18:00 08 January 2008
NewScientist.com news service
Tom Simonite
Atom-thick sheets of a carbon compound called graphene should smash the record for room-temperature conductivity, say UK researchers.
The fact that the near-2D layers lets electrons travel so freely means the sheets could allow a new generation of super-fast microelectronics, they say.
Prototype devices like transistors have already been made from graphene, but its basic properties are still being explored.
Graphene is the name given to a sheet of carbon atoms arranged in a hexagon pattern. Stacks of such sheets make the pencil-core ingredient graphite, but until recently it had been extremely difficult to isolate single layers.
The new research was carried out by scientists at the University of Manchester – where graphene was first isolated in 2004 – and colleagues from Russia, the Netherlands, and the US.
The team calculated that pure graphene should allow electrons to travel more easily than in any other material, including gold, silicon, gallium arsenide, and carbon nanotubes.
Electronic qualityThe mobility of charge in a semiconductor is known as its "electronic quality" and governs the speeds the material is able to provide in electronics.
For example, gallium arsenide is used in cellphone transmitters because its higher electronic quality means it can operate at greater frequencies than the silicon used for most other applications.
At room temperature, gallium arsenide has an electronic quality of 8500 cm2/Vs compared with just 1500 cm2/Vs for silicon. But good quality graphene without impurities should reach up to 200,000 cm2/Vs, according to the new research.
In experiments, the team showed that two different factors were slowing down the movement of charge.
The first factor is a "built-in" speed limit that cannot be changed: ripples in the sheets trap vibrations from heat passing through the graphene, which in turn slow down the travelling electrons.
The second source of electron congestion is impurities in the graphene. These could be removed, however, via better manufacturing, meaning the material's electronic quality should reach the proposed record-breaking levels.
Manufacturing problem"Graphene exhibits the highest electronic quality among all known materials," says Andre Geim of the Manchester University team. "Our work singles it out as the best possible material for electronic applications."
Walt de Heer at Georgia Institute of Technology, US, says that the projected figure agrees with what he had expected, based on the behaviour of similar materials like nanotubes.
But he adds the result highlights the main barrier between graphene and the electronics industry – it is hard to isolate pure layers of graphene in sheets large enough for industrial manufacture. "They need a workable material presented in large wafers like silicon," he says.
The experimental devices used in the new research were made by carefully peeling off layers of graphene from chunks of graphite using sticky tape. That technique, while useful in the lab, is of little use to semiconductor companies.
De Heer and colleagues are working to overcome this practical problem. They can already cover areas with a few layers of graphene by heating silicon carbide wafers up to 1300 ºC – the heat breaks down the material, leaving the graphene behind.
"We are able to 'grow' a canvas of material that has similar if not identical electrical properties," says de Heer.
The new research will appear in a forthcoming edition of the journal Physical Review Letters
Nanotechnology – Follow the emergence of a new technology in our continuously updated special report.
Sunday, February 24, 2008
Magnetic Switch Flips On Immune Cell
Few things can make a scientist’s day like a phone call from the Defense Advanced Research Projects Agency, or DARPA. A once-obscure branch of the Pentagon, DARPA gained wider recognition in the late 1990s as a kind of cloak-and-dagger agency, seeking out scientists doing the most audacious, out-of-the-box science it could find and enlisting them in projects to ensure national security.
Liza Green, HMS Media Services
“Magnetism is always at the basis of any magic trick—it’s under the table or wherever,” said Donald Ingber. “Magnetism also has definite effects in the body.” Ingber (front) is shown with (clockwise from left) Flavia Cassiola, Martin Montoya-Zavala, and Robert Mannix.
In the aftermath of the Gulf War, it was especially concerned with finding ways to fend off the threat of biological weapons and was convinced that answers lay in the cellular world. Cells carry out a host of functions—they can sense changes in the environment, process information, mount responses, and transmit signals. Immune cells, in particular, can detect and immediately respond to pathogens. DARPA wanted to harness this power to create small, cell-based, wearable devices that might protect American soldiers, and even civilians, from harmful biological agents. But they needed a portable, low-energy toggle that would quickly turn the cells on and off. They became intrigued by the possibility of creating a magnetic switch. It was around this time, early 2002, that DARPA phoned Donald Ingber.
