16:17 19 October 2006
An invisibility cloak that works in the microwave region of the electromagnetic spectrum has been unveiled by researchers in the US. The device is the first practical version of a theoretical set-up first suggested in a paper published earlier in 2006.
The cloak works by steering microwave light around an object, making it appear to an observer as if it were not there at all. Materials that bend light in this way do not exist naturally, so have to be engineered with the necessary optical properties.
Earlier in 2006, John Pendry, a theoretical physicist at Imperial College London, UK, and colleagues showed how such an invisibility cloak could, in theory, be made (see Physicists draw up plans for real 'cloaking device'). Now David Smith and colleagues at Duke University in North Carolina, US, have proved the idea works.
In recent years, materials scientists have made rapid progress in making so-called "metamaterials", which can have exotic electromagnetic properties unseen in nature. These are made up of repeating structures of simple electronic components such as capacitors and inductors.
In 2001, Smith built a metamaterial with a negative
refractive index, which bends microwaves in a way impossible for ordinary
lenses. Now he has gone one step further.
"It's a real breakthrough," says Ulf Leonhardt, a physicist at the University of St Andrews in Scotland. "Paradoxically, it turns out to be easier to construct an invisibility cloak than it was to make the negative lens."
To simplify the problem, Smith's cloak works in only two dimensions. It is about the size of a movie reel canister and consists of a series of concentric rings, each housing a set of simple electronic components that distort an electromagnetic field as it passes through.
"Rather than the cloak's material properties being the same everywhere, its material properties vary from point to point and vary in a very specific way," says Smith. The combined effect of the rings is to steer microwaves around the central region of the device in which Smith hid a copper ring.
To study the effect of his cloak, Smith took images
of microwaves flowing through the rings, like water waves moving across
a pond. Without the cloak in place, the microwaves were reflected and diffracted
by the copper ring. But with the cloak in place, the distortion was dramatically
"It's not perfect," says Leonhardt. "If you could see in the microwave region of the spectrum, the copper ring would not quite disappear. You'd see perhaps a shadow and some slight distortion where the copper ring ought to be."
The device has another important limitation – it works only at a single specific frequency of microwave. "How it might be possible to make a device that works over a range of frequencies is an open problem," says Leonhardt. But Smith now hopes to build a 3D structure that could hide an object completely from view.
So far, the technology works only in the microwave region of the spectrum. The problem with visible light is that it has a much smaller wavelength, meaning an optical metamaterial would have to be built on the nanoscale, which is beyond the limits of current nanotechnology. It, too, would only work at a specific frequency.
"It's not yet clear that you're going to get the invisibility that everyone thinks about with Harry Potter's cloak or the Star Trek cloaking device," says Smith.
Journal reference: Science: (DOI: 10.1126/science.1133628)
Invisibility cloak leaves the realm of magic at last
03 June 2006
HIDING objects inside a cloak that channels light around them to make it look as if they aren't there may soon be possible thanks to a breakthrough idea by materials scientists. It raises the prospect of invisibility shields that could hide objects sitting right under your nose.
Objects are visible simply because light scatters off their surfaces and into your eyes. So in theory, a cloaking device could work by steering light around an object so that you see only the light from behind it, and not the object itself. Now John Pendry, a theoretical physicist at Imperial College London, and his colleagues have worked out how this could be done with a spherical cloak that channels light around an object hidden at its centre (see Diagram).
The stuff that makes this plausible is a new generation of "metamaterials", which can be tailored to have exotic electrical and magnetic properties not found in nature. The metamaterials developed so far consist of complex arrays of metal washer-like shapes and wires. The metal shapes are smaller than the wavelength of light and so interact with it, explains Pendry. "On these scales, it is not the chemical properties of the metal that determine how it interacts with light, it's the metal's structure."
The new idea is to build a sphere of metamaterial whose components are arranged in such a way that they bend radiation around the central cavity before sending it on its way, like a ring road diverting traffic around a town.
Team member David Smith at Duke University in North Carolina has already created a metamaterial that bends microwaves, and is now putting the cloaking idea into practice to make a microwave invisibility shield. "The theory tells us the material properties we need at each point," says Smith. "The challenge is to match those theoretical requirements in the actual material, point-by-point." The team hopes to complete it within a year.
