Photo credit: Ed Scholes
Photo credit: Ed Scholes

Researchers in the USA have investigated the microstructures that make the feathers of some species of bird of paradise one of the blackest materials known able to absorb almost all of the light that hits them. The details could lead to novel meta materials for photonics and other applications.

Sexual dimorphism in birds of paradise, the Paradisaeidae, is quite extreme. The female, as in many species of bird, is usually plain in appearance, often just speckled brown with little to distinguish her from her forest surroundings. The males, on the other wing, can produce quite astonishing courtship displays to attract the females. One species in particular, the Superb Bird of Paradise, Lophorina superba, shakes his tail (and other) feathers to create a shimmering, iridescent slash of bright blue-green against the blackest backdrop you can imagine. In fact, his feathers are so black that they absorb a staggering 99.95% of directly incident light that is, light striking the feather at a 90 degree angle.

"Structurally absorbing, 'super black' materials (which have extremely low, broadband reflectance) have important applications for a wide range of optical, thermal, mechanical, and solar technologies, including thin solar cells and the lining of space telescopes," the team writes in the journal Nature Communications [DOI: 10.1038/s41467-017-02088-w].

It is well known that chemical pigments can generate color in plants and animals, iridescence and structural coloration are commonly generated by microscopic structural features of a surface, such as a feather or butterfly wing. However, certain types of structure can produce the "opposite" of color - blackness - by absorbing much of the light incident upon them.

Dakota McCoy (Harvard University), Teresa Feo (Smithsonian Institution), Todd Harvey (Yale University), and Richard Prum (Yale) have investigated the nature of the blackness of various birds of paradise and in the case of L. superba have shown just how special are his feathers. Most feathers are flat and fractal in nature - viewing them at increasingly smaller scale reveals hierarchical, yet feather-like structure, within feather-like structure. However, in the case of the male L. superba, to the naked eye his feathers look feathery, but under the scanning electron microscope we do not see feathery frond upon feathery frond, but something resembling more the structure of a coral reef, bottle brushes or leafy trees.

Knowledge of the structures of these super black feathers might assist scientists in developing new materials to rival the carbon nanotube material Vantablack, which absorbs 99.96% of the light that hits it. We find it difficult to focus on such materials, including the black parts of L. superba. It feels like one is looking into a void rather than at a solid object. One has to wonder what the female of the species "sees" when she is choosing a mate.

"We are now doing a few things: we are hoping to start a collaboration to investigate potential applications, we are looking for additional examples of ultra-absorbent materials in nature (and ultra-reflective), we are investigating the evolutionary dynamics of sexual selection that produce such an extreme trait," McCoy told Materials Today.

David Bradley