“They asked, ‘Do you think this is feasible and how would you go about doing it?’” said Ingber, the Judah Folkman professor of vascular biology in the Department of Pathology at Children’s Hospital Boston. Ingber would spend the next few months coming up with a blueprint for such a device. Over the past five years, funded by a DARPA grant, he, Robert Mannix, Sanjay Kumar, and colleagues have been working to turn it into a reality.
In the January Nature Nanotechnology, they report that they have created a nanomagnetic cellular switch—one that can rapidly and reliably activate mast cells, a class of immune cell. Turning on the switch resulted in the release of intracellular calcium, which is exactly what happens when the mast cells are activated naturally.
Even more remarkable is how the switch mimics true mast cell activation. Mast cells are immune workhorses. They detect an extraordinarily wide variety of antigens—from pathogens and environmental contaminants—and attack them by releasing calcium, leading to the export of histamines and other substances. Like many cells, mast cells are not activated one receptor at a time. Antigens bind not just to one but several receptors on the cell surface, essentially drawing them together into clusters, or scaffolds, onto which activating molecules can fit.
Ingber’s concept was to recreate this clustering by attaching a single tiny iron oxide bead to each mast cell receptor and exposing the cells to a magnetic field. Once exposed, the beads would become magnetized, attracting one another and, at the same time, pulling the receptors into scaffold-like clusters (see figures).
“The idea that the biochemistry is really about binding sites in spatial proximity, that it’s really the physicality of bringing them together—that was the gamble, the hypothesis.”
“The idea that the biochemistry is really about binding sites in spatial proximity, that it’s really the physicality of bringing them together—that was the gamble, the hypothesis. If we could do this with magnets, would that be enough to trigger activation?” Ingber said.
Though the answer appears to be yes, the switch is still very much in the prototype stage. “It’s early in the process, and I think it needs to be taken a bit further to really have a specific target,” he said. But receptor clustering is a very common process and, down the road, the nanomagnetic switch could open the door to an array of cell-based devices.
“Imagine you engineer a cell to produce an antidote to some chemical biological agent,” Ingber ventured. “You cou.ld inject these cells subcutaneously—they’re basically factories that don’t normally make the antidote. With these particles on their receptors and a detector that says the agents are coming, you can switch the cells on to produce the antidote.” He envisions this might be done with a local magnet, lodged perhaps in a wristwatch. “The whole point is magnetic fields travel across the skin,” he said.
Getting Physical
Ingber is known for this kind of exuberant thinking—and, indeed, it is not surprising that DARPA called on him. Over the past three decades, he has been a lone voice for a different, often contrarian way of looking at cells and molecules—one that combines cutting-edge technology with an almost old-fashioned reverence for physicality (see Focus Sept. 3, 1999, and Feb. 25, 2005).
Years ago, in a set of classic experiments initiated while working as a postdoc with the late Judah Folkman, Ingber found that by varying the degree to which cells were stretched on a patch of extracellular matrix, he could get them to either grow, differentiate, or divide. It was while working on these experiments that he made his first foray into the field of magnetism. To get cells to stretch, he plated them on a bed of tiny magnetic beads, which the cells engulfed, and then exposed the cells to a magnet.
“I put my magnet underneath and all the beads aligned vertically, like little actin polymers. And the cells just sat between them—I knew nothing about magnetics,” he said. The beads had turned into tiny magnets and aligned in north–south fashion. He realized that for the beads to actually pull on cells required exposing them to a differential force, or magnetic gradient. He would go on to use the gradient-exposed iron oxide beads to explore the effect of mechanical force on integrins and other cell surface structures.
