Structural color is caused by the interaction of light with structures of nanoscale periodic structure, with geometries on the order of magnitude of visible light wavelengths. Light that encounters these minute structures is subject to optical phenomena including thin film interference, multilayer interference, diffraction grating effects, photonic crystal effects, and light scattering. These phenomena lead to selective reflection of particular light wavelengths through constructive and destructive interference. This type of coloration is quite different from what we may deem electronic absorption color; that is, color derived from the selective absorption of discrete wavelengths of the visible spectrum by electrons temporarily promoted to higher energy orbitals. Unlike electronic absorption coloration, structural color is not a material property, but is rather a function of a material’s geometry.
Color in Nature
The brilliant hues found in the feathers of the peacock are the result of periodic arrangement of melanin rods and air spaces in a two-dimensional photonic structure. Light waves reflected by this structure are selectively reinforced and cancelled, ultimately producing what our eyes perceive as a single predominant color. By simply varying the geometric periodicity of these melanin structures, the perceived color is dramatically shifted. If these feathers were finely ground, the periodicity of their nanostructure would be lost and an essentially colorless powder would remain. Such examples of structural color are found throughout the natural world, from insect exoskeletons to opal gemstones.
Structural color has been studied since the time of Newton and Hooke, who observed structural solor in soap bubbles, films of oil and bird feathers. Our modern understanding of light as a propogating wave allows us to understand why Newton observed all the visible colors, as well as dark black colored regions, upon the surface of the oil films. When light waves propagate in phase with adjacent waves, they reinforce one another; conversely, when they are 90o out of phase, they cancel each other. Light waves slightly out of phase are partially cancelled. With this in mind, one can readily understand why waves of light of a given wavelength travelling through a film and reflecting from its inner surface may emerge in phase, partially out of phase, or completely out of phase with light beams being reflected from the outer film surface.
Polymer Thin Films
A polymer thin film of appropriate thickness may display structural color, independant of the polymer’s intrinsic electronic absorption color. Light striking a thin film reflects from the film surfaces and penetrates through the film and reflects from the underlying substrate. The hues of these films shift according to Bragg’s Law, in which the optical path length distance, d, is a function of film thickness and index of refraction. Based upon d, light emerging from the film posesses a particular phase that may constructively or destructively interfere with an adjacent beam of light being reflected at the air-film interface. By varying the speed at which thin films are spun onto substrates, it is possible to control film thickness and film color.