From Perseus's helmet to Harry Potter's cloak, the idea of an object that can render a person invisible has been a staple of fiction and mythology for eons. But for about the past year, physicists and materials scientists have been publishing papers that reveal real progress in turning science fiction into fact.
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The landmark announcement came in October 2006 when David R. Smith's team at Duke University's Pratt School of Engineering published a paper in the journal Science announcing that they had created a device capable of rendering objects invisible to a small bandwidth of microwave beams. The first working "invisibility cloak" was actually a small ring of concentric circles made from precisely arranged "metamaterials".
Metamaterials are constructed in a laboratory from nano-sized elements, and the size, shape, and pattern of elements gives them the ability to interact with electromagnetic waves such as microwaves and visible light differently from ordinary materials. In this case, they exhibited a low refractive index, which allowed researchers to create a structure that guides microwaves around a "cloaked" object and past it. The reflection and shadow of the object is minimal, and so the object becomes effectively transparent to that wavelength. The ability to hide things from microwaves could give a boost to radar cloaking technology, and metamaterials that can bend microwaves and radio waves in odd ways have already lead to improvements in wireless antenna technology.
Still, since we can't see microwaves – and they're thousands of times bigger than visible light waves – the use of the term "invisible" felt like something of a misnomer for non-scientists and Harry Potter fans. But the announcement by the Duke engineers laid the groundwork for a "real" invisibility cloak: one that works in the optical wavelength.
Most recently, a group at the University of Stuttgart in Germany published a paper in the journal Nature Materials, announcing the creation of a metamaterial that operates in the infrared range. Infrared light is not actually visible to people, but it's considered part of the visible light range by physicists.
Their metamaterial is made from horseshoe-shaped gold nanowires that are arranged in a square pattern and perfectly stacked above each other. It is not a cloak, but it does demonstrate two advancements. First, the structure is designed to interact with electromagnetic waves in three dimensions. Previous experiments have been tabletop designs that work in only two dimensions, in other words, from the sides of an object, but not the top and bottom.
Second, it demonstrates negative permeability in the infrared range, which lead researcher Harald Geissen says is the harder part of achieving a negative refractive index. The device is small, but it represents a method of manufacturing 3D metamaterials. An approach that Geissen says may have future applications in creating perfect lenses and optical cloaks. In the meantime, we'll have a to wait a little longer for that invisibility cloak…
The Duke project was published October 19, 2006, in Science Express, the advance online publication of the journal Science. It was funded by the Intelligence Community Postdoctoral Fellowship Program.
The Stuttgart project was published in the January 2008 issue of Nature Materials. It was funded by Deutsche Forschungsgemeinschaft, Baden-Württemberg, and BMBF (Federal Ministry of Education and Research).
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