Invisibility cloak now within sight: scientists (Update 2)

August 11, 2008
Shown is a schematic and two scanning electron microscope images with top and side views of a metamaterial developed by UC Berkeley researchers. The material is composed of parallel nanowires embedded inside porous aluminum oxide. As visible light passes through the material, it is bent backwards in a phenomenon known as negative refraction. Credit: Image by Jie Yao, UC Berkeley

( -- Scientists at the University of California, Berkeley, have for the first time engineered 3-D materials that can reverse the natural direction of visible and near-infrared light, a development that could help form the basis for higher resolution optical imaging, nanocircuits for high-powered computers, and, to the delight of science-fiction and fantasy buffs, cloaking devices that could render objects invisible to the human eye.

Two breakthroughs in the development of metamaterials - composite materials with extraordinary capabilities to bend electromagnetic waves - are reported separately this week in the Aug. 13 advanced online issue of Nature, and in the Aug. 15 issue of Science.

On the left is a schematic of the first 3-D "fishnet" metamaterial that can achieve a negative index of refraction at optical frequencies. On the right is a scanning electron microscope image of the fabricated structure, developed by UC Berkeley researchers. The alternating layers form small circuits that can bend light backwards. Image by Jason Valentine, UC Berkeley

Applications for a metamaterial entail altering how light normally behaves. In the case of invisibility cloaks or shields, the material would need to curve light waves completely around the object like a river flowing around a rock. For optical microscopes to discern individual, living viruses or DNA molecules, the resolution of the microscope must be smaller than the wavelength of light.

The common thread in such metamaterials is negative refraction. In contrast, all materials found in nature have a positive refractive index, a measure of how much electromagnetic waves are bent when moving from one medium to another.

In a classic illustration of how refraction works, the submerged part of a pole inserted into water will appear as if it is bent up towards the water's surface. If water exhibited negative refraction, the submerged portion of the pole would instead appear to jut out from the water's surface. Or, to give another example, a fish swimming underwater would instead appear to be moving in the air above the water's surface.

Other research teams have previously developed metamaterials that function at optical frequencies, but those 2-D materials have been limited to a single monolayer of artificial atoms whose light-bending properties cannot be defined. Thicker, 3-D metamaterials with negative refraction have only been reported at longer microwave wavelengths.

"What we have done is take two very different approaches to the challenge of creating bulk metamaterials that can exhibit negative refraction in optical frequencies," said Xiang Zhang, professor at UC Berkeley's Nanoscale Science and Engineering Center, funded by the National Science Foundation (NSF), and head of the research teams that developed the two new metamaterials. "Both bring us a major step closer to the development of practical applications for metamaterials."

Zhang is also a faculty scientist in the Material Sciences Division at the Lawrence Berkeley National Laboratory.

Humans view the world through the narrow band of electromagnetic radiation known as visible light, with wavelengths ranging from 400 nanometers (violet and purple light), to 700 nanometers (deep red light). Infrared light wavelengths are longer, measuring from about 750 nanometers to 1 millimeter. (A human hair is about 100,000 nanometers in diameter.)

For a metamaterial to achieve negative refraction, its structural array must be smaller than the electromagnetic wavelength being used. Not surprisingly, there has been more success in manipulating wavelengths in the longer microwave band, which can measure 1 millimeter up to 30 centimeters long.

In the Nature paper, the UC Berkeley researchers stacked together alternating layers of silver and non-conducting magnesium fluoride, and cut nanoscale-sized fishnet patterns into the layers to create a bulk optical metamaterial. At wavelengths as short as 1500 nanometers, the near-infrared light range, researchers measured a negative index of refraction.

Jason Valentine, UC Berkeley graduate student and co-lead author of the Nature paper, explained that each pair of conducting and non-conducting layers forms a circuit, or current loop. Stacking the alternating layers together creates a series of circuits that respond together in opposition to that of the magnetic field from the incoming light.

Valentine also noted that both materials achieve negative refraction while minimizing the amount of energy that is absorbed or "lost" as light passes through them. In the case of the "fishnet" material described in Nature, the strongly interacting nanocircuits allow the light to pass through the material and expend less energy moving through the metal layers.

"Natural materials do not respond to the magnetic field of light, but the metamaterial we created here does," said Valentine. "It is the first bulk material that can be described as having optical magnetism, so both the electrical and magnetic fields in a light wave move backward in the material."

The metamaterial described in the Science paper takes another approach to the goal of bending light backwards. It is composed of silver nanowires grown inside porous aluminum oxide. Although the structure is about 10 times thinner than a piece of paper - a wayward sneeze could blow it away - it is considered a bulk metamaterial because it is more than 10 times the size of a wavelength of light.

The authors of the Science paper observed negative refraction from red light wavelengths as short as 660 nanometers. It is the first demonstration of bulk media bending visible light backwards.

"The geometry of the vertical nanowires, which were equidistant and parallel to each other, were designed to only respond to the electrical field in light waves," said Jie Yao, a student in UC Berkeley's Graduate Program in Applied Science and Technology and co-lead author of the study in Science. "The magnetic field, which oscillates at a perpendicular angle to the electrical field in a light wave, is essentially blind to the upright nanowires, a feature which significantly reduces energy loss."

The innovation of this nanowire material, researchers said, is that it finds a new way to bend light backwards without technically achieving a negative index of refraction. For there to be a negative index of refraction in a metamaterial, its values for permittivity - the ability to transmit an electric field - and permeability - how it responds to a magnetic field - must both be negative.

