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Gold Nanobricks Pave The Way For Advances In Invisibility Cloaks

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I am currently in Athens, having given a lecture on the Materials Science of Superheroes. In it, I discuss a range of superpowers that were previously consigned to science fiction, but are rapidly becoming science fact. One of the superpowers people always talk about is the power to become invisible. As a Harry Potter fan, I love the idea of throwing an invisibility cloak over me and going on secret adventures, but surely that is far from a reality?

Actually, it isn’t too far-fetched an idea at all. For a long time now, scientists have been working on metamaterials – materials that display properties that we do not naturally encounter in nature. These properties are often achieved not by developing the material itself, but by tweaking and optimising the microstructure and nanostructure of the material to allow it to interact with electromagnetic waves very differently to the bulk material.

One such consequence of playing around with these materials would be to allow them to bend light around an object. As an observer, this would effectively mean that the object being concealed by this material would be entirely invisible to the observer. Previous developments in this field were able to bend much larger electromagnetic waves with a greater wavelength around objects before, but these wavelengths did not fall within the visible range of the electromagnetic spectrum – the wavelengths of light that humans see in. As such, true invisibility as we know and love it was not achieved. The other problem with many of these metamaterials is their inflexibility, which limits their ability to be wrapped around or draped over an object that they are concealing.

Recently however, advancements in this area have been made by materials scientist Professor Xiang Zhang and his group at the Lawrence Berkeley National Laboratory in UC Berkeley, United States. Rather than bending light around an object, Zhang and his group have created a material that can be draped over an object, transforming said object into a perfect flat mirror. In doing so, light is reflected back almost perfectly, while any light that is scattered by the object does so in such a way that the location of the object cannot be determined. The phase and frequency of reflected light is maintained; as there is no change, there is no visible colour change due to interference.

The cloak itself is made of a 50 nm thick film of magnesium fluoride that has been covered with gold brick-like antennae. The gold bricks are 30 nm thick, but vary in their width and length. These nanobricks are the key to almost perfectly reflecting light back off the surface. So far, this cloak of invisibility has been able to conceal very small objects. Larger objects pose new challenges, particularly the issue of them creating shadows, however the flexibility of this material and the method it employs makes it a good contender for the first real life invisibility cloak. In addition to this, the gold bricks on this material can be finely tuned to provide other methods of concealment, from appearing to blend in with the background like camouflage, to projecting the image of an entirely different object to the observer – from Harry Potter’s invisibility to Professor McGonagall’s transfiguration, this research proves that science really can make magic happen.