To invisibility… and beyond!
The mystic superpowers of superman, wonder-woman and the like have captured the imagination of generations of children across the world. Now, thanks to a team from the University of California in Berkeley, science is one step closer to making comic book fiction a reality: An invisibility shield to hide in broad daylight.
That’s because scientists are becoming increasingly better at shaping the path of light like The Incredible Hulk could mold steel, a requirement for an invisibility shield.
Indeed, an object is only visible because light rays hitting its surface are reflected and sent back into the eye. That’s why we need a flashlight to guide ourselves through a pitch-black campsite, and why crystal clear skyscraper windows are the bane of birds flying in New York City. No light rays reflected from the object? Nothing to see.
But making an object invisible is not as simple as blocking light: A plant under an opaque cardboard box is merely hidden, not invisible. That means that an invisibility shield requires building a shield that doesn’t actually block light. Instead, it must force the light rays to bend around the object like the water in a river gently curves around a rock.
Unfortunately, Mother Nature provides materials that bend light in one and one direction only: A spoon placed in a glass of water always appears to be bent upwards. And for an invisibility shield to really work, light must be deflected in many directions such that the shield could truly curve light rays around the object like rock in water. This was something that was considered impossible… until the team led by Dr Xiang Zhang presented their special material.
By sandwiching thin sheets of conducting metal imprinted with holes one thousand times smaller than the width of a human hair, the team was able to build a metamaterial - from the Greek ‘meta’ or ‘beyond’.
Sending a light field – like a laser beam – through the sheet causes the billion holes to act like tiny antennas emitting a unique magnetic field, one like no other found in nature. The field interacts with the light, and causes it to bend in a direction determined by the frequency, or color, of the light.
To illustrate the flexibility and range of bending directions, the team fabricated a prism out of their funky material. While a normal prism produces a rainbow of colors in always the same order (redder colors on top, bluer ones on the bottom), the team’s prism behaved quite differently: The bluer colors went from being in the bottom part of the rainbow to being in its upper layer.
Until now, the potential of 3-dimensional metamaterials had been shown to work at microwave frequencies only. Metamaterials for visible light had been demonstrated before, but they couldn’t work unless they were made from one single atom-thin layer of metal, too impractical for real devices. The innovation of Dr. Zhang’s team lies in implementing the theoretical concept of drilling tiny holes in 3-dimensions in a material, a scientific first.
For now, an invisibility shield is still years away from being available at a corner store, or even in Dr. Zhang’s high-tech lab. Indeed, the metamaterial only works for a narrow range of colors, which means that an invisibility shield built for bluish light wouldn’t work for reddish light. Since broad daylight contains all colors, the shield in daylight would act more like a color filter than an invisibility cloak.
Yet, this is one huge step towards designing super devices for monochromatic (one-color) applications. “Shaping light will allow scientists to create super lenses, improve communication networks, medical imaging devices, and more. This is really just the beginning for us.”
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