Biological inspiration from photonic structures of firefly light organ

Abstract

Cuticular photonic structures found in insect effectively manage light within an ultrathin layer. For last decade, these cuticular photonic structures have been inspired for many novel imaging or display applications. Nature also implements such efficient photonic structures for out-coupling bioluminescent light from the body. However, optical functions of the photonic structures were not well concluded and little effort on engineering biomimetics for illumination has yet been made. In this study, we report optical functions and engineering inspirations of the photonic structures found in the firefly light organs. The characteristics of firefly light organs were investigated such as anatomical features, spectrum of firefly bioluminescence, and photonic structures on the lantern cuticle. The lantern consists of a cuticle, a photogenic layer and a dorsal layer. The spectral peak is around 560 nm in male. The cuticular nanostructures were found on a lantern cuticle of a firefly, Luciola lateralis Motschulsky. The period, height, and width roughly range near 250 nm, 110 nm, and 150 nm, respectively. The nanostructures serve as highly efficient light extraction by the optical index matching and diffraction. The cuticular hierarchical structures were found on a lantern cuticle of a firefly, Pyrocoelia rufa. The lantern has hierarchical structures, which consist of asymmetric microstructures and nanostructures. The width of asymmetric microstructures roughly range near $10 \mu m$ and the physical dimensions of nanostructures are similar to those of the firefly, Luciola lateralis Motschulsky. The hierarchical structures improve light extraction by reducing total internal reflection and Fresnel reflection. The cuticular nanostructures found on a lantern cuticle of a firefly, Luciola lateralis Motschulsky, were mimicked by implementing nanostructures on a curved lens surface, i.e., bioinspired LED lens. The nanofabrication was done by colloidal lithography, reactive ion etching, and reconfigurable nanomolding. The experimental results unveil that highly ordered nanostructures on the lantern cuticle substantially contribute to increase light transmission as a low index effective medium. The bioinspired LED lens features the nanostructures on a curved lens surface. This lens demonstrates the maximum transmittance of 98.3 % at 560nm and also 3 % over visible ranges higher than that of a smooth surface lens, also comparable to that of conventional AR coating of magnesium fluoride. The bioinspired LED lens has the maximum transmission when all rays from a LED light source are normally incident to the lens surface. The cuticular hierarchical structures found on a lantern cuticle of a firefly, Pyrocoelia rufa, were mim-icked by implementing hierarchical structures on a LED lighting panel. We developed a simple and monolithic fabrication method, termed geometry-guided resist reflow, for asymmetric microstructures. The slope profiles can be freely formed as a concave, convex, or linear shape and the slope angle can also be tuned from 7 to 68 degrees. The micro-nanofabrication was done by geometry-guided resist reflow, replica molding, and PDMS oxidation. The experimental results reveal that hierarchical structures on the lantern cuticle substantially contribute to increase light extraction by reducing total internal reflection. The hierarchical structures provide higher light extraction than the asymmetric microstructures with same inclined angles of structures. The bioinspired hierarchical structures improve light extraction up to 70.37 % at 560 nm, compared to smooth surface. The asymmetric microstructures and hierarchical structures precisely controlled the illumination angles by varying inclined angles. In summary, this work reports optical functions and engineering inspirations of the photonic structures found in the firefly light organs. Both the calculated and experimental results unveil that cuticular nanostruc-tures and cuticular hierarchical structures on the lantern cuticle substantially contribute to increase light ex-traction efficiency by reducing Fresnel reflection and total internal reflection. These biological inspirations can offer new opportunities for LED and OLED based highly efficient lighting or display systems.