The principle is exactly the same for visible light, but you may have to wait a little while for your invisibility bubble: nobody has yet succeeded in making metamaterials that work at optical wavelengths. However, "many teams are already involved in shrinking metamaterials down to these scales," says Smith.
Another hurdle is that the materials can only steer light in a narrow band of wavelengths, which is fine for microwave radar, for example, but not the entire spectrum of visible light (Science, DOI: 10.1126/science.1125907).
Bending Light Backwards
Bending Light Backwards
THE controversy is over. After years of argument, physicists have shown you really can build materials that will bend light the opposite way from normal, reversing the way refraction usually works. But whether or not such materials can fulfil the prediction that they will act as "perfect lenses", capable of focusing features smaller than the wavelength of light, is still up for debate.
When light enters or leaves a transparent material such as glass at an angle, the rays get refracted, or bent. The light always bends the same way. But in 1968, theorist Victor Veselago at the Lebedev Physics Institute in Moscow speculated that it might be possible to build a material that would bend light the opposite way.
He was thinking of the electric and magnetic fields of a ray of light. The planes of these fields are normally perpendicular to each other and to the direction in which the light is moving. But Veselago calculated that if you could flip the planes of the two fields, light would bend the opposite way, and you would have a "left-handed" material.
Years later, it fell to theorist John Pendry of Imperial College, London, to work out how you might do that. He predicted in 1996 that an array of thin parallel wires would reverse light's electric field, while an array of copper rings should reverse the magnetic field. The same would be true for any electromagnetic radiation. Sure enough researchers who tested out the device with microwaves said it bent them in the opposite direction to normal (New Scientist, 14 April 2001, p 35). But last year the idea came under attack from groups who claimed that there were problems with Pendry's theory and who argued that the experimental results could have been misinterpreted (New Scientist, 18 May 2002, p 11).
Now two other groups have resolved the debate. Andrew Houck from Harvard and his colleagues from MIT built a prism made of interlocking fibreglass strips patterned with copper wires and rings and fired a beam of microwaves at it (see Graphic). When they measured the electric field inside and outside the prism, they found the direction in which the beam was refracted had flipped. Patanjali Parimi and colleagues at Northeastern University in Boston saw similar effects after firing a beam of microwaves at an array of copper rods.
The results convinced other physicists at the meeting. "Contouring electromagnetic fields in this way is very powerful," says Clifford Krowne of the Naval Research Laboratory in Washington DC. He believes left-handed materials could eventually be used to build new components for optical telecommunications equipment.
But the prospects for making a perfect lens are less certain. In 2000, Pendry predicted that a lens made of a left-handed material should create images far sharper than usual. In a conventional lens, "evanescent" waves carrying the finest details of an image die off rapidly. But in a flat slab of left-handed material, Pendry said these waves should get amplified and focused into a "perfect" image.
Houck's team followed microwaves spreading out from a tiny antenna as they hit a flat slab of their left-handed material. The slab focused the microwaves, but the image was much more blurred than Pendry had predicted. Some researchers argue that a perfect lens is impossible, but Pendry says the latest result just means more work is needed. "Producing a near-perfect image requires even more perfect materials," he says.
Physicists draw up plans for real 'cloaking device'
19:00 25 May 2006
Physicists have drawn up blueprints for a cloaking device that could, in theory, render objects invisible.
Light normally bounces off an object's surface making it visible to the human eye. But John Pendry and colleagues at Imperial College London, UK, have calculated that materials engineered to have abnormal optical properties, known as metamaterials, could make light pass around an object as so it appears as if it were not there at all.
Metamaterials are exotic composites made of electronic components such as wires and inductors that can be engineered to precisely control the way light travels through them.
Pendry's team has drawn up plans for a spherical metamaterial
structure that would render an enclosed object invisible. "The theory tells
us the material properties we need at each point," says team member David
Smith, from Duke University in North Carolina, US. "The challenge is to
match those theoretical requirements in the actual material, point-by-point."
Other designs for invisibility cloaks have been drawn up in the past. One idea is to calculate exactly how an object scatters light and design a surrounding material to exactly cancel this out.
But such cloaking devices could not be used for more than one object. "Using our method you can hide different objects under the same cloak, or move around within the cloak, and remain hidden," says Pendry.