Divvying the Beads
But at five microns, the beads were far too bulky for the DARPA challenge. Ingber’s concept was to have one bead per receptor, which meant they needed to be infinitesimally smaller, about 30 nanometers. Also, the beads were supposed to tug on one another, pulling the receptors with them. Ingber wanted them to turn into tiny magnets when exposed to a magnetic field. In fact, to act as true on-off switches, they should be capable of being repeatedly magnetized and demagnetized, which sent Mannix, an HMS research fellow in pathology at Children’s, on a search.
“I began looking like crazy on the internet for beads and found a little company which stopped making the beads soon after I purchased them,” he said.
Beads in hand, Mannix, Kumar, then a postdoc in Ingber’s lab, and Ingber, faced another thorny challenge—making sure that each mast cell receptor had only one bead. They solved this by placing a non-binding ligand on 29 of the bead’s 30 binding sites, leaving only one with an actively binding ligand. Each receptor had been bound to a complementary antibody. The researchers mixed the ligand-coated beads with the antibody-bound mast cells. “Statistically, you can expect only one bead per receptor,” Mannix said. In fact, follow-up with scanning electron microscopy (SEM) by Flavia Cassiola, HMS research fellow in surgery, showed that this was the case.
Beads of a feather. Like many cells, mast cells are not activated one receptor at a time, but instead by the clustering of receptors. Multivalent ligands bind to not just one but several receptors on the mast cell surface, essentially drawing them together into clusters, or scaffolds, onto which activating molecules can fit (top). The researchers mimicked this receptor clustering by attaching a single tiny iron oxide bead to each mast cell receptor and exposing the cells to a magnetic field. Once exposed, the beads became magnetized, attracting one another and, at the same time, pulling the receptors into scaffoldlike clusters (middle). Scanning electron micrographs vividly show the mast cell receptors before and after clustering (bottom).
Mannix and Kumar, now an assistant professor of bioengineering at UC-Berkeley, then loaded the mast cells with a calcium-revealing fluorescent dye and placed them on a microscope stage. Placing the tip of a magnetic needle—crafted by HMS instructor in surgery Martin Montoya-Zavala—near the cells, they applied a magnetic field at regular time intervals. Analyzing photos taken during the experiment, the researchers could see that the nanomagnetic switches worked: when the magnetic field was applied, the cells fluoresced, indicating they had been activated. SEM of the mast cell surface showed that the fluorescing coincided with the clustering of receptors.
DARPA’s challenge was to develop a generic switch—one that could be used on a variety of cells. It remains to be seen whether the new switch works on other cells. But given that most cells employ receptor clustering, it should. Though their DARPA grant has run out, Ingber believes a follow-up project is an example of the kind of research that might be conducted under the auspices of the new Harvard Institute for Biologically Inspired Engineering, which he cofounded.
Meanwhile, Ingber and the Department of Defense (DoD) have forged a new and somewhat unexpected relationship. It turns out the DoD is one of the biggest funders of breast cancer research, and Ingber was recently granted one of their Breast Cancer Innovator Awards, suggesting his days as a maverick are far from over. But with the formation of a new institute, his is no longer a lone voice.
“In the past, bioengineers took engineering principles to solve biological problems, but the boundaries between living and nonliving are breaking down,” he said. “It’s really exciting.”
—Misia Landau
The Chinese Government's Plans for Nanotechnology
By Alexis Madrigal February 17, 2008 | 4:29:52 PMCategories: AAAS 2008, Nanotechnology
BOSTON, MA - China aims to leapfrog the United States in technological development with substantial investment in nanotechnology, but whether those efforts will actually pay off is still unclear. That was the message from University of California at Santa Barbara researchers presenting their findings on the state of Chinese nanotechnology here at the AAAS annual meeting.
Richard Applebaum and Rachel Parker from the Center for Nanotechnology in Society at UCSB conducted about sixty interviews with Chinese officials to piece together a picture of the current state of Chinese nanotechnology. Applebaum set the specific research effort within the context of China's stated overarching goal to "leapfrog" the West by using a combination of learning from the West (i.e. technology transfer) and increasing domestic research capacity ("indigenous innovation" or zizhu chuangxin).
