Simanaitis Says

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CAN YOU SEE ME NOW?

SCIENCE IS moving forward to match fiction. In Harry Potter and the Philosopher’s Stone, J.K. Rowling’s hero receives an invisibility cloak, a silky garment that makes him disappear. Now, Science magazine, published by the American Association for the Advancement of Science, reports that researchers are improving their understanding of real-world invisibility.

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Xiang Zhang and colleagues at the University of California, Berkeley; Lawrence Berkeley National Laboratory; and King Abdulaziz University, Jeddah, Saudi Arabia give details in “An Ultrathin Invisibility Skin Cloak for Visible Light” in Science, September 18, 2015.  An abstract of their paper is available, along with an article by Adrian Cho, which summarizes the research, “Skintight Invisibility Cloak Radiates Deception.”

Way back in 2006, invisibility devices were described by Sir John Pendry et al, of Imperial College London and Duke University, U.S.A. They likened the devices to water flowing around a stick in a stream. A metamaterial, in a sense, grabs light and diverts it around the object.

The original invisibility device worked at only a single wavelength of light; later ones routed visible light and longer wavelengths around the object being hidden. However, as Professor Pendry put it, these devices were much larger than the objects they hid: “less like Harry Potter’s cloak and more like Harry Potter’s shed,” he said.

Instead of diverting light around an object, Xiang Zhang and his team work to modify the reflected light. Cho’s summary offers an example with a bump on a surface. Ordinarily, light waves bouncing off the bump give away its presence.

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At left, incoming light rays encounters an object. At right, the directions of reflected waves are dictated by the object’s shape. These and the following images from Science, September 18, 2015.

For invisibility, the reflections are modified by an extremely thin metasurface cloaking the object. This metasurface is transparent and arrayed with tiny rectangles of gold, most less than a micrometer, 0.001 mm, 0.000039 in., on a side. Each gold patch acts like an antenna and, depending on its dimensions, its redirected light can be tuned to model the underlying surface, not the object.

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With a nanocloak of calibrated pieces of gold, the reflected light renders the object invisible.

Cho writes, “To prove the ‘skin cloak’ works, the researchers made one to hide a landscape of random bumps a few micrometers wide and about a micrometer tall. The cloak was 80 nanometers thick, one-ninth the wavelength of the red light used in the experiment.”

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A: a 3D illustration of a metasurface skin cloak. B: Its nanoantennas modify reflections, now corresponding to the flat surface, not the bump.

There’s plenty of work still to be done before Harry Potter would be satisfied. The antenna pattern now must be tuned specifically to the object. You could wear it, Zhang says, “but you can’t move or it won’t work anymore.”

However, researchers say it’s a significant advance. Pendry suggests a practical application: A fighter plane, for example, might be covered with a metasurface designed for radar, not to make it invisible, but rather to make it resemble a freighter. “It might be easier to make one thing look like something else,” Pendry says, “and that might be good enough.”

PotterFriends

If Harry wished, he could masquerade as Professor Severus Snape or Hermione Granger. ds

© Dennis Simanaitis, SimanaitisSays.com, 2015

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