Invisibility cloak on the way....

1
BBC wrote:Invisibility cloak 'step closer'

Scientists in the US say they are a step closer to developing materials that could render people invisible.

Researchers at the University of California in Berkeley have developed a material that can bend light around 3D objects making them "disappear".

The materials do not occur naturally but have been created on a nano scale, measured in billionths of a metre.

The team says the principles could one day be scaled up to make invisibility cloaks large enough to hide people.

Stealth operations
The findings, by scientists led by Xiang Zhang, were published in the journals Nature and Science.

The light-bending effect relies on reversing refraction, the effect that makes a straw placed in water appear bent.

Previous efforts have shown this negative refraction effect using microwaves—a wavelength far longer than humans can see.

The new materials instead work at wavelengths around those used in the telecommunications industry—much nearer to the visible part of the spectrum.

Two different teams led by Zhang made objects made of so-called metamaterials—artificial structures with features smaller than the wavelength of light that give the materials their unusual properties.

One approach used nanometre-scale stacks of silver and magnesium fluoride in a "fishnet" structure, while another made use of nanowires made of silver.

Light is neither absorbed nor reflected by the objects, passing "like water flowing around a rock," according to the researchers. As a result, only the light from behind the objects can be seen.

Cloak and shadow
"This is a huge step forward, a tremendous achievement," says Professor Ortwin Hess of the Advanced Technology Institute at the University of Surrey.

"It's a careful choice of the right materials and the right structuring to get this effect for the first time at these wavelengths."

There could be more immediate applications for the devices in telecommunications, Prof Hess says.

What's more, they could be used to make better microscopes, allowing images of far smaller objects than conventional microscopes can see.

And a genuine cloaking effect isn't far around the corner.

"In order to have the 'Harry Potter' effect, you just need to find the right materials for the visible wavelengths," says Prof Hess, "and it's absolutely thrilling to see we're on the right track."

Re: Invisibility cloak on the way....

2
...or not:
Ars wrote:New meta-material doesn't actually render anything invisible

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.

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.

Re: Invisibility cloak on the way....

3
or maybe so:
Ars wrote:Cloaking devices inch towards visible wavelengths
Researchers have developed a type of cloaking device that operates in a broad spectrum of the near-infrared, and suggests that only manufacturing limits kept it from operating at visible wavelengths.

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.