Nanotechnology research is one of four Chinese "science Megaprojects" that have the central purpose of catching the country up to US research by 2020. Still, for all the big talk, the actual government investment is not overwhelming. The researchers estimated that the Chinese government only invested $400 million from 2002 to 2007, although that investment is expected to rise considerably.
They highlighted several international partnerships related to nanotechnology including the Tsinghua-Foxconn Nanotechnology Research Center and the Zheijang-California NanoSystems Institute, but didn't go into much detail about what types of projects are being developed in those centers.
Right now, most nanotech research is being pushed by the central and regional governments with little private capital contributing to the national output. There are a lot of questions about whether or not that is a sustainable model for developing a high-tech industry, Applebaum noted. (It should also be noted, though, that some would question whether the venture capital model is sustainable either.)
It also leads to strange applications of nanotechnology in high-profile venues. Parker said that the Olympic village parking lots being constructed in Beijing will have a nanopolymer coating that will absorb exhaust. It was just an off-hand mention, but I am officially intrigued by the idea of coating our parking lots with pollution absorbing material. I can't vouch for the true environmental-safety of that solution, but I'd love to know how they're doing it. The coating could be something like this pollution absorbing concrete that uses titanium dioxide to degrade pollutants.
BOSTON, MA - China aims to leapfrog the United States in technological development with substantial investment in nanotechnology, but whether those efforts will actually pay off is still unclear. That was the message from University of California at Santa Barbara researchers presenting their findings on the state of Chinese nanotechnology here at the AAAS annual meeting.
Richard Applebaum and Rachel Parker from the Center for Nanotechnology in Society at UCSB conducted about sixty interviews with Chinese officials to piece together a picture of the current state of Chinese nanotechnology. Applebaum set the specific research effort within the context of China's stated overarching goal to "leapfrog" the West by using a combination of learning from the West (i.e. technology transfer) and increasing domestic research capacity ("indigenous innovation" or zizhu chuangxin).
Nanotechnology research is one of four Chinese "science Megaprojects" that have the central purpose of catching the country up to US research by 2020. Still, for all the big talk, the actual government investment is not overwhelming. The researchers estimated that the Chinese government only invested $400 million from 2002 to 2007, although that investment is expected to rise considerably.
They highlighted several international partnerships related to nanotechnology including the Tsinghua-Foxconn Nanotechnology Research Center and the Zheijang-California NanoSystems Institute, but didn't go into much detail about what types of projects are being developed in those centers.
Right now, most nanotech research is being pushed by the central and regional governments with little private capital contributing to the national output. There are a lot of questions about whether or not that is a sustainable model for developing a high-tech industry, Applebaum noted. (It should also be noted, though, that some would question whether the venture capital model is sustainable either.)
It also leads to strange applications of nanotechnology in high-profile venues. Parker said that the Olympic village parking lots being constructed in Beijing will have a nanopolymer coating that will absorb exhaust. It was just an off-hand mention, but I am officially intrigued by the idea of coating our parking lots with pollution absorbing material. I can't vouch for the true environmental-safety of that solution, but I'd love to know how they're doing it. The coating could be something like this pollution absorbing concrete that uses titanium dioxide to degrade pollutants.
UA Optical Scientists Add New, Practical Dimension to Holography
Tucson, AZ | Posted on February 6th, 2008
University of Arizona optical scientists have broken a technological barrier by making three-dimensional holographic displays that can be erased and rewritten in a matter of minutes.
The holographic displays - which are viewed without special eyewear - are the first updatable three-dimensional displays with memory ever to be developed, making them ideal tools for medical, industrial and military applications that require "situational awareness."
"This is a new type of device, nothing like the tiny hologram of a dove on your credit card," UA optical sciences professor Nasser Peyghambarian said. "The hologram on your credit card is printed permanently. You cannot erase the image and replace it with an entirely new three-dimensional picture."
