David Smith: Creating New Materials that Go Beyond Nature
In 1967, Russian scientist Victor Veselago published theoretical research on a bizarre type of material that, while possible according to the known laws of physics, didn't exist anywhere on the planet as far as anyone could tell. Veselago was describing materials with a negative refraction, which, relative to the positive refraction all known materials had, would have dramatic impacts on electromagnetic radiation--from microwaves to visible light--leading to a variety of alien properties. Some 30 years later, as a side project while working as a post-doctoral researcher at the University of California, San Diego, David Smith, now a Duke professor, made a material that fit the description. The work was met with great skepticism, even by Veselago himself, but was in time proven solid, and it changed the course of Smith's entire research career.
The term metamaterials is now used to describe materials such as Smith's that are tuned using electronic components to manipulate their electric and magnetic properties. For the past decade or so, Smith's and other research groups have been exploring a variety of increasingly advanced metamaterials with potential applications from improved antennas to the world's best optical lenses. Smith has even proven that metametarials can be designed to cloak objects to prevent detection using microwave signals, and further work could enable the possibility of hiding objects from radar or even visible light. That would open the possibility of a cloak reminiscent of those found previously only in the most fantastical of fiction, including Harry Potter.
Besides continued metamaterials research, Smith has branched into the hot, relatively new field of plasmonics, which involves the strange and only recently exploited ability of metals, under proper conditions, to conduct light waves in truly unique ways. It's a new direction for Smith, but a natural progression, because, as with metamaterials, plasmonics is about manipulating electromagnetic waves, in this case light waves exclusively. This plasmonics work also offers potential for improvements in communications and lens technologies.
"In a nutshell, what we try to do is take advanced properties and exploit them to create new types of devices or photonic components," says Smith. As a result of its science fictionesque potential, the work has received wide national and international media coverage, but for Smith, it's simply a matter of exploring the possibilities. "You can go well beyond what nature has given you," he says.
Negative Refraction: Making Light Take a Left Turn
In simplest terms, refraction is the way that light or other electromagnetic waves bend when they go from one medium into another. The bent shape we perceive when looking at a spoon in a cup of tea, for instance, is caused by differences between the speed that light travels through the tea and the air. Changing mediums have similar effects on everything in the electromagnetic spectrum from radio waves to gamma rays.
The changes in velocity that cause refraction are in turn caused by the way that the electrons in a material interact with the electric and magnetic fields in electromagnetic waves. Usually these interactions lead to positive refraction, though based on the famous Maxwell's Equations, which provide the definitive mathematical description of electromagnetic wave behavior, cientists have known for more than a century that the laws of physics do not preclude negative refraction. It's just that no one had ever worked out a way to pull it off.
Negatively refractive materials have at times been called left-handed materials, which simply refers to their refraction being in the opposite direction of the norm. "It's all a little hard to picture," says Smith, "So just imagine things moving opposite to the direction that's expected."
Visible light is the easiest entry point for thinking about the concept, so consider that if tea had a negative refraction, the spoon would bend the opposite way from what it normally does, and if water had negative refraction, a swimming pool would look deeper than it really is, rather than shallower, when you look into it. But effects on light are only the beginning as there are, for instance, similar effects on radio waves and other bands of the electromagnetic spectrum used in communications. "Negative refraction gives a twist to everything electromagnetic, " says Smith, "It makes us reconsider everything we know about electromagentism."
Intrigued by Veselago's concept, Smith set out as a graduate student to create the first crude negative refraction material. Negative refraction demands that a material impact both the electric and magnetic fields in an electromagnetic wave, which are perpendicular to each other. To do this, Smith used standard circuit board technologies to combine a series of paired metal rods, which interacted with the electric fields, and metal loops that affected the magnetic fields. Though the basic concepts and technologies needed had been known for decades, Smith says what had been lacking was the computer power and techniques to generate the engineering scheme for the material.
The experiment was a success, and Smith was able to convince himself that microwaves entering his new material were negatively refracted, but it would take a bit more time to convince others. "It definitely caused a stir," says Smith of his first paper on the topic in Physical Research Letters in 2000, "because it pointed out that metamaterials can give you something that seems to be absent in nature." A number of groups questioned whether the feat had really been accomplished. Ironically, even Veselago doubted the results.
It was during this period that Duke professor Steve Cummer became interested in the negative refraction work because of ties to his own electromagnetics-focused research. Cummer began doing modeling work to show that negative refraction was possible, and ended up meeting Smith, then still at UC San Diego, at a meeting. They went on to work together, and when Smith arrived at Duke in 2004, their relationship grew into a full collaboration.
