TECH SPECIAL...Mimicking fireflies improves LED tech

Mimicking fireflies improves LED tech  By utilising a newly discovered pattern of jagged scales on the fireflies’ abdomens, researchers have made light-emitting diodes 1.5 times more efficient than the original     The nighttime twinkling of fireflies has inspired scientists to modify a light-emitting diode (LED) so it is morethanoneandahalftimesasefficient as the original. 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. Theoverlayer,whichincreasedLEDlight extraction by up to 55 per cent, could be easily tailored to existing diode designs to help humans light up the night while using less energy. The work is published in the journal O p tic s E x p r e s s. “The most important aspect of this work is that it shows how much we can learn by carefully observing nature,” says Annick Bay, of the University of Namur in Belgium who studies natural photonic structures, including beetle scales and butterfly wings. When her advisor, Jean Pol Vigneron, visited Central America to conduct field work on the Panamanian tortoise beetle (Charidotella egregia), he also noticed clouds of twinkling fireflies and brought some specimens back to the lab to examine in more detail.     Fireflies create light through a chemical reaction that takes place in specialised cells called photocytes. The light is emitted through a part of the insect’s exoskeleton called the cuticle.     Light travels 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 minimise internal reflections, meaning morelightescapestoreachtheeyesofpotential firefly suitors.     In Optics Express papers, Bay, Vigneron, and colleagues first describe the intricate structures they saw when they examined firefly lanterns and then present how the same features could enhance LED design.     Using scanning electron microscopes, the researchers identified structures such as nanoscale ribs and larger, misfit scales, on the fireflies’ cuticles.     When the researchers used computer simulations to model how the structures affectedlighttransmissiontheyfoundthatthe 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.     “We refer to the edge structures as having afactory roof shape,” says Bay. “The tips of the scales protrude and have a tilted slope, like a factory roof.” Theprotrusionsrepeatapproximately every 10 micrometers, with a height of 3micrometers. “In the beginning we thought smaller nanoscale structures would be most important, but surprisingly in the end we found the structure that was the most effective in improving light extraction was this big-scale structure,” says Bay. Human-made light-emitting devices like LEDs face the same internal reflection problems as fireflies’ lanterns and Bay and her colleagues thought a factory roofshaped coating could make LEDs brighter.

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