Scientific Method / Science & Exploration

Invisibility cloaks closer to living up to their name

A new metamaterial with a negative refractive index can misdirect light in …

Metamaterials, which are used to produce the items that are termed "invisibility cloaks," have occasionally been targets of skepticism because they don't actually work in visible wavelengths. A paper published in Nature this week describes the structure of a new metamaterial with a negative refractive index that can misdirect light in most of the visible spectrum, up through blue light, bringing the cloaks a step closer to imparting actual invisibility. 

The metamaterials that compose the invisibility cloaks work because they have a negative refractive index, which causes light entering them to bounce in an unusual way. Recently, scientists had found that the physical fabrication limits of their metamaterials were too close to the wavelengths of visible light, and they could only bend waves on the red end of the electromagnetic spectrum. The "invisibility cloak" name had been writing checks its functionality couldn't cash; to make visually undetectable material, scientists needed a new approach.

They found they were able to bend visible light by using a two-dimensional array of coaxial waveguides arranged in a hexagonal configuration. The waveguides were composed of two layers of silver sandwiching a gallium-phosphorus insulator; they could maintain a negative refractive index for light of wavelengths as short as 450-500 nanometers, corresponding to the color blue.

The literal invisibility cloak is stll incomplete—as it stands, the metamaterial described here would be able to bend away every color except violet, so anything "cloaked" would just be highlighted purple. Even if researchers do eventually conquer violet, an invisibility cloak might still be noticeable to the human eye. Still, the ability to bend shorter wavelengths brings scientists a step closer to a new way of beating the diffraction limit and creating superlenses that can see features even smaller than the wavelengths we use.

Nature, 2010. DOI: 10.1038/NMAT2747  (About DOIs).

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