Many of the colorful beasts found in nature aren’t actually colored at all. That is, they don’t have any pigments on their feathers, wings, etc., that produce color the way we typically think – the absorption of light in every wave frequency except the one that is reflected back to our eyes. A lot of animals instead produce colors using microscopic structural patterns to diffract light.
Think of a prism that separates white light into the colors of the rainbow, like on that Pink Floyd poster. In the animal kingdom, this iridescence is caused by the structural layering of cells. As light hits the top layer, some gets reflected and the rest passes through to the next. After repeating this process a few times, the beams of light amplify each other to produce colors. You can see non-pigmented colors shining bright off the wings of butterflies, the backs of jewel beetles, and the feathers of many birds.
This trick is trying to be mimicked by scientists the world over. The thought being that if you could produce colors based on structures and get those structures to constantly rearrange, you could create thin, flexible display screens for applications like electronic paper or E-readers. And because the colors don’t rely on a backlight, instead being created by natural, ambient light, the power requirements of such a device would be close to nil.
In a recent study, researchers from the University of Michigan have come one step closer to making this dream a reality. They’ve managed to create reflected hues that don’t shift colors based on the viewer’s perspective angle, an issue that has plagued the field for years and one most people who have ever seen a butterfly flap its wings or a peacock ruffle its tail feathers remember well.
The team created metallic grooves that funnel and trap light within. The grooves are so small, that they are shorter than the wavelengths of the colors they trap, producing stable and relatively easy-to-make color. The color that is trapped depends on the size of the groove, and putting these grooves in particular patterns can create the entire spectrum of visible light.
To demonstrate their device, the researchers etched nanoscale grooves in a plate of glass with the technique commonly used to make integrated circuits, or computer chips. Next, they coated the plate with a thin layer of silver, which boosts the local electric field near the slit by being charged by polarization effects caused by light.
Although only static images have been created so far, the team hopes to develop a moving picture version in the near future.
The paper, “Angle-insensitive structural colours based on metallic nanocavities and coloured pixels beyond the diffraction limit,” was published in Nature Scientific Reports by lead author Jay Guo, professor of electrical engineering and computer science at the University of Michigan.