Improved light extraction in the bioluminescent lantern of a Photuris firefly (Lampyridae)

A common problem of light sources emitting from an homogeneous high-refractive index medium into air is the loss of photons by total internal reflection. Bioluminescent organisms, as well as artificial devices, have to face this problem. It is expected that life, with its mechanisms for evolution, would have selected appropriate optical structures to get around this problem, at least partially. The morphology of the lantern of a specific firefly in the genus Photuris has been examined. The optical properties of the different parts of this lantern have been modeled, in order to determine their positive or adverse effect with regard to the global light extraction. We conclude that the most efficient pieces of the lantern structure are the misfit of the external scales (which produce abrupt roughness in air) and the lowering of the refractive index at the level of the cluster of photocytes, where the bioluminescent production takes place.Comment: 13 pages, 11 figures, published in Optics Expres

Optimal overlayer inspired by Photuris firefly improves light-extraction efficiency of existing light-emitting diodes

In this paper the design, fabrication and characterization of a bioinspired overlayer deposited on a GaN LED is described. The purpose of this overlayer is to improve light extraction into air from the diode's high refractive-index active material. The layer design is inspired by the microstructure found in the firefly Photuris sp. The actual dimensions and material composition have been optimized to take into account the high refractive index of the GaN diode stack. This two-dimensional pattern contrasts other designs by its unusual profile, its larger dimensions and the fact that it can be tailored to an existing diode design rather than requiring a complete redesign of the diode geometry. The gain of light extraction reaches values up to 55% with respect to the reference unprocessed LED.Comment: 9 pages, 9 Figures, published in Optics Expres

Light-extraction enhancement for light-emitting diodes: a firefly-inspired structure refined by the genetic algorithm

ABSTRACT The efficiency of light-emitting diodes (LED) has increased significantly over the past few years, but the overall efficiency is still limited by total internal reflections due to the high dielectric-constant contrast between the incident and emergent media. The bioluminescent organ of fireflies gave incentive for light-extraction enhancement studies. A specific factory-roof shaped structure was shown, by means of light-propagation simulations and measurements, to enhance light extraction significantly. In order to achieve a similar effect for light-emitting diodes, the structure needs to be adapted to the specific set-up of LEDs. In this context simulations were carried out to determine the best geometrical parameters. In the present work, the search for a geometry that maximizes the extraction of light has been conducted by using a genetic algorithm. The idealized structure considered previously was generalized to a broader variety of shapes. The genetic algorithm makes it possible to search simultaneously over a wider range of parameters. It is also significantly less time-consuming than the previous approach that was based on a systematic scan on parameters. The results of the genetic algorithm show that (1) the calculations can be performed in a smaller amount of time and (2) the light extraction can be enhanced even more significantly by using optimal parameters determined by the genetic algorithm for the generalized structure. The combination of the genetic algorithm with the Rigorous Coupled Waves Analysis method constitutes a strong simulation tool, which provides us with adapted designs for enhancing light extraction from light-emitting diodes

A biomimetic approach for improved light extraction from solid-state sources: from fireflies to light-emitting diodes

Since their first use, solid-state lighting devices have received a growing interest and popularity from engineers, physicists, industry and consumers. Their promising internal efficiencies, i.e. the high conversion rates of injected current to produced light, make them very appealing for the replacement of incandescent or fluorescent light sources. The major challenge for the development of these light sources is to enhance the external light extraction: the active material of light emitting diodes is in general a semiconductor with a high refractive index, which considerably limits the extraction of light into free space due to total reflection. This lack of high external efficiency has generated an important amount of research and publications. Very different approaches have been considered to enhance the external light extraction efficiency: photonic crystals, photonic structures inside and above the active material, graded refractive index layer to reduce the high index contrast between the active material and air, plasmonic resonances and more. In the present research, the bioluminescent organ's morphology of a Panamanian firefly is used as a source of inspiration to improve the light extraction efficiency of light-emitting devices. The internal efficiencies of light-emitting devices are nowadays very high, yet, the overall efficiency is limited by the total internal reflection due to the high refractive index of the incident medium. Fireflies emit light in their dense bioluminescent organ and light extraction is equally limited by total internal reflection. During their million years of evolution, the bioluminescent organ had the time to improve both, its internal functioning and its light extraction efficiency. The thorough analysis of the lantern's morphology shows several structures added on the light path, that could influence the light extraction efficiency. The jagged scales form a micrometric, two-dimensional and asymmetric structure which is shown, through simulations, to influence the light extraction positively. The new arrangement of the interface leads to a considerable increase in the light extraction efficiency. A similar analysis is then carried out with a two-dimensional and a three-dimensional symmetric structure. From the simulations it has been concluded that the firefly inspired structure is the most suitable for fabrication on light-emitting diodes. In the framework of a collaboration with microelectronics experts, the jagged-scale structure inspired from fireflies has been adapted to enhance the light-extraction efficiency of an GaN-based LED. Measurements confirm tendency that light extraction increases, as predicted by the simulations.Les diodes électroluminescentes ont reçu un intérêt croissant de la part des ingénieurs, physiciens, industriels et consommateurs, depuis leur première utilisation. Leur efficacité interne élevée, c'est-à-dire le taux de conversion élevé de courant injecté en lumière émise, les place en bonne position pour remplacer les sources lumineuses incandescentes et fluorescentes. Une limitation persiste néanmoins sur l'efficacité globale de la diode: les matériaux actifs ont des indices de réfraction élevés par rapport au milieu émergent (l'air), ce qui limite considérablement l'extraction de lumière, et ainsi l'efficacité externe, à cause du phénomène de réflexion totale. Ce manque d'efficacité externe a généré une activité de recherche considérable avec une grande variété d'approches différentes pour résoudre le problème: cristaux photoniques, structures photoniques dans et au dessus du milieu actif, couches d'indice de réfraction adaptative pour réduire le contraste élevé entre le milieu incident et émergent. L'organe bioluminescent d'une luciole du Panama est utilisé comme source d'inspiration pour l'amélioration de l'efficacité d'extraction de lumière des diodes dans la recherche présentée dans ce manuscrit. La lumière émise par les lucioles est générée chimiquement dans son organe bioluminescent, qui présente un matériau dense par rapport à milieu émergent (l'air). Pendant les millions d'années d'évolution, l'organe bioluminescent a pu avoir le temps d'améliorer son fonctionnement interne ainsi que l'extraction de lumière, c'est-à-dire son efficacité externe. L'étude morphologique de la lanterne a montré différentes structures qui peuvent influencer la propagation de la lumière entre l'organe émetteur et l'air libre. Les écailles inclinées forment un réseau micrométrique, bidimensionnel et asymétrique. Les simulations associées montrent que cette structure augmente considérablement l'extraction de lumière. Une analyse similaire est conduite pour deux modèles inspirées: une structure triangulaire (bidimensionnelle et symétrique), ainsi qu'une structure pyramidale (tridimensionnelle et symétrique). Les simulations permettent de conclure que la structure à écailles inclinées de la luciole est la mieux adaptée à la fabrication. Dans le cadre d'une collaboration avec des experts en microélectronique, la structure à écailles inclinées, inspirée par les lucioles, a été adaptée pour augmenter l'efficacité d'extraction de lumière d'une diode basée sur le GaN. La mesure réalisée confirme la tendance prédite par les simulations.(DOCSC01) -- FUNDP, 201