Structural color, which is the foundation that makes things like a peacock's tail feathers appear iridescent, has been an area of study for scientists as they try to adapt it for use in everyday technologies – only without the “rainbow effect” that makes the colors unstable depending on the angle of view. Now, Researchers at the University of Michigan have mimicked the peacock's color mechanism in an approach that could lead to high resolution reflective color displays and have implications for data storage, cryptography and counterfeiting.

Structural coloration is caused by the interference of light as it travels through and reflects off several microscopically fine layers of a surface structure, thus differing to the production of color by chemical pigments absorbing parts of visible light. The grooves that are also found within a structural formation must be small enough to interfere with the wavelength. The net result of this is that each different hairline groove reflects certain parts of the visible spectrum. By thinking of rainbow-play on soap bubbles, some sea-shells or even the underside of a compact disc, we can see its effects within everyday objects.

The problem with replicating such a structure is that its inherent complexity results in “shimmering” or shifting color. Engineers and researchers have long tried to harness the properties of structural color, and this is especially true in more recent times regarding its use within reflective displays such as e-book readers and next-gen electronic paper displays. In a step forward, a team of University of Michigan researchers, led by Jay Guo, professor of electrical engineering and computer science, have been able to “lock in” certain parts of a wavelength, making the reflected hues hold true despite changes in the angle of view.

To achieve this, the researchers etched nanoscale grooves in a plate of glass using a technique commonly used to manufacture computer chips. The etching itself was a tiny reproduction of the well-known symbol of the Olympic rings. Each ring was smaller than a human hair, at about the width of 20 microns. The glass plate was then coated in a thin layer of silver. Light itself is made of electric and magnetic field components, and when it reaches the coated surface the electric element creates a polarization charge at the surface, increasing the electric field around the slit that then pulls a particular wavelength of light in, trapping it there.

Based off the industry-standard print model of cyan, magenta and yellow, the research team had to determine the appropriate size each slit had to be in order to catch the different colors. They found that at a groove depth of 170 nanometers and spaced at 180 nanometers, a slit 40 nanometers wide will trap red light and reflect a cyan color, while a groove 60 nanometers across will trap green and produce a magenta hue and 90 nanometers traps blue and produces yellow.

The diffraction limit, theorized by German physicist Ernst Abbe back in 1873, had previously long been thought of as the smallest point a beam of light could be focused onto. But as head researcher Jay Guo put it, “That’s the magic part of the work. Light is funneled into the nanocavity, whose width is much, much smaller than the wavelength of the light. And that’s how we can achieve color with resolution beyond the diffraction limit. Also counterintuitive is that longer wavelength light gets trapped in narrower grooves.”

In contrast to backlit screens such as tablets, LCD TVs and computer monitors that are hard to read in daylight, the research could eventually lead to color e-books and electronic papers that don’t require their own light to be properly visible. This also cuts the power use of a device, as those who are familiar with E-ink e-readers will attest to. Beyond this, the researchers claim the technology could be used in a variety of fields, including data storage, cryptography and invisible anti-counterfeiting techniques for documents.

So far, the process has only made static pictures, but the researchers hope to move on to developing moving pictures in the future. The project is funded by the Air Force Office of Scientific Research and the National Science Foundation, and the released paper is extensively titled as: “Angle-Insensitive Structural Colors based on Metallic Nanocavities and Colored Pixels beyond the Diffraction Limit.”

The team's paper is published in Scientific Reports.

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