The news has broken: we are all going to be able to purchase cloaks of invisibility in a few years. Or perhaps not. Some recent research from Berkeley is a big step and will, no doubt, find many applications, but invisibility is not among them. We take a look at what the researchers achieved and use the Ars meta-crystal ball™ to predict where this will end up being employed.

Over the past few years, there has been a lot of theoretical and experimental work on a class of structures called meta-materials. A meta-material is used to get around the limitations of natural materials by structuring them in special ways. This can best be described using quartz as an example. A single quartz crystal is pretty much transparent to visible light; a few percent of the light is reflected from the crystal surface, but otherwise it all goes through. However, if the wavelength is much shorter (on the order of the spacing between molecules), then the crystal is still transparent, but light is scattered into a pattern. Similarly, if we take a handful of quartz crystals, all the little reflections from the many surfaces scatter even visible light in every direction, resulting in white sand that is no longer possible to see through. If the quartz crystals were all the same size and shape and arranged in an orderly fashion, however, it would be possible to use them to guide visible light down specific paths as well.

Quartz demonstrates how order and scale matter in how light propagates through a material. Meta-materials take advantage of this by deliberately engineering materials to have order on scales comparable to the wavelength of light that they want to influence. This can be done by alternating layers of materials (as in antireflection coatings), by crafting arrays of holes in an otherwise solid material, or by building crystal-like structures composed of small spheres.

In certain circumstances, a meta-material can exhibit a negative refractive index (in nature, the refractive index is always positive), which causes light to bend in unexpected directions. This has proven to be difficult to achieve at visible wavelengths due to a number of challenges. A negative refractive index has to have the appropriate scale (~30nm features), which is difficult, but not impossible.

However, the electric field phase of the light also needs to be controlled. To do that, the material must respond to the light's electric field with a large opposing field that can slows the light's advance. In effect, this requires metallic structures that allow the electrons to resonate with the light field, allowing a large opposing field to build up. Making a single layer of these structures has been done before, but it's really hard to define the bulk properties of a meta-material when it's only a single layer thick.

That is what makes the new research so important. For the first time, researchers have managed to make a negative index meta-material* that is more than a single layer thick. This allowed them to directly measure its bulk properties and confirm some of the predictions for these materials.





Nature, J.Valentine et al

To construct their negative index material, the researchers grew alternating layers of silver and magnesium fluoride. This provides the bulk periodicity required, but it doesn't provide a resonant structure that will slow the light. To obtain that, the researchers drilled holes through the structure, creating a series of silver rings. Each ring acts like an electrical inductor, retarding the phase of the light's electric field locally. The inductors in adjacent layers all couple together, creating one giant inductor. This has the effect of reducing the retardation per inductor, but increasing the range of light colors that the rings will retard.

The meta-material used in this demonstration had 21 layers (ten of magnesium fluoride and 11 of silver) with 484 holes drilled through it. The holes are about 500nm in diameter, and the whole structure has a face of just 5 micrometers on a side. One face was angled so that the material acted like a prism. The researchers' measurements showed that the prism did indeed have a negative refractive index over about one-third of the near-infrared spectrum (1.45 to 1.8 micrometers, to be precise).

Although this is quite small scale, it is a very important achievement. Let's deal with what it isn't first. It will not act as a cloaking material because of absorption. There is a lot of silver in it, and silver absorbs and reflects radiation quite efficiently, meaning that not much light makes it through the material—the sample described in Nature, which was less than one micrometer thick, absorbed 35 percent of the incident radiation.

Under ideal fabrication conditions, this might be reduced to as little as six percent per micrometer. Nevertheless, when you consider that a windowpane reflects about four percent of the incident radiation, you can see why even six percent is too high to render anything invisible. If anything, it will be easy to spot, as humans pick out reflections and flashes, meaning that this would probably draw more attention than good camouflage.

So, if this isn't a material for making you invisible what is it good for? It's the basic material we need for beating the diffraction limit. Negative index materials are not subject to the diffraction limit, meaning that light can be focused to smaller volumes. This implies that we would be able to illuminate cellular machinery at the level of individual molecules or perhaps even individual atoms. It would even be possible to use direct chemical imaging, instead of relying on fluorescent labels. This one application, which may well be attainable with the technology the authors used in their demonstration, should be enough for anyone to get excited about.

*Negative index materials for microwave radiation have already been demonstrated.

Nature, 2008, DOI: 10.1038/nature07247

Science, 2008, DOI: 10.1126/science.1157566