However, Pendry's team’s design could currently only
work at wavelengths larger than visible light. Designing a cloaking device
for visible wavelengths could be tricky as it would involve creating nanoscale
metamaterials. "At these levels it is far more difficult to control the
metal's properties," says Smith. Nonetheless, he believes that optical
cloaking devices could be become a reality within the next decade.
Will Stewart, an independent optics expert at the University of Southampton, UK, is less convinced. He believes that it may prove too difficult to overcome these problems within such a timeframe. "It's great fun and a lovely idea, but I don't think it can literally be taken and applied to make an optical cloak," he says.
But Stewart says the approach could work well with a narrow band of wavelengths and could, for example, shield an object from radar. Pendry's team is, in fact, working on just such a device made from millimetre-sized metal units, which they hope to complete within a year.
"It looks like Star Trek was right," Stewart says, referring to the invisibility shield famously used by Klingon spaceships in the science fiction show.
Journal reference: Science (DOI: 10.1126/science.1125907)
Super-lens brings fine features into focus
30 April 2005
ONE of the most intriguing ideas in optics - that a "super-lens" can focus features that are much smaller than the wavelength of light illuminating the object - has been independently verified by two experiments.
The key to a super-lens lies in the so-called "evanescent" waves that objects emit along with visible light. Conventional optical devices are unable to focus these waves because they decay rapidly as they pass through normal lenses. Five years ago, John Pendry of Imperial College London predicted that materials that refract light the opposite way to ordinary materials could focus these evanescent waves.
Now, two teams have shown that extremely thin silver foil has a negative refractive index and so acts as a super-lens. Xiang Zhang of the University of California in Berkeley used a layer of silver 35 nanometres thick to form 60-nanometre-wide images of nanowires - about one-sixth of the wavelength of the ultraviolet light illuminating the wires (Science, vol 308, p 534). Richard Blaikie of the University of Canterbury in Christchurch, New Zealand, used a similar silver lens to focus features 70 nanometres wide (Optics Express, vol 13, p 2127).
The technique is of more than just theoretical interest. Such lenses can be used to fabricate microchips with much finer features than is possible with today's best optics, says David Smith of Duke University in Durham, North Carolina.
14 April 2001
MORE than three decades ago, an obscure Russian physicist invented a freakish material. It was not a real material that could be bent or broken. Instead, Victor Veselago had dreamed up an imaginary substance, and wondered what unworldly properties it might have. A harmless pursuit, you might think, for someone working at the General Physics Institute of the Russian Academy of Sciences in Moscow. But Veselago's work was no an idle flight of fancy.
On the contrary, it was built on the soundest of scientific foundations—the laws of electromagnetism laid down by James Clerk Maxwell one hundred years before, that describe how light behaves as it passes through any medium. Veselago's whim was to change the value of two of the constants in Maxwell's equations from positive to negative. He wanted to know what properties such a material might have, and also hoped to have a little fun finding out.
He immediately realised that his creation would behave in some very strange ways. Veselago even came to Europe in the early 1970s to promote his ideas, but his work was regarded as little more than a curiosity.
That changed last year when physicists announced that they had finally created his fantasy material. It is unlike any substance ever seen. It has a structure like a distorted honeycomb and contains wires and strange electronic components rigidly suspended in repeating patterns. The announcement prompted a flurry of theoretical and practical work on its properties. And amazingly, this has revealed that Veselago's material, and others like it, are more remarkable and useful than even he had imagined.
Over a century before Veselago's thought experiment, Maxwell had developed his equations to describe the way electromagnetic waves such as light pass through things like glass, water and air. But rather than working out exactly how the two components of light—oscillating electric and magnetic fields—vary at every point in a particular material, Maxwell realised that it was possible to simplify the calculations by taking a kind of average of the fields throughout the material.
He found that for a given frequency of light, this averaging introduces a constant factor into his equations. In the case of the magnetic field, this averaging factor is called the magnetic permeability, and for the electric field it is the electrical permittivity. These constants are especially useful to scientists and engineers because they describe the overall behaviour of light as it passes through the material and can be used to determine quantities such as the speed of light in that medium and its refractive index—how much it bends light.
