let out the most light. The finding was confirmed experimentally when the researchers observed the edges glowing the brightest when the cuticle was illuminated from below.

Human-made light-emitting devices like LEDs face the same internal reflection problems as fireflies’ lanterns, and researchers thought a factory roof-shaped coating could make LEDs brighter.

In the second Optics Express paper, the researchers describe the method they used to create a jagged overlayer on top of a standard gallium nitride LED. They deposited a layer of light-sensitive material on top of the LEDs and then exposed sections with a laser to create the triangular factory-roof profile.

Since the LEDs were made from a material that slowed light even more than the fireflies’ cuticle, the scientists adjusted the dimensions of the protrusions to a height and width of 5 micrometers to maximize the light extraction.

The paper authors say the technique is easy, and doesn’t require creating new LEDs. It only takes a few additional steps to coat and laser patterns on an existing LED.

Photo Credit: Optics Express



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