An Introduction to the Ultra-thin Invisibility Cloak
Invisibility in Pop Culture
There are numerous (sometimes extremely geeky) references to invisibility in movies, television shows, and even literature. Harry Potter inherited an invisibility cloak from his father and the crew members of Star Trek’s USS Enterprise could cloak their communications and ship from prying eyes. Even the famous super-spy created by Ian Fleming, James Bond, has a car that disappears from the naked eye at the press of a button. In addition, who can forget Wonder Woman’s invisible jet or Bilbo Baggins and the One Ring from the Lord of the Rings and the Hobbit? Over the years, whether in literature or science fiction, the concept of invisibility has continued to captivate audiences around the world.
The Creation of the Ultra-thin Invisibility Cloak
One of the largest problems with materials that have the capacity to bend light around an object is that these materials are not only hard to shape but also only work from one vantage point. That is, if you viewed an object coated with one of these materials from elsewhere – it might be visible. Researchers from the United States Department of Energy’s Lawrence Berkeley National Laboratory and the University of California Berkeley, led by Xiang Zhang constructed a thin film of a 50-nanometer thick layer of magnesium fluoride topped by a pattern of gold antennas that were each 30 nanometers thick. The antennas varied in size, ranging from 30 to 220 nanometers long and 90 to 175 nanometers wide.
Testing the “Cloak”
To test their cloak prototype, the researchers wrapped up an irregularly shaped, extremely tiny (~30 microns across) object in the cloak. Then, they shined a light (near infrared in the electromagnetic spectrum) on the object. They discovered that when light scattered from the cloak bounced off of the object, it reflected back almost perfectly and without revealing the object’s location. The edges of the object were also invisible.
How Was “Invisibility” Achieved?
The gold antennas built into the cloak controlled the scattering of the light that reflected off of the cloak. Ordinarily, light reflecting off of an object will scatter slightly, especially if the object is irregular in shape. Sometimes the light will even create interference patterns, resulting in the light appearing in different colors or reflection. The gold antennas of the cloak reflected the light in such a manner that the light’s phase (the angle measuring how far along a light wave a point is) and frequency were both preserved. This causes the cloak to act as a mirror and reflect back to the viewer — almost as if there were nothing behind.
One problem with this type of cloak is that the object donning it would have to be stationary since the cloak could only be tuned to match one background. In addition, larger objects have shadows that would break the illusion of a reflection. Making a cloak that would work similar to James Bond’s car would be close to impossible – and even if it were possible – would be prohibitively expensive.
The Takeaway: The Future of Invisibility Cloak is Unlimited
While the cloak may not look exactly like Harry Potter’s, the technology’s reflectivity can be applied to displays. Any projection, like in a movie theater, has to use a flat surface. But if the phase and frequency of the light reflected from the surface were controlled, the issue of shape would cease to matter. Any surface could be used to project an image or media. The next step for the invisibility cloak would be manufacturing it at an industrial scale with the antennas tuned to different waves of light. Advances made in these areas would mean the future of invisibility cloak technology has endless possibilities.