A broadband invisibility cloak: where is my spare dimension

11 01 2009

By Chris Lee | Published: January 09, 2009 – 11:39AM CT

In late 2008, I wrote an article about using plasmonics to develop a broadband invisibility cloak. The authors of that paper concluded that you could either have perfect invisibility at one color, or imperfect invisibility at three colors. Now, a new paper, published in Scienceshows that broadband invisibility may well be possible.

These results were obtained using the idea of transformation media, which I also discussedin late 2008. The basic idea is that optical elements are really just ways of bending space as far as light is concerned. If we then concentrate on what bends are necessary to achieve particular optical effects, then designing a corresponding optical device becomes much much easier—although the theory says nothing about how to actually construct such devices.

Even using transformation media, broadband invisibility was thought to be impossible because, at certain locations, light is required to have an infinite speed, something that might be difficult to physically realize. Undeterred by such findings, researchers from the recently opened Physics Department at the National University of Singapore have pursued the problem and have come up with a broadband invisibility cloak.

Their solution is ingenious but not simple. In a two dimensional example, the researchers construct a plane that causes light to converge on a specific region of space. At this point, a mirror reflects the light from the planar space onto a spherical space. The light then travels around the circumference of the sphere and, no matter what direction the light enters the sphere surface, it always ends up at the same location. A second mirror can then divert the light back onto the planar space, where it exits as if nothing had ever happened.

Note that the two dimensional example required a three dimensional object. As the authors note, their three dimensional example involves a four dimensional hypersphere. Here’s the thing: I can’t picture how to make a three dimensional object that bends space in four space-like dimensions… I just can’t do it, and I can’t see how it is possible.

Nevertheless, it’s an interesting finding, which someone much cleverer than me might be able to implement. However, don’t expect to learn much from the two page paper—to actually see how the results were obtained, you need to read the 20 odd pages of supplementary information.




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