Mechanics of structural color
Last spring, we got to enjoy the re-arrival of the birds and insects that migrated or hibernated during the winter. These insects and birds come in a wide variety of exotic colors, many of which are caused by a special type of coloration called structural coloration. While most materials are colored using dyes or pigments that absorb certain wavelengths of light and reflect others to create the colors we perceive, structural coloration relies heavily on the structure of a material to create the desired appearance that, unlike dyes and pigments, can be made resistant to fading.
The mechanics behind structural coloration relies heavily on the diffraction and interference of light due to the structure of a material on the micro- and nanoscales. Diffraction gratings and thin-film interference are just two of many photonic mechanisms of structural coloration that occur naturally. These are the primary causes of iridescence seen in things such as opals and pearls. Some engineers have started looking into the structures behind these colorations to try to recreate it artificially. The Menelaus blue morpho butterfly has an excellent nanostructure that allows what are actually brown scales on its wings to look blue to our eyes.
The ridges discussed in this video function as something called a diffraction grating. Diffraction gratings are components with periodic structures that have objects or slits in them comparable in size to the wavelength of a wave that hits it. The grating diffracts light into several different beams that travel in different directions depending on their size and spacing. While there is a lot of quantum physics that determine the exact colors and patterns that result from these gratings, the main concept behind the diffraction grating is interference. The various angles of diffraction caused by the light bending around these objects or slits can create either constructive or destructive interference for a given wavelength, which means that a specific color can either be amplified or cancelled out.
While these butterflies use diffraction gratings to create the blue color displayed on their wings, there are different mechanisms to get these types of exotic colors, such as thin-film interference. Thin-film interference is a type of structural coloration in which two or more semi-transparent surfaces are layered, causing constructive and destructive interference similar to diffraction gratings. The thickness of the layers determines the interference pattern, which can vary from just a couple colors to entire sections of the visible light spectrum. The concept of thin-film interference has been used to design solutions for various optical applications, such as antireflection coatings used on glasses and camera lenses.
Nature seems to have an endless number of things to teach us. While scientists have been studying these phenomena in animals since the mid-1600s, structural coloration is just one of many mysteries nature has, and the science community has only scratched the surface.