Miniature Super-Iridescent Optics

A miniature peacock spider with rainbow-iridescence. The colours of the iridescent patches almost change to their complementary colours between the two different views, from blue to red, and from purple to yellow green. (Source: J. C. Otto)

Brightly colored Australian peacock spiders (Maratus spp.) captivate even the most arachno­phobic viewers with their flamboyant courtship displays featuring diverse and intricate body colora­tions, patterns and movements – all packed into miniature bodies measuring less than 5 mm in size for many species. However, these displays aren’t just pretty to look at, they also inspire new ways for humans to produce color in tech­nology. One species of peacock spider – the rainbow peacock spider (Maratus robinsoni) – is parti­cularly impres­sive, because it showcases an intense rainbow iridescent signal in males’ courtship displays to the females. This is the first known instance in nature of males using an entire rainbow of colors to entice females to mate. But how do males make their rainbows?

Figuring out the answer was inherently inter­disciplinary, so Bor-Kai Hsiung – now at Scripps Insti­tution of Oceano­graphy at the University of Cali­fornia San Diego – assembled a team that included biologists, physicists and engineers while he was a Ph.D. student in The University of Akron’s Integrated Bio­science Ph.D. program under the mentor­ship of Todd Blackledge and Matthew Shawkey and supported by UA’s Biomimicry Research and Innovation Center. The team inves­tigated the spider’s photonic structures using techniques that included light and electron micro­scopy, hyper­spectral imaging, imaging scattero­metry and optical modeling to generate hypo­theses about how the spider’s scale generates such intense rainbows. The team then used cutting edge nano 3D printing to fabricate different proto­types to test and vali­date its hypo­theses.

In the end, the team found that the intense rainbow iri­descence emerged from specia­lized abdominal scales on the spiders. These scales combine an airfoil-like micro­scopic 3D contour with nanoscale diffrac­tion grating structures on the surface. It is the inter­action between the surface nano-diffraction grating and the micro­scopic curvature of the scales that enables separation and isolation of light into its component wavelengths at finer angles and smaller distances than are possible with current engineering tech­nologies. “Who knew that such a small critter would create such an intense iri­descence using extremely sophis­ticated mechanisms that will inspire optical engineers,” said Dimitri Deheyn with excitement. Deheyn is the postdoc mentor for Hsiung at Scripps Oceano­graphy.

„From a bio­mimicry perspective, to fully under­stand and address a question, one has to take extremes from both ends into consi­deration. Hence, I purposefully chose to study these tiny spiders with intense iridescence after having inves­tigated the non-iridescent blue taran­tulas,” said Hsiung. The mechanism behind these tiny rainbows may inspire new color tech­nology, but wouldn’t have been discovered without research combining basic natural history with physics and engi­neering.

“Bringing together such diverse research expertise to under­stand the incredible diversity of nature and then applying that knowledge to human tech­nology is exactly what UA’s Biomimicry Research and Inno­vation Center is all about,” said Blackledge, a professor of biology. “We sometimes forget that mathematical optical models, while critical tools, are hypotheses that need to be tested,” noted Shawkey, when asked how this research could change the way researchers inves­tigate bio­logical photonic structures in the future. “Nanoscale 3D printing allowed us to experimen­tally validate our models, which was really exciting. We hope that these techniques will become common in the future.”

“As an engineer, what I found fascinating about these spider structural colors is how these long-evolved complex structures can still out­perform human engi­neering,” added Radwanul Hasan Siddique, a post­doctoral scholar at Caltech. “Even with high-end fabri­cation techniques, we could not replicate the exact struc­tures. I wonder how the spiders assemble these fancy struc­tural patterns in the first place!”

Inspi­ration from these super iri­descent spider scales can be used to overcome current limi­tations in spectral mani­pulation, and to reduce the size of optical spectro­meters for appli­cations where fine-scale spectral reso­lution is required in a very small package, notably instru­ments on space missions, or wearable chemical detec­tion systems. In the end, peacock spiders don’t just produce nature’s smallest rainbows, they could also have impli­cations for a wide array of fields ranging from life sciences and bio­technologies to material sciences and engi­neering. (Source: U Akron)

Reference: B.-K. Hsiung et al.: Rainbow peacock spiders inspire miniature super-iridescent optics, Nat. Commun. 82278 (2017); DOI: 10.1038/s41467-017-02451-x

Link: Dept. of Biology and Integrated Bioscience Program, The University of Akron, Akron, USA

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