Cloaking devices and metamaterials are hot topics in the realm of science where optics blurs into materials science. By crafting materials that can interact with specific wavelengths of light, researchers have been able to steer that light around small objects, essentially cloaking the object at those wavelengths. The problem so far is that many of these materials are very specific about the wavelengths they work at, and none of those were in the visible spectrum. Now, researchers have designed a cloaking material that operates across a range of the near-infrared, and suggest it should be possible to bring things down to the visible spectrum.

The material was used in a test setup that's similar to one that was used in past work in the microwave range. The setup can basically be described as a mirror with a bump. When light hits it directly, the bump acts a bit like a funhouse mirror, distorting the reflection. The cloaking device can be put down on top of the bump and steer light waves in a way that makes it look as if neither the bump nor the cloak were there, producing reflections as if there were a smooth, undistorted mirror in place.

It's simple in principle, but actually steering the light waves is quite challenging. Metamaterials can do the job, but only within a fairly narrow range of wavelengths, and some mathematical calculations suggest that doing much better than that might be impossible.

To create a cloak, the researchers had to craft a material with a carefully controlled index of refraction. They did that by carefully drilling an irregular lattice of holes in a silicon substrate—the arrangement of the holes was calculated using what the authors term quasi-conformal mapping, and the holes were kept smaller than the wavelength of the incident light. Once fabricated, the object was coated in gold in order to give it a reflective surface.

The authors found that their device could successfully cloak the bump in their test setup. It didn't reflect the incident light with full efficiency (only 58 percent efficiency was obtained), but the loss was ascribed to defects in the material rather than an inevitable outcome of the technique. The most striking feature of their device, however, was the fact that it was effective across a wide range of wavelengths, from 1,400nm to 1,800nm. All of that's in the infrared range, but it's not too far from the visible spectrum. The dropoff below 1,400 nm is also a product of the fact that the wavelength of light is approaching the size of the holes drilled in the cloaking device; smaller holes would drop things down into the visible range, provided the manufacturing technique was sufficiently precise.

It looks like there is going to be a race to visible wavelengths, too. At the end of the publication, there's a "note added in proof," which typically is used to recognize the publication of related material during the awkward period between when a paper has passed peer review and before it's formatted for publication. This note points to a document which has been deposited in the arXiv that describes a related cloaking device.

Many details of that second publication are the same—using a "carpet" style cloak, made of a silicon-on-insulator material, that masks the presence of an underlying bump—but there are two key differences. For one, instead of holes, the authors take the opposite approach, fashioning a forest of carefully spaced posts that protrude from the underlying surface. The device also gets just to the edge of what's a typically visible wavelength, cloaking everything from 950nm to 1500nm. The visible spectrum is typically considered to end at 700nm.

So, it appears that we're only a small development in process technology away from having devices edge down into the visible wavelengths. Of course, at some point, someone will have to figure out a way to apply this to something other than a bump on a mirror.

Nature Materials, 2009. DOI: 10.1038/NMAT2461