"Holography has been around for decades, but holographic displays are really one of the first practical applications of the technique," UA optical scientist Savas Tay said.
Dynamic hologram displays could be made into devices that help surgeons track progress during lengthy and complex brain surgeries, show airline or fighter pilots any hazards within their entire surrounding airspace, or give emergency response teams nearly real-time views of fast-changing flood situations or traffic problems, for example.
And no one yet knows where the advertising and entertainment industries will go with possible applications, Peyghambarian said. "Imagine that when you walk into the supermarket or department store, you could see a large, dynamic, three-dimensional product display," he said.
Tay, Peyghambarian, their colleagues from the UA College of Optical Sciences and collaborators from Nitto Denko Technical Corp., of Oceanside, Calif., report on the research in the Feb. 7 issue of the journal Nature.
Their device basically consists of a special plastic film sandwiched between two pieces of glass, each coated with a transparent electrode. The images are "written" into the light-sensitive plastic, called a photorefractive polymer, using laser beams and an externally applied electric field. The scientists take pictures of an object or scene from many two-dimensional perspectives as they scan their object, and the holographic display assembles the two-dimensional perspectives into a three-dimensional picture.
The Air Force Office of Scientific Research, which has funded Peyghambarian's team to develop updatable holographic displays, has used holographic displays in the past. But those displays have been static. They did not allow erasing and updating of the images. The new holographic display can show a new image every few minutes.
The 4-inch-by-4-inch prototype display that Peyghambarian, Tay and their colleagues created now comes only in red, but the researchers believe much larger displays in full color could be developed. They next will make 1-foot-by-1-foot displays, then 3-foot-by-3-foot displays.
"We use highly efficient, low-cost recording materials capable of very large sizes, which is very important for life-size, realistic 3-D displays," Peyghambarian said. "We can record complete scenes or objects within three minutes and can store them for three hours."
The researchers also are working to write images even faster using pulsed lasers.
"If you can write faster with a pulsed laser, then you can write larger holograms in the same amount of time it now takes to write smaller ones," Tay said. "We envision this to be a life-size hologram. We could, for example, display an image of a whole human that would be the same size as the actual person."
Tay emphasized how important updatable holographic displays could be for medicine.
"Three-dimensional imaging techniques are already commonly used in medicine, for example, in MRI (magnetic resonance imaging) or CT scan (computerized tomography) techniques," Tay said. "However, the huge amount of data that is created in three dimensions is still being displayed on two-dimensional devices, either on a computer screen or on a piece of paper. A great amount of data is lost by displaying it this way. So I think when we develop larger, full-color 3-D holograms, every hospital in the world will want one."
####
About University of Arizona
The University of Arizona is a premier, student-centered research institution. Established in 1885 as the first university in the Arizona Territory and the state's only land grant institution, the UA embraces its three-fold mission of excellence in teaching, research and public service. Now in its second century of service to the state, the UA has become one of the nation's top 20 public research institutions. It is one of only 62 members in the Association of American Universities, a prestigious organization that recognizes universities with exceptionally strong research and academic programs. With world class faculty in fields as diverse as astronomy, plant science, biomedical science, business, law, music and dance, The University of Arizona offers a rewarding educational experience to all who choose to focus on excellence.
For more information, please click here
Contacts:
Nasser Peyghambarian
520-621-4649
nnp@u.arizona.edu
Savas Tay
520-245-9722
savas.tay@gmail.com
Tuesday, February 5, 2008
Rounding up gases, nano-style
A new process for catching gas from the environment and holding it indefinitely in molecular-sized containers has been developed by a team of University of Calgary researchers, who say it represents a novel method of gas storage that could yield benefits for capturing, storing and transporting gases more safely and efficiently.
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“This is a proof of concept that represents an entirely new way of storing gas, not just improving on a method that already exists,” said U of C chemistry professor George Shimizu. “We have come up with a material that mechanically traps gas at high densities without having to use high pressures, which require special storage tanks and generate safety concerns.”