So what happened when Veselago created his imaginary substance by changing the value of these constants to -1? Inside his fantasy material, the usual relationship between the electric and magnetic fields was reversed.
Light consists of an electric field and a magnetic field oscillating at right angles to each other and to the direction in which the light is travelling (see Diagram). Physicists have a trick for remembering how these three are oriented, known as the right-hand rule. Arrange the thumb, forefinger and middle finger of the right hand in such a way that they point at right angles to each other. The thumb represents the direction that the light is travelling, the forefinger represents the plane of the electric field and the middle finger the plane of the magnetic field.
In Veselago's material, however, these quantities obey a left-hand rule, as if they were reflected in a mirror—which is why they're called "left-handed materials".
A left-handed material should have some strange properties. It should have a negative refractive index, for example, which means that light entering it would bend in the opposite direction to light passing through conventional materials. And the Doppler effect would also work in reverse: normally the frequency of light passing through glass increases slightly when the source is getting closer. In left-handed materials, though, its frequency would decrease.
But Veselago never got his hands on a left-handed material. Although the permittivity of some natural materials such as ionised gases can be negative, the permeability of naturally occurring materials never drops below zero. So Veselago was unable to prove his predictions with real materials.
But in 1996, John Pendry, a theoretical physicist at Imperial College, London, began thinking about designing new materials. He and his team investigated the effect that a regular array of thin, parallel conducting wires would have on a particular kind of electromagnetic radiation—microwaves. They predicted that the average effect of the wires would make it look as if the waves were moving through a material with negative electrical permittivity.
Pendry reasoned that if simple wires could produce such an exotic medium, why not arrays of other electrical components? So he began to calculate the bulk properties of other strange concoctions. In 1999, he published theoretical calculations for a number of other materials. One in particular caught the eye of David Smith and his colleagues at the University of California in San Diego.
"It was a very unusual and very unexpected effect," says Smith. What Pendry was predicting was that a structure consisting of a periodic arrangement of simple electronic components would have a magnetic permeability that was negative at microwave frequencies. The electrical components were split ring resonators—C-shaped circuits about the size of the capital Cs in this article. Smith and his colleagues immediately set about trying to prove Pendry's results experimentally.
Creating an array of split ring resonators is not difficult. The team simply printed C-shaped copper circuits onto a sheet of fibreglass using the conventional lithographic techniques used to make circuit boards. They then cut up the sheets and stacked the slices, like a loaf of sliced bread. But they decided they might as well go further and create a material with both negative permittivity and permeability—a truly left-handed material.
Thanks to Pendry's earlier work, modifying the material to give it a negative permittivity turned out to be relatively straightforward. Smith simply inserted arrays of fine wires into the gaps between the sheets of resonators. Without the wires, the material absorbs microwaves at more or less all frequencies. But when the arrays were inserted, the group discovered that microwaves of around 5 gigahertz could pass through. They also found evidence that the orientation of the electric and magnetic fields was reversed, just as Veselago had predicted. The conclusion was inescapable. Smith and his colleagues had created the world's first left-handed material.
Their results were published last year to general acclaim, and new studies are underway. Meanwhile, Pendry continues to make new predictions for these materials.
Last October, he published the results of an investigation into the electromagnetic properties of left-handed materials on a scale of a few nanometres—in a region known as the "near field". The near field is a kind of twilight zone close to the light source in which the electric and magnetic fields behave wildly. On this scale there are all kinds of components to the field that you never normally see, says Pendry. These components are called evanescent waves and they die away rapidly as they travel from the source, such as an excited atom in a light bulb. After a few wavelengths the evanescent waves are so weak that physicists can safely ignore them.
But evanescent waves cause one of the biggest headaches in optical physics. When evanescent waves fade away, the fine detail they contain about the source is lost. So no matter how good a lens is at focusing light, it can never create a perfect image of the source without the near field—there will always be an inherent "fuzziness", which means scientists can't focus light to a spot much smaller than its wavelength. This restriction is known as the diffraction limit and it causes problems in all kinds of applications. For example, it limits the size of transistors that can be carved using photolithographic techniques and the amount of information that DVDs can store.
