January 13, 2016

Shiny Fish Skin Inspires Nanoscale Light Reflectors

The transmission electron microscopy (TEM) image (a) shows a cross section of ribbonfish skin
(scale bar, 5 mm). Figure (b) illustrates a superlattice of cytoplasm and guanine crystal layers
matching the dashed red line in (a). Figure (c) illustrate a five-stage Cantor bar.
Changes introduced into the fractal lead to a pattern that resembles the superlattice in Figure (b).
Image: Douglas Werner lab / Penn State

(January 13, 2016)  A nature-inspired method to model the reflection of light from the skin of silvery fish and other organisms may be possible, according to Penn State researchers.

Such a technique may be applicable to developing better broadband reflectors and custom multi-spectral filters for a wide variety of applications, including advanced optical coatings for glass, laser protection, infrared imaging systems, optical communication systems and photovoltaics, according to Douglas Werner, John L. and Genevieve H. McCain Chair Professor in Electrical Engineering, Penn State.

The proposed model also contributes to the understanding of the reflective layering in the skin of some organisms. The shiny skins of certain ribbonfish reflect light across a broad range of wavelengths, giving them a brilliant metallic appearance. The reflectivity is the result of stacked layers of crystalline organic compounds embedded in their skin's cytoplasm. Some organisms with metallic sheens have layers that are stacked in a regular pattern, while others, including the ribbonfish, have stacking patterns described as "chaotic" or random. The Penn State team determined that the stacking is not completely random and developed mathematical algorithms to replicate those patterns in semiconductor materials.

"We are proposing a model that uses fractal geometry to describe the layering in the biological structure of silvery fish," says Jeremy Bossard, postdoctoral researcher in electrical engineering, Penn State. "While we are not trying to reproduce the structure found in nature, the same model could guide the design of devices such as broadband mirrors."

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