In a paper published in the current online version of the world’s leading material science journal Nature-Materials, Shimizu, fellow U of C professor David Cramb, chemistry graduate student Brett Chandler and colleagues from the National Research Council describe their invention of “molecular nanovalves.”
Using the orderly crystal structure of a barium organotrisulfonate, the researchers developed a unique solid structure that is able to convert from a series of open channels to a collection of air-tight chambers. The transition happens quickly and is controlled simply by heating the material to close the nanovalves, then adding water to the substance to re-open them and release the trapped gas. The paper includes video footage of the process taking place under a microscope, showing gas bubbles escaping from the crystals with the introduction of water.
“The process is highly controllable and because we’re not breaking any strong chemical bonds, the material is completely recyclable and can be used indefinitely,” Shimizu said.
The team intends to continue developing the nanovalve concept by trying to create similar structures using lighter chemicals such as sodium and lithium and structures that are capable of capturing the lightest and smallest of all gases – hydrogen and helium.
“These materials could help push forward the development of hydrogen fuel cells and the creation of filters to catch and store gases like CO2 or hydrogen sulfide from industrial operations in Alberta,” Cramb said.
The paper “Mechanical gas capture and release in a network solid via multiple single-crystalline transformations” is available in the Advanced Online Publication of the journal Nature-Materials.
Source: University of Calgary
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“This is a proof of concept that represents an entirely new way of storing gas, not just improving on a method that already exists,” said U of C chemistry professor George Shimizu. “We have come up with a material that mechanically traps gas at high densities without having to use high pressures, which require special storage tanks and generate safety concerns.”
In a paper published in the current online version of the world’s leading material science journal Nature-Materials, Shimizu, fellow U of C professor David Cramb, chemistry graduate student Brett Chandler and colleagues from the National Research Council describe their invention of “molecular nanovalves.”
Using the orderly crystal structure of a barium organotrisulfonate, the researchers developed a unique solid structure that is able to convert from a series of open channels to a collection of air-tight chambers. The transition happens quickly and is controlled simply by heating the material to close the nanovalves, then adding water to the substance to re-open them and release the trapped gas. The paper includes video footage of the process taking place under a microscope, showing gas bubbles escaping from the crystals with the introduction of water.
“The process is highly controllable and because we’re not breaking any strong chemical bonds, the material is completely recyclable and can be used indefinitely,” Shimizu said.
The team intends to continue developing the nanovalve concept by trying to create similar structures using lighter chemicals such as sodium and lithium and structures that are capable of capturing the lightest and smallest of all gases – hydrogen and helium.
“These materials could help push forward the development of hydrogen fuel cells and the creation of filters to catch and store gases like CO2 or hydrogen sulfide from industrial operations in Alberta,” Cramb said.
The paper “Mechanical gas capture and release in a network solid via multiple single-crystalline transformations” is available in the Advanced Online Publication of the journal Nature-Materials.
Source: University of Calgary
Invisibility cloaks and perfect lenses - the promise of optical metamaterials
The idea of an invisibility cloak - a material which would divert light undetectably around an object - captured the imagination of the media a couple of years ago. For visible light, the possibility of an invisibility cloak remains a prediction, but it graphically illustrates the potential power of a line of research initiated a few years ago by the theoretical physicist Sir John Pendry of Imperial College, London. Pendry realised that constructing structures with peculiar internal structures of conductors and dielectrics would allow one to make what are in effect new materials with very unusual optical properties. The most spectacular of these new metamaterials would have a negative refractive index. In addition to making an invisibility cloak possible one could in principle use negative refractive index metamaterials to make a perfect lens, allowing one to use ordinary light to image structures much smaller than the limit of a few hundred nanometers currently set by the wavelength of light for ordinary optical microscopy. Metamaterials have been made which operate in the microwave range of the electromagnetic spectrum. But to make an optical metamaterial one needs to be able to fabricate rather intricate structures at the nanoscale. A recent paper in Nature Materials (abstract, subscription needed for full article) describes exciting and significant progress towards this goal. The paper, whose lead author is Na Liu, a student in the group of Harald Giessen at the University of Stuttgart, describes the fabrication of an optical metamaterial. This consists of a regular, three dimensional array of horseshoe shaped, sub-micron sized pieces of gold embedded in a transparent polymer - see the electron micrograph below. This metamaterial doesn’t yet have a negative refractive index, but it shows that a similar structure could have this remarkable property.
