The natural world is a profusion of colors, the most ubiquitous of which is the pleasing green of plants. Plants produce their colors by pigments which reflect a portion of the light spectrum while absorbing the rest. In particular, green pigments like chlorophyll reflect the green part of the spectrum and absorb the longer red and yellow wave lengths as well as shorter blue ones.
Animals can’t produce their own pigments. Yet, the animal kingdom has some of the most stunning creatures in terms of color. How do they do it? Some animals derive their colors from the food they eat. This is the case for the bright reds and yellows of birds, which mostly come from carotenoid pigments in their food. But blue presents a challenge to animals because there are few foods in nature which provides sufficient blue pigments. Yet we have blue-winged leaf bird (pictured above), the blue-ringed octopus, blue jays, blue dart frogs, blue limpets and everyone’s favorite – the jewel-like blue morpho butterfly.
How do these blue beauties get their color? The short answer is that each species has evolved special micro-engineering mechanisms to mimic what plants do through pigments. These mechanisms take the form of tiny structures that scatter light and then interacting those scattered wavelengths to reinforce some colors while suppressing others. The colors produced this way are known as structural colors.
Consider the blue-rayed limpet (a mollusk) which has distinctive bright blue stripes on its shell. These are produced by layers of calcium carbonate crystals embedded in the shell. The crystals are arrayed as multiple microscopic sheets, with each layer diffracting and reflecting a sliver of light. The diffracted light waves interact with each other, and depending on the thickness of each layer and the wavelength of the light, the waves either add up or cancel out. Incredibly, the layers of crystals on the limpet’s shell are exactly of the right thickness (100 nanometers) to cancel all wavelengths but blue.
How about the blue feathers of birds? At high magnification, birds’ feathers contain barbs (filaments) which have small bubbles of air suspended in keratin protein. The light scattering off each air bubble interacts with light bouncing off neighboring bubbles, making the colors of the feathers look blue or turquoise or ultraviolet, according to Richard Prum, an expert on bird-feather coloration at Yale University.
Almost the same principle is at work in producing the striking blue color of the morpho butterfly except that in place of barbs, the wings of the morpho butterfly are sculpted with layers of microscopic grooves lined with tree-like protrusions. Because light reflecting from the top layer is out of phase with light reflecting from the bottom, this causes a shift in hue when viewed from different angles, giving the sparking visual effect known as iridescence.
Other animals exploit a more dynamic feature of structural coloring. Octopuses for instance employ color-changing tricks by thickening and thinning layers of specialized color cells known as chromatophores just below their skin, changing the wavelengths of the colors they show to the world.
Lighting Up Our World
Back in the human world, scientists and engineers are learning how structural colors can make point the way for making better materials for photonic applications, such as more efficient fiber optic cables and high-resolution microscopy imaging. For example, using fiber optic cables to transmit blue light offers several advantages to the more commonly used infrared light. These include high bandwidth, minimal signal degradation over long distances, resistance to electromagnetic interference, improved data security, and the ability to carry large amounts of data at very high speeds, making it ideal for applications like high-definition video transmission and large data centers. To overcome the higher attenuation of blue light within the fiber, engineers are now designing fiber-optic cables lined with blue-reflecting materials like those found in blue leafbirds so that no blue photons can escape, thus solving the attenuation problem.
Other teams are working to develop improved methods of dark-field imaging microscopy, inspired by the iridescent structures found in the insect world. A dark field microscope uses a special condenser to illuminate a specimen with oblique light, causing the sample to appear bright against a dark background, which enhances contrast and allows for clear observation of transparent or weakly contrasting samples such as algae and bacteria. Inspired by the microscopic scale structure in the wings of the Papilio butterfly, engineers at MIT [1] have developed a small, mirrored chip that helps to produce dark-field images, without the need for expensive components. Slightly larger than a postage stamp and as thin as a credit card, the chip is placed on a microscope’s stage where it emits a hollow cone of light that can be used to generate detailed dark-field images of algae, bacteria, and similarly translucent tiny objects. A key component of this chip is a Bragg mirror — a structure made from alternating nanoscale layers of transparent materials, with distinctly different refractive intensities that is also the source of iridescence on a Papilio butterfly’s wings. Says Cecile Chazot, a graduate student in MIT’s Department of Material Science and Engineering and the lead research of the project, “the butterfly’s wing scales feature really intriguing egg crate-like structures with a Bragg mirror lining which gives them their iridescent color.” In these and many other innovations, the animal world is showing the way.
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[1] https://meche.mit.edu/news-media/mirrored-chip-could-enable-handheld-dark-field-microscopes