Fireflies make LEDs shine brighter

　　A gallium nitride LED coated with a new type of cover. This cover mimics the structure of the firefly's light source and can increase the light extraction efficiency of the LED by more than 50%. Image: Nicolas Andr The flickering light at the tail of a firefly has inspired scientists.

Recently, they improved the light-emitting diode (LED) based on the structure of the light source in the tail of the witch firefly, which can increase its efficiency by more than 50%.

　　Researchers from Belgium, France and Canada published two articles in Optics Express, an Optical Society (OSA) journal, introducing their bionics research results. When they were studying the light emitters on the tails of fireflies, they accidentally discovered a zigzag arrangement of scales that can increase the brightness of the light emitters. Scientists then applied it to LED design and made an LED covering layer that mimics the natural structure of firefly light emitters. This covering layer can increase the LED light extraction efficiency by up to 55%, and can be easily applied to existing diode designs, saving more energy for humans.

　　"This study shows us that we can learn a lot by carefully observing nature," said Annick Bay, a doctoral student at the University of Namur in Belgium, who studies natural photonic structures, including beetle scales and butterfly wings. While doing field work in Central America, her advisor, Jean Pol Vigneron, discovered several groups of fireflies and brought some samples of the genus Photuris back to the lab for further study.

　　Fireflies produce fluorescence through a chemical reaction. This chemical reaction occurs in a special type of light-emitting cell. The fluorescence is emitted from a part of the firefly's exoskeleton - the cuticle. Light travels slower in the cuticle than in the air. The difference in the speed of light in these two different media means that some of the light is reflected back to the light source, making the light source relatively dim. However, the unique geometric surface of the cuticle of these firefly specimens can minimize reflection. This makes the light they emit stronger and easier to be found by the opposite sex.

　　In their paper in Optics Express, Bay, Vigneron and their colleagues describe for the first time the complex structure of this light emitter and explain how it could improve LED design. Using a scanning electron microscope, the researchers discovered a nanoscale skeleton and larger scales on the firefly cuticle. When the researchers used computers to simulate how this structure affects light propagation, they found that most of the light was emitted from the sharp edges of these jagged scales. The researchers then confirmed this result experimentally, showing that these edges were brightest when they illuminated the cuticle from below.

　　Schematic diagram of the factory roof structure. Image: locallocalhistory.co.uk

　　"These edge structures have the shape of a factory roof," Bei said. The ends of the scales protrude outward and have a slope, just like a factory roof. "There is a protrusion every 10 microns, and its height is about 3 microns. Initially we thought that the smaller nanostructures were the most important, but in the end we were surprised to find that it was the big ones that were most effective in improving light extraction efficiency."

　　Artificial light-emitting devices like LEDs suffer from the same problem of internal reflection as the light-emitting organs of fireflies. Bei and her team thought that a set of coverings, like those on factory roofs, could make LEDs brighter. Their second article, published in Optics Express, describes a method for adding a jagged covering to a standard gallium nitride LED. Nicolas André of the University of Sherbrooke in Canada coated a standard gallium nitride LED with a layer of photosensitive material and then used a laser to illuminate the cross-section to create triangular slopes similar to those of a factory roof. Because light travels more slowly in an LED than in a firefly's cuticle, the scientists adjusted the size of these "scales" to maximize light extraction efficiency. "The good thing about this technology is that the process is simple and there is no need to make new LEDs. It only takes a few more processing steps for us to add coverings and laser patterning to the LEDs."

　　Other research groups have studied the photonic structures of firefly emitters, but they have focused on nanoscale structures. The team led by Bei is the first to discover micrometer-scale photonic structures. The structures are larger than the wavelength of visible light, but surprisingly they improve light extraction efficiency even better than smaller nanostructures. The researchers tested the factory roof-like covering and found that it can increase light extraction efficiency by more than 50%. The researchers estimate that this novel design, which can be achieved by adapting existing manufacturing techniques, could be used in LED production within a few years.

　　The firefly species that inspired the researchers to design the new, efficient LED cover belong to the genus Phalaenopsis, which is common in Latin America and the United States. Bay said she and her colleagues will continue to explore the great diversity of nature to find new sources of knowledge and inspiration. "The Phalaenopsis fireflies are very efficient, but I'm sure there are other species that are even more efficient. Our work is not over yet."