An optical metamaterial consisting of split rings of gold in a polymer matrix. Electron micrograph from Harald Giessen’s group at 4. Physikalisches Institut, Universität Stuttgart.
To get a feel for how these things work, it’s worth recalling what happens when light goes through an ordinary material. Light, of course, consists of electromagnetic waves, so as a light wave passes a point in space there’s a rapidly alternating electric field. So any charged particle will feel a force from this alternating field. This leads to something of a paradox - when light passes through a transparent material, like glass or a clear crystal, it seems at first that the light isn’t interacting very much with the material. But since the material is full of electrons and positive nuclei, this can’t be right - all the charged particles in the material must be being wiggled around, and as they are wiggled around they in turn must be behaving like little aerials and emitting electromagetic radiation themselves. The solution to the paradox comes when one realises that all these waves emitted by the wiggled electrons interfere with each other, and it turns out that the net effect is of a wave propagating forward in the same direction as the light thats propagating through the material, only with a somewhat different velocity. It’s the ratio of this effective velocity in the material to the velocity the wave would have in free space that defines the refractive index. Now, in a structure like the one in the picture, we have sub-micron shapes of a metal, which is an electrical conductor. When this sees the oscillating electric field due to an incident light wave, the free electrons in the metal slosh around in a collective oscillation called a plasmon mode. These plasmons generate both electric and magnetic fields, whose behaviour depends very sensitively on the size and shape of the object in which the electrons are sloshing around in (to be strictly accurate, the plasmons are restricted to the region near the surface of the object; its the geometry of the surface that matters). If you design the geometry right, you can find a frequency at which both the magnetic and electric fields generated by the motion of the electrons is out of phase with the fields in the light wave that are exciting the plasmons - this is the condition for the negative refractive index which is needed for perfect lenses and other exciting possibilities.
The metamaterial shown in the diagram has a perfectly periodic pattern, and this is what’s needed if you want a uniform plane wave arriving at the material to excite another uniform plane wave. But, in principle, you should be able to design an metamaterial that isn’t periodic to direct and concentrate the light radiation any way you like, on length scales well below the wavelength of light. Some of the possibilities this might lead to were discussed in an article in Science last year, Circuits with Light at Nanoscales: Optical Nanocircuits Inspired by Metamaterials (abstract, subscription required for full article) by Nader Engheta at the University of Pennsylvania. If we can learn how to make precisely specified, non-periodic arrays of metallic, dielectric and semiconducting shaped elements, we should be able to direct light waves where we want them to go on the nanoscale - well below light’s wavelength. This might allow us to store information, to process information in all-optical computers, to interact with electrons in structures like quantum dots, for quantum computing applications, to image structures using light down to the molecular level, and to detect individual molecules with great sensitivity. I’ve said this before, but I’m more and more convinced that this is a potential killer application for advanced nanotechnology - if one really could place atoms in arbitrary, pre-prescribed positions with nanoscale accuracy, this is what one could do with the resulting materials.