Both Cummer and Smith published papers that shot down arguments against negative refraction. Bolstering their arguments, various military labs, the Mayo Institute, and Boeing, which Smith had teamed up with for further research, all eventually confirmed Smith's original results. Smith was vindicated, and the work was chosen in 2003 as one of the top ten breakthroughs for the year.
"We are now putting a fair amount of effort into understanding what other kinds of material properties we can get out of these materials when you imbed active or controllable elements," says Cummer. As the theory and modeling work progresses, the researchers are working to create ever more interesting demonstrations of the principles involved. Smith is also working to explore a range of applications for negative refraction with partners such as Boeing and Sensormetrix, a company he co-founded.
Smith's first metamaterial took him solidly into the realm of science fiction in the sense that it brought Veselago's vision of a material no one was sure was even possible to life, but to those outside the fields of physics and engineering, it was rather esoteric stuff. However, some of the applications Smith and Cummer now explore are much more easily grasped, because they sound like they've fallen straight out of Harry Potter and Tom Clancy novels.
Putting Unnatural Materials to Use
Smith's lab has long since moved from the initial stage of just trying to create metamaterials, to exploiting their unique and bizarre properties to build new devices. Potential uses include the creation of better lenses than have ever been possible, and various communications improvements.
Much of this more recent work is dependant on the fact that the properties of negative refraction metamaterials can be tuned so that they manipulate electromagnetic waves in useful, controlled ways that vary within a particular piece of material. For example, by varying the properties of the components, a metamaterial can steer waves like the curved surfaces of conventional lenses in eyeglasses, but still remain flat and lightweight. "We rely completely on the properties of the material rather than shaping the interface," says Smith.
In collaboration with Sensormetrix, Smith has already created structures known as gradient index (GRIN) lenses, which are lenses whose focusing powers are controlled through internal variations rather than the shape of the surface. GRIN lenses are found in copiers and scanners and can be made by chemically manipulating properties within glass. But metamaterial GRIN lenses offer a wider range of options and should be significantly cheaper to produce.
The sample materials made by Sensormetrix include some 9,000 different component cells with 1,000 unique cell designs. "It really was an effort in design and simulation to get this," says Smith, "because each cell has to be tuned or changed a little bit to achieve the exact electric and magnetic properties we want."
Light weight and elimination of the need to grind lenses means cheaper manufacturing costs, but beyond economic benefits, metamaterial lenses can also outperform their traditional counterparts in other ways. For instance, traditional lenses are plagued by the inevitable imperfections in natural materials such as glass and plastic that can limit their quality. Ultimately this keeps waves from focusing quite as well as might be desired. But a metamaterial can be manufactured with such precision that they open the possibility for what Smith's colleague and collaborator Sir John Pendry, at London's Imperial College, has dubbed a "perfect lens".
As another proof of the general metamaterials concepts, Smith is working with Boeing to create a high-gain, or directional, antenna. Just as an optical lens focuses light, a high gain antenna focuses radio waves so that a signal's strength can be focused in a particular desired direction.
Smith says communications applications are one of the most likely uses for the technology his group is developing, and numerous companies such as Qualcomm and Lucent have approached him to discuss possibilities. Because every range of electromagnetic wavelengths is used for something, the potential applications for this and related work are vast.
Just a few years after demonstrating the first metamaterial, Smith and Cummer, in collaboration with Sir John Pendry of Imperial College in London, set out to explore arguably the ultimate level of electromagnetic control--manipulating waves in order to hide an object. Though Smith refers to the pursuit more technically as transformation optics, it's most commonly known as cloaking, as in the stuff of science fiction and fantasy. Even here in reality, it's conceivable that airplanes or ships could be made completely invisible to radar, as opposed to current stealth planes, which have minimized profiles that are difficult, but not impossible, to detect. Further down the road, a device akin to Harry Potter's famous invisibility cloak isn't out of the question.
The basic idea in cloaking is to make waves travel around a given point, which, like Smith's other pursuits, is a concept well grounded in basic physics. "Stick that coordinate system into Maxwell's equations, and out pops a description for an invisibility cloak or whatever type of structure you're interested in," says Smith. All the math required could have been done 80 or more years ago, he says, but researchers then would not have been able to create the materials needed to accomplish what the equations could describe.
