Can't you see? Invisibility cloak may be reality

The age-old fantasy of making yourself invisible has taken a step toward reality, with scientists saying they have created three-dimensional materials that can bend visible light.

For the moment, the vanishing act takes place on a nanoscale, measured in billionths of a metre. But there is no fundamental reason why the same principles cannot be scaled up one day to make invisibility cloaks big enough to hide a person, a tank or even a tanker, the scientists say.

The groundbreaking experiments, led by Xiang Zhang at the University of California at Berkeley, were reported simultaneously on Sunday in the British journal Nature and the United States-based journal Science.

Recent advances have created other so-called metamaterials, artificially engineered structures with optical properties that bend light in unnatural ways.

But previous attempts had three serious limitations.

One was that they only worked on the microwave range of the light spectrum, bending wavelengths much too long to be visible to the human eye. The earlier technology was also limited to two-dimensional systems no thicker than a single layer of atoms.

Finally, a large portion of the light was absorbed rather than refracted, a form of energy loss that reduced the “invisibility” factor.

The new material, by contrast, produces the negative refractive needed to work within a visible light spectrum and in three dimensions.

Negative refraction—or “left-handed”—materials deflect light in a way that breaks the standard “right-handed” rules of electromagnetism.

Unhindered, light will travel in a straight line.
But it will change paths when it travels from one medium to another. A pencil sticking out of a glass of water, for example, appears to bend slightly starting at the interface of air and liquid.

But with metamaterials, light travels opposite the direction it would normally when it hits that interface—the pencil now seems to bend backwards, out of the water, an optical effect that does not occur in nature.

The most immediate application of this new technology will be the construction of special lenses for optical microscopes that can focus on things as tiny as molecules.

“There is a significant need in biology to be able to image at high resolution,” Jason Valentine, a graduate student at Berkeley and lead author of the Nature paper, explained in an email.

The resolution of traditional high-powered tools such as the electron microscope cannot exceed half of a wavelength of light, and have a tendency to cause damage to living cells, he said.

“Metamaterials would be a better optical imaging method since it would be non-invasive,” said Jie Yao, lead author of the Science article.

But the holy grail of metamaterial research has become the kind of magic shroud of invisibility that has fired the human imagination from HG Wells’s Invisible Man to the adventures of Harry Potter.

“In the case of such cloaks or shields, the material would need to curve light waves completely around the object like a river flowing around a rock,” Zhang said. “An observer looking at the cloaked object would then see light from behind it, making it seem to disappear.”

The US military, which funded the research, is especially keen to develop materials that could usher in an entire new generation of stealth technology.

But it isn’t going to happen overnight. “To scale up such a material to sizes visible to the naked eye will be very time-consuming and very costly,” said Yao.

One of the two metamaterials developed by Zhang and his colleagues has a multilayered fishnet structure composed of alternating layers of silver and magnesium fluoride.

“The fish-net structure is counter-intuitive because, as we stack the material in layers, the energy loss is reduced instead of increased,” Yao said.

Previous experiments has shown that the bulkier the material, the higher the loss.

The second material, reported in Science, is composed of silver nanowires grown inside a porous aluminium matrix.—Sapa-AFP

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