The idea of an invisibility cloak - a material which would divert light undetectably around an object - captured the imagination of the media a couple of years ago. For visible light, the possibility of an invisibility cloak remains a prediction, but it graphically illustrates the potential power of a line of research initiated a few years ago by the theoretical physicist Sir John Pendry of Imperial College, London. Pendry realised that constructing structures with peculiar internal structures of conductors and dielectrics would allow one to make what are in effect new materials with very unusual optical properties. The most spectacular of these new metamaterials would have a negative refractive index. In addition to making an invisibility cloak possible one could in principle use negative refractive index metamaterials to make a perfect lens, allowing one to use ordinary light to image structures much smaller than the limit of a few hundred nanometers currently set by the wavelength of light for ordinary optical microscopy. Metamaterials have been made which operate in the microwave range of the electromagnetic spectrum. But to make an optical metamaterial one needs to be able to fabricate rather intricate structures at the nanoscale. A recent paper in Nature Materials (abstract, subscription needed for full article) describes exciting and significant progress towards this goal. The paper, whose lead author is Na Liu, a student in the group of Harald Giessen at the University of Stuttgart, describes the fabrication of an optical metamaterial. This consists of a regular, three dimensional array of horseshoe shaped, sub-micron sized pieces of gold embedded in a transparent polymer - see the electron micrograph below. This metamaterial doesn’t yet have a negative refractive index, but it shows that a similar structure could have this remarkable property.
An optical metamaterial consisting of split rings of gold in a polymer matrix. Electron micrograph from Harald Giessen’s group at 4. Physikalisches Institut, Universität Stuttgart.
To get a feel for how these things work, it’s worth recalling what happens when light goes through an ordinary material. Light, of course, consists of electromagnetic waves, so as a light wave passes a point in space there’s a rapidly alternating electric field. So any charged particle will feel a force from this alternating field. This leads to something of a paradox - when light passes through a transparent material, like glass or a clear crystal, it seems at first that the light isn’t interacting very much with the material. But since the material is full of electrons and positive nuclei, this can’t be right - all the charged particles in the material must be being wiggled around, and as they are wiggled around they in turn must be behaving like little aerials and emitting electromagetic radiation themselves. The solution to the paradox comes when one realises that all these waves emitted by the wiggled electrons interfere with each other, and it turns out that the net effect is of a wave propagating forward in the same direction as the light thats propagating through the material, only with a somewhat different velocity. It’s the ratio of this effective velocity in the material to the velocity the wave would have in free space that defines the refractive index. Now, in a structure like the one in the picture, we have sub-micron shapes of a metal, which is an electrical conductor. When this sees the oscillating electric field due to an incident light wave, the free electrons in the metal slosh around in a collective oscillation called a plasmon mode. These plasmons generate both electric and magnetic fields, whose behaviour depends very sensitively on the size and shape of the object in which the electrons are sloshing around in (to be strictly accurate, the plasmons are restricted to the region near the surface of the object; its the geometry of the surface that matters). If you design the geometry right, you can find a frequency at which both the magnetic and electric fields generated by the motion of the electrons is out of phase with the fields in the light wave that are exciting the plasmons - this is the condition for the negative refractive index which is needed for perfect lenses and other exciting possibilities.
The metamaterial shown in the diagram has a perfectly periodic pattern, and this is what’s needed if you want a uniform plane wave arriving at the material to excite another uniform plane wave. But, in principle, you should be able to design an metamaterial that isn’t periodic to direct and concentrate the light radiation any way you like, on length scales well below the wavelength of light. Some of the possibilities this might lead to were discussed in an article in Science last year, Circuits with Light at Nanoscales: Optical Nanocircuits Inspired by Metamaterials (abstract, subscription required for full article) by Nader Engheta at the University of Pennsylvania. If we can learn how to make precisely specified, non-periodic arrays of metallic, dielectric and semiconducting shaped elements, we should be able to direct light waves where we want them to go on the nanoscale - well below light’s wavelength. This might allow us to store information, to process information in all-optical computers, to interact with electrons in structures like quantum dots, for quantum computing applications, to image structures using light down to the molecular level, and to detect individual molecules with great sensitivity. I’ve said this before, but I’m more and more convinced that this is a potential killer application for advanced nanotechnology - if one really could place atoms in arbitrary, pre-prescribed positions with nanoscale accuracy, this is what one could do with the resulting materials.
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