Pendry discovered that, in theory, left-handed materials can have a remarkable restorative effect on evanescent waves and even focus them to create a perfect image of the source. Nobody was more surprised at this than Pendry himself. "It's a very unusual and surprising idea and I didn't expect it to work," he says. But his mathematics was checked and rechecked and the work was published in Physical Review Letters (vol 85, p 3966).
The focusing works very differently from a conventional lens. Imagine a light source placed just nanometres from a thin slab of left-handed material. The source emits light, including evanescent waves, which enter the material making electrons in it oscillate. Since the slab is very thin, the light and evanescent waves pass through, causing electrons on the far surface to oscillate as well. The movement of electrons on one surface affects movement on the other, through the electric field, and at one particular frequency they begin to resonate. It is this resonance that amplifies the evanescent waves as they leave the medium.
The material has a negative refractive index, so the flat slab focuses the light like a lens, creating an image. But unlike conventional lenses, which can't focus evanescent waves, the image is perfect—so Pendry has coined the term superlenses to describe his slabs of left-handed material. Of course, to catch the evanescent waves the lens has to be placed a lot less than a wavelength from the source. For visible light, that would be within a few tens of nanometres. This could be a challenge, says Pendry, but it is already possible to position objects with this kind of precision using atomic force microscopy, for example. And if you work with radio waves, the near field stretches for many centimetres, so the tolerances for positioning are more forgiving.
Strange materials like those made by Smith should work as superlenses for microwaves and radio waves. In fact, Smith and a team from UCSD have just created the first superlens with a negative refractive index for microwaves. But Pendry reckons that a much more common material should do the same for visible light. Metals such as silver exhibit negative permittivity at the frequency of visible light, but have a positive permeability. However, Pendry calculates that for evanescent waves the permeability can be ignored. It turns out that a slab of silver just 40 nanometres thick placed 20 nanometres from the source should act as a superlens (New Scientist, 28 October 2000, p 22). "The reason we haven't seen this before is that nobody has looked," he says. "We're not sure what we can do with it but we have a few ideas."
Pendry's other ideas are already finding applications. One of the most promising is in the field of magnetic resonance imaging. MRI machines work by placing a sample—usually a patient—inside a powerful magnetic field. A blast of high-frequency radio waves forces nuclei in water molecules to spin in a particular way and to re-emit radio waves at a different, characteristic frequency. This signal is used to build a computer image that is useful for diagnosing diseases such as cancer. However, these machines require powerful superconducting magnets that are expensive and bulky. The size of the machine prevents access to the patient, so it can't be used to monitor surgery in real time.
But Pendry's superlenses could change all that. In February, Pendry and his colleagues published the results of preliminary work in which they tailored the properties of a left-handed material to channel radio signals to a detector outside the machine. The team made it by rolling a thin sheet of conducting material onto a plastic rod 20 centimetres long, giving it a spiral cross section like a Swiss roll, and then packing a number of these rods tightly together into a bundle. Instead of working like a lens, the material acts like a waveguide carrying radio waves from the nuclei in the sample to a detector. The team has published an image of a thumb produced in this way. Being able to use smaller detectors outside the machine should allow MRI scanners to be smaller and cheaper. Eventually superlenses could even focus radio waves onto tiny parts of the sample, allowing a much more detailed image to be built up by scanning the focused radio waves across a patient, for example.
Eventually, superlenses could also have a dramatic impact on the way silicon chips are made. The size of transistors is currently limited by the wavelength of light used in photolithography. Since superlenses can focus details in a way that doesn't depend on the wavelength of light, they magically remove this barrier and could help chip makers to build smaller circuits. The amount of information stored on compact discs and DVDs is also restricted by the ability of conventional lenses to focus light to a small spot. Here, too, superlenses could help.
They could even improve the way radio antennas work. Radar beams produce extra radiation at right angles to the main beam that create noise and reduce the radar's efficiency. Pendry believes that superlenses could clean up the side lobes and produce a more directed main beam. In addition, Ely Yablonovitch at the University of California, Los Angeles, is making a left-handed material to help reduce exposure to radiation from mobile phones.
Pendry is convinced that these materials have an exciting future. "I'm still at the gawping stage, but people will gradually think of applications," he says optimistically. "They couldn't think what to do with lasers when they were first invented—but now look at them."
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