Iridescent Spiders Provide Inspiration for Optics Design

The iridescent signal produced by miniature Australian peacock spiders during courtship displays could provide inspiration for the development of light-dispersive components that would be able to perform under irradiances and at scales not currently possible. An international research team identified the mechanisms by which the spiders actively displayed isolated wavelengths within the visible spectrum during courtship.

The team hypothesized that the rainbow iridescence in Maratus robinsoni and Maratus chrysomelas was produced by specialized abdominal scales that functioned as three-dimensional reflective diffraction grating structures. Using hyperspectral imaging (HSI), scanning and transmission electron microscopy (SEM/TEM) and imaging scatterometry, the team quantitatively characterized the color and nanostructure of both peacock spider species.

A miniature peacock spider with rainbow-iridescence. a) An adult male Maratus robinsoni. b) A M. robinsoni resting on a human fingernail: the spider is only about 2.5 mm in size. The iridescent abdomen of the spider is indicated by the black arrow. c) A zoom-in view (scale bar: 200 µm) of the same spider abdomen as shown in the dashed square of a), but with different viewing angle. Note the colors of the iridescent patches almost change to their complementary colors between the two different views, from blue to red (red arrows), and from purple to yellow green (blue arrows). Courtesy of Jürgen C. Otto.

The researchers used analytical and finite-element optical simulation to identify the mechanism of color production in the scales, and validated the mechanism using bioinspired, actual-size physical prototypes fabricated via two-photon nanolithography.

The team demonstrated that the intense rainbow iridescence was produced by specialized 3D airfoil-shaped nanograting scales. These scales increased the resolving power of the diffraction grating through the synergistic effects of their vertically orientated surface nanogratings and the microscopic curvature of their airfoil-shape, to enable separation and isolation of light into its component wavelengths at finer angles and smaller distances than are possible with current engineering technologies.

Light micrograph of rainbow-patterned M. robinsoni scales. Black center square: 4×4 µm. Courtesy of Bor-Kai Hsiung, co-author.

“As an engineer, what I found fascinating about these spider structural colors is how these long evolved complex structures can still outperform human engineering," said Caltech researcher Radwanul Hasan Siddique. Even with high-end fabrication techniques, we could not replicate the exact structures.”

The team concurs that the rainbow-iridescent scales of M. robinsoni and M. chrysomelas could help researchers design miniature light-dispersive components with ultrahigh resolving power, using computer-aided optical design processes. This bioinspired approach could allow engineers to develop optical devices, especially spectrometers, with at least 50 percent smaller length scale for applications where fine-scale spectral resolution is required in a very small package, such as instruments on space missions or wearable chemical detection systems.

A zoom-in view of abdomen of Peacock spider Maratus robinsoni. Courtesy of Jürgen Otto, co-author.

“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, researcher at Scripps Oceanography.

“We sometimes forget that mathematical optical models, while critical tools, are hypotheses that need to be tested,” said researcher Matthew Shawkey, who worked on the research while at the University of Akron. “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.”

The research team comprised researchers from the following universities and institutions: University of Akron; Scripps Institution of Oceanography at the University of California San Diego; California Institute of Technology (Caltech); University of Nebraska-Lincoln; University of Ghent; and University of Groningen.

The research was published in Nature Communications (doi:10.1038/s41467-017-02451-x).