The twinkling of fireflies in the night sky inspired an international team of scientists to take the flashes on bioluminescent insects’ abdomens and apply them to a coating to increase LED efficiency. Their method proved more than one and a half times as efficient as the original LED. Other research groups have studied the photonic structures in firefly lanterns — the organs on the insects’ abdomen that flash to attract mates — as well, and have even mimicked some of the structures to enhance light extraction in LEDs, but their work focused on nanoscale features. Now, researchers from Belgium, France and Canada are the first to identify micrometer-scale photonic features that are larger than the wavelength of visible light, but that, surprisingly, improve light extraction better than the smaller nanoscale features. A firefly specimen from the genus Photuris, which is commonly found in Latin America and the US, served as the inspiration for the effective new LED coating developed by scientists from Belgium, France and Canada. Courtesy of Optics Express. The team studied the structure of the lanterns of the Photuris firefly — a very effective light-emitting firefly specimen commonly found in Latin America and the US — and identified an unexpected pattern of jagged scales that enhanced the lanterns’ glow. The researchers applied this knowledge to create an LED overlayer that mimicked the natural structure, increasing LED light extraction by up to 55 percent. The overlayer could be easily tailored to existing diode designs to help humans light up the night while using less energy, they say. “The most important aspect of this work is that it shows how much we can learn by carefully observing nature,” said Annick Bay, a doctoral student at the University of Namur who studies natural photonic structures, including beetle scales and butterfly wings. Fireflies create light through a chemical reaction that takes place in specialized cells called photocytes, and this light is emitted through a part of the insects’ exoskeleton called the cuticle. Light travels more slowly through the cuticle than through air, and the mismatch means that 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, enabling more light to escape to reach the eyes of potential firefly suitors. The misfit scales found on the lantern of the Photuris firefly. Researchers found that the sharp edges of the scales emit the most light. Courtesy of Optics Express. Bay, her adviser Jean Pol Vigneron and colleagues observed the intricate structures while examining firefly lanterns under a scanning electron microscope. They identified structures such as nanoscale ribs and larger, misfit scales, on the fireflies’ cuticles. Computer simulations used to model how the structures affected light transmission found that sharp edges of the jagged misfit scales let out the most light. These findings were 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 a factory-roof shape,” Bay said. “The tips of the scales protrude and have a tilted slope, like a factory roof.” The protrusions repeat approximately every 10 µm, with a height of approximately 3 µm. “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.” A GaN LED, coated with a “factory roof” pattern modeled on the fireflies’ scales. The bio-inspired LED coating increased light extraction by more than 50 percent. Courtesy of Nicolas André. Man-made LEDs face the same internal reflection problems as fireflies’ lanterns, and Bay’s team thought a factory-roof-shaped coating could make LEDs brighter. Nicolas André of the University of Sherbrooke created a jagged overlayer on top of a standard gallium nitride LED by depositing a layer of light-sensitive material on top of the LEDs, then exposing sections with a laser to create the triangular factory-roof profile. Because 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 µm to maximize light extraction. “What’s nice about our technique is that it’s an easy process, and we don’t have to create new LEDs,” Bay said. “With a few more steps, we can coat and laser-pattern an existing LED.” The factory-roof coating increased light extraction by more than 50 percent, a significantly higher percentage than other biomimicry approaches achieved to date. The researchers speculate that, with achievable modifications to current manufacturing techniques, it should be possible to apply these novel design enhancements to current LED production within the next few years. The work was published in a pair of papers in Optics Express and Energy Express. For more information, visit: www.fundp.ac.be/en, www.usherbrooke.ca or www.esrf.eu