A team of researchers from Belgium, Canada, and France have developed a more efficient gallium nitride (GaN)-based LED using a design inspired by the firefly. The design, fabrication, and characterization of this modified GaN-based LED is described in two papers recently published in Optics Express.
Fireflies are bioluminescent and chemically produce their own light from the abdomen’s“latern” organ. When researchers examined the structure of firefly laterns under a scanning electron microscope (SEM) they saw the abdomen cuticle consists of scales arranged in a jagged pattern. This misfit between scales produces a sharp edge that contributes to light scattering (i.e. overall light extraction) .
“We refer to the edge structures as having a factory roof shape,” said Annick Bay, the paper’s lead author and a Ph.D. student at the University of Namur in Belgium, in a public release. “The tips of the scales protrude and have a tilted slope, like a factory roof.”
Light travels more slowly in the firefly cuticle than through air. This mismatch means some of the light produced from the firefly is reflected back into the lantern making the firefly less bright than they could be. But the researchers found the surface pattern the fireflies’ cuticles can help minimize internal reflections, allowing more light to escape (i.e. brighter fireflies).
Low light extraction efficiency occurs when there’s total internal reflection. The structure of the surface can be modified to increase light extraction efficiencies. The surface of GaN LEDs is typically modified by adding roughness or nanostructures to form a graded refractive index on the surface.
The new LED consists of a layer of light-sensitive material on top of a standard GaN LED. Sections of the light-sensitive layer are then exposed to a laser, creating the factory-roof structure. Since light travels even slower in the standard GaN LEDs than the firefly, the dimensions of the protrusions on the LED surface were optimized (height and width of 5 micrometers) to improve the amount of light that can be extracted from the LED. Researchers were surprised the micrometer sized protrusions were so effective in improving light extraction efficiency because convention optimization methods are based on protrusions in the submicron scale.
The researchers performed a series of experiments with coated (light-sensitive layer) and patterned (factory-roof like protrusions) and found:
1. A coated pattered LED was 54% more efficient than a bare LED
2. A coated patterned LED was 33% more efficient than a coated LED
3. A coated LED was 16% more efficient than a bare LED
While the patterned surface definitely improves the light extraction from LEDs, researchers believe the light-sensitive coating alone already gives a slight improvement because of the gradual transition in how quickly light travels from the LED to the air, minimizing the total internal reflection.
The technique is promising according to the researchers because of its simplicity and scalability, allowing any protrusion size to be made on a precise area of a large surface.
Bay, A., André, N., Sarrazin, M., Belarouci, A., Aimez, V., Francis, L., & Vigneron, J. (2013). Optimal overlayer inspired by Photuris firefly improves light-extraction efficiency of existing light-emitting diodes Optics Express, 21 (S1) DOI: 10.1364/OE.21.00A179
Bay, A., Cloetens, P., Suhonen, H., & Vigneron, J. (2013). Improved light extraction in the bioluminescent lantern of a Photuris firefly (Lampyridae) Optics Express, 21 (1) DOI: 10.1364/OE.21.000764\
Featured image source: Animal Planet