The benefits of having a true negative index of refraction, such as the one achieved by the fishnet metamaterial in the Nature paper, is that it can dramatically improve the performance of antennas by reducing interference. Negative index materials are also able to reverse the Doppler effect - the phenomenon used in police radar guns to monitor the speed of passing vehicles - so that the frequency of waves decreases instead of increases upon approach.

But for most of the applications touted for metamaterials, such as nanoscale optical imaging or cloaking devices, both the nanowire and fishnet metamaterials can potentially play a key role, the researchers said.

"What makes both these materials stand out is that they are able to function in a broad spectrum of optical wavelengths with lower energy loss," said Zhang. "We've also opened up a new approach to developing metamaterials by moving away from previous designs that were based upon the physics of resonance. Previous metamaterials in the optical range would need to vibrate at certain frequencies to achieve negative refraction, leading to strong energy absorption. Resonance is not a factor in both the nanowire and fishnet metamaterials."

While the researchers welcome these new developments in metamaterials at optical wavelengths, they also caution that they are still far off from invisibility cloaks and other applications that may capture the imagination. For instance, unlike the cloak made famous in the Harry Potter novels, the metamaterials described here are made of metal and are fragile. Developing a way to manufacture these materials on a large scale will also be a challenge, they said.

Nevertheless, the researchers said achieving negative refraction in an optical wavelength with bulk metamaterials is an important milestone in the quest for such devices.

Provided by University of California - Berkeley

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3.3 / 5 (4) Aug 11, 2008
"Invisibility cloak now within sight"....must say, the people responsible for these headings must be on something.....

On a more serious note,what would happen if you lost your harry potter invisibility cloak? how will you start searching for it? by using IR???
not rated yet Aug 11, 2008
ahh ok, so now we only need to figure out how to bend light like a river...can't be to difficult.
1 / 5 (2) Aug 11, 2008
Well think of the practical nonmilitary applications. First one: diverting the attention of "rubberneckers" in traffic. Diverting attention from crime scenes, etc. Get rid of bystander interference and prevent unnecessary traffic backups.
2.1 / 5 (7) Aug 11, 2008
This could be an extremely bad thing to invent. There are so many evil/criminal uses for it. Do the benifits outweigh these negatives?
Also it says 'one day' - I hate those words.. It Could mean 1 day or 100 years from now.
4 / 5 (6) Aug 11, 2008
"One possible application would be the construction of special lenses for optical microscopes that could focus on things as tiny as molecules."
That alone would be amazing enough. Imagine the uses for an optical microscope exceeding the resolution of electron microscopes, able to resolve the workings of living cells almost down to the molecular scale. No more crude simulations! Just being able to look and see why something works in a test tube but not in the actual cell would be awesome.
4 / 5 (1) Aug 11, 2008
This could be an extremely bad thing to invent. There are so many evil/criminal uses for it. Do the benifits outweigh these negatives?
Also it says 'one day' - I hate those words.. It Could mean 1 day or 100 years from now.

Yeah but look at it this way. If everyone has it readily available then invisibility counters itself.

Can't steal something you can't see.
1.7 / 5 (3) Aug 11, 2008
Colossal Storage wrote article ~10 years ago about using reprogramable ferroelectric metamaterials for invisibility.

Zhang wants to call himself the originator ?

Then Californias are still inventing the same way as usual.
not rated yet Aug 11, 2008
"In a classic illustration of how refraction works, the submerged part of a pole inserted into water will appear as if it is bent up towards the water's surface."

Anyone else see this statement as a problem!?
not rated yet Aug 11, 2008
Is this true classical refraction that is negative? It appears to be the bending of light waves by the principle used for focusing and bending radio wave frequencies, wave guides. True negative refraction does not exist.
4 / 5 (1) Aug 13, 2008
"If water exhibited negative refraction, the submerged portion of the pole would instead appear to jut out from the water's surface. Or, to give another example, a fish swimming underwater would instead appear to be moving in the air above the water's surface."

Well, I would like to see how the author of this will draw a ray diagram that would show a fish above the water. Also, this kind of paper - on an invisibility cloak - is supposed to show a pretty simulation of an electromagnetic or scalar wave nicely flowing around a spherical object almost without any distortion or scattering. Perhaps, they are included in the original papers; without that it does not impress at all.
not rated yet Aug 14, 2008
Stern: it seems like the ray diagram would work out fine. If the two rays from your two eyes cross above the water, and then the water reverses their horizontal directions, then the water will "uncross" them so that they meet again under the surface at a point more or less under the first crossing (or directly under it, if the index is -1). So a fish at that second meeting point would appear to be floating above the water at the first meeting point.

Since the ray diagram must be rendered in 3D in order for the crossing and uncrossing to be clearly illustrated, that makes it challenging to put on a web page.
not rated yet Aug 14, 2008
"Or, to give another example, a fish swimming underwater would instead appear to be moving in the air above the water's surface."

Practical holograms?
not rated yet Aug 16, 2008
With some adaptation this technology could have a counterpart in gravity manipulation. What I know about general relativity says forget it. What I know about quantum theory says go for it.
1 / 5 (1) Aug 16, 2008
i think this is all about coating aircraft so mm wave radar will not find reflections.
not rated yet Aug 21, 2008
The invisibility cloak is a tired headline to grab attention to a step-wise improvement. This particular step is more visible than glass.

The sub-wavelength microscopy is where the real juice is.

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