The Smith team announced its theoretical work on cloaking against microwaves in May of 2006, eliciting some interesting reactions. "We got a lot of not just interest, but almost suspicion that we were actually cloaking battleships and fighter planes in here, " says Smith, "So we really felt pressure to quickly do an experiment that would illustrate exactly where we were." So they did, and the details on the proof of concept were published in 2006.
This first project was a system that successfully cloaked an object in such a way that it became invisible to microwaves. The cloaking device itself was a cylinder covered in electromagnetic manipulation cells similar to those found inside the GRIN lens, though only ten different types of cells were required for cloaking compared to the GRIN's 1,000. For cloaking, the rods and rings used to manipulate the waves are designed and arranged to precisely steer the microwaves around an object and then shift the waves back to their original path on the other side.
Because of the science fiction angle, the work was widely reported in the popular press and even led to an interview with Smith and Cummer on the Today Show. But for now, a key limitation of the work is that while the same principles could be applied to cloaking visible light to hide something or someone from view, this cannot be accomplished using current techniques and technologies. Though Maxwell's equations scale just fine to shift from dealing with longer microwaves to shorter visible light waves, the properties of the units used to manipulate waves are not similarly adaptable. The problem is that current metamaterials are not good at conducting light's shorter waves, meaning loss of light and preventing their emergence on the other side of a cloaked object unaffected. In other words, the object wouldn't be cloaked and instead viewers would see a spot.
Smith says light invisibility would probably require new materials that could actively add back the energy lost by the light waves as they travel around the cloaked object, and there's no clear path for accomplishing this. "However, there's nothing that fundamentally rules out a Harry Potter cloak, which is good news" says Smith, "it just may take 30, 40, or 50 years or who knows how long."
Plasmonics: A New Way to See the Light
During the late 1990s, scientists discovered that light can interact in some strange, previously unrealized ways with metals. When light travels through the air or through fiber optic strands, it bounces around but maintains its shape. But under the right conditions, when light hits the surface of a metal, it can transform into a mode somewhere between light and electrical current called a plasmon. "They're not just light anymore," says Smith, "These plasmons are a really unique way of propagating energy."
Metals have electrons that wander freely on their surface, and the plasmons are waves of these electrons resonating. Though plasmons oscillate at the same frequency as the original lightwave, their actual wavelength is condensed, meaning that the plasmons are a more compact version of the original lightwave that travel at a slower speed. But, plasmons can travel along the metal and then be emitted again as normal light of the original wavelength, or shifted to a different wavelength due to properties of the metal. Though they didn't know it, for centuries now stained-glass artisans have been taking advantage of this phenomenon, which governs how light is absorbed and reflected by metal particles added to glass to give it color.
Smith's lab group now dedicates a significant chunk of its research efforts to the emerging field of plasmonics, with hopes for a huge range of eventual benefits well beyond pretty windows, including advances in communications, medical, and other applications.
Though plasmonics research is still in its infancy, it has already been put to use. Companies such as Nanosphere produce gold-coated particles as small as 13 nanometers in diameter (thousands of times shorter than the width of a human hair) tuned to resonate in specific ways that enable various forms of biological sensing.
At this stage, though, Smith's research in the area is focused on fundamental science questions, rather than specific applications. His group is working with both particles similar to those used in medical diagnostics, and more elaborate structures. With particles, the team is looking at questions such as how their affects on light change when particles are grouped together. They are also exploring ways that interactions between the particles and light could one day enhance explosives or other detection technologies beyond medical applications.
Another key area of plasmonics interest for Smith is photonic circuit components such as metallic wave guides. Right now, light used in various computing capacities is ushered around chips in its visible form using wave guides made of transparent plastics and other non-conductors. Most internet data, for instance, is transmitted through fiber optic networks in the form of light, but then has to be converted into an electronic form before it can appear on a computer screen. These back and forth conversions between light and electrons are a major efficiency barrier.
Metallic wave guides could offer a more attractive option for accomplishing such modulation, and could provide numerous other benefits based on the special characteristics of plasmons, including their ability, in effect, to shrink light waves. This could be critical given the fact that the individual transistors and other components of chips and circuits are now much smaller than the wavelengths of visible light. Sending the light into circuits as plasmons could eliminate the need for conversion, and would also mean that the same metal leads that carry current around a chip could also carry light.
Smith's and other groups are also pursuing numerous other applications for plasmonics, for instance the possibility that plasmons' ability to focus light could be used to make better lenses, in ways related to the perfect lens concept tied to negative refraction materials.
"We're looking at fundamental questions," says Smith, "but they also have practical applications."