Beyond Materials: From Invisibility Cloaks to Satellite Communications
By Ken Kingery
Duke-developed “metamaterials” are carefully designed structures that control all sorts of physical waves in previously impossible ways. Our researchers are poised to make these devices a household name.
When the editors of Physical Review Letters first received the manuscript, they immediately rejected it. Their form letter said the work didn’t seem important enough to even send it out for review. The editors didn’t realize that the contents would revolutionize the manipulation of electromagnetic waves that dominate today’s technology.
Then again, neither did the author.
The year was 2000, and David R. Smith was a research scientist at the University of California, San Diego, who was interested in tinkering with the properties of materials. He had a crazy notion that by controlling the structure of materials rather than the chemistry one could engineer materials with properties never seen in nature.
Smith’s imagined materials were based on collections of little conducting elements, which he had read about in papers from Sir John Pendry, a theoretical physicist at Imperial College London. By placing a series of carefully designed metal wires or rings in specific arrangements, Pendry had proposed, one could create materials with tailored and unusual electromagnetic properties.
“Just for fun, our group decided to see if this was true,” recalled Smith. “This was nothing that anyone would have funded and probably very few would have published.”
Pendry had been intrigued by the notion of materials with negative response — a condition that was well-known in optics, and which leads to a host of remarkable optical phenomena. By creating artificial materials, Pendry hoped to extend the unique properties of negative response optics across the electromagnetic spectrum.
The concept turned out to hold some water. The combination of split-ring resonators and wires which Pendry had proposed to combine negative magnetic response and negative electric response, both behaved as predicted.
The combination of the two properties has a curious effect: It bends light backward.
This was the finding that Physical Review Letters found entirely uninteresting and not worth publishing. The rejection prompted Smith to search the literature on materials with these properties. To his surprise, a single paper popped up — one written in 1968 by a Russian physicist named Victor Veselago.
And it was a doozy.
“That paper said that if this property could ever be realized, you’d have all these crazy physics effects,” said Smith. “You’d turn light backward, you’d change the Doppler shift, Snell’s law and Cherenkov radiation — it would really be a fundamental paradigm shift. So I just changed the abstract using some of Veselago’s language and they accepted it in a snap.”
Thus began the field of metamaterials.
“The definition of metamaterials now is a little bit difficult to pin down because it’s evolved over the years,” continued Smith, who moved his research to Duke in 2004. “Our group focuses on electromagnetic properties, but a metamaterial could also mean designing acoustic or thermal properties — really anything.”
We rely on material properties every day. Styrofoam keeps coffee hot on the morning commute. Copper carries electricity to a car’s spark plugs. Glasses bend and focus light to better see the road. These properties — thermodynamic, electrical and optical, respectively — arise due to the specific elements and arrangements of the atoms in a material.
But structures can also give material specific properties. A coil of copper wire with a magnet rotating inside will create an electric current, whereas a straight copper wire won’t. A smooth sheet of silver is reflective, while a surface coated with tiny silver spheres is black.
The Greek preposition “meta” means “beyond,” so a metamaterial is a type of material engineered to have properties beyond those provided by nature. In Smith’s and Pendry’s work, the materials receive their unusual properties through their structure rather than just their chemistry.
“Materials are made up of atoms or molecules with certain properties that create the overall properties for the material,” said Smith. “The idea of metamaterials is to duplicate that, but with artificial manmade structures that can give us material properties unlike any that exist.”
In Smith’s seminal 2000 paper, the role of the atom is played by a designed structure, called a cell, which contains a couple of concentric C-shaped wires made from semiconductor materials plus a straight wire. These cells are much larger than an atom, of course, and are carefully designed to mimic, but significantly alter, the way an atom’s positively charged nucleus and negatively charged electrons would respond to electromagnetic waves.
When exposed to electromagnetic waves tuned to the metamaterial cell’s shape and size, the charges and currents within the metamaterial elements can’t keep pace with the swiftly changing fields and appear to reverse their normal behavior. For example, a negatively charged particle will usually move toward an electric field. However, if the particle is bound, like a mass on a spring, and the frequency of the wave is high enough, the charged particle will appear to move away from the field. This reversal in a metamaterial leads to the interesting effect of bending light backwards from its expected path, and many other strange behaviors.
“Veselago took a lot of flak for writing the paper describing what these effects would mean because it seemed so unrealistic and unachievable,” said Smith. “A lot of people told him he did it just to publish a paper.”
There are still some major limitations to metamaterials, of course.
They can only manipulate electromagnetic waves that are larger or roughly the same size as each individual cell. Otherwise it’s like trying to surf on the ripples made by a splashing pebble in a pond. And these effects can only be tailored to one narrow range of frequencies at a time.
This means that a metamaterial cannot affect both microwaves and millimeter waves at the same time. It also means that making metamaterials that work in the visible spectrum of light — which has wavelengths just hundreds of nanometers long — is extremely difficult because the cells would have to be only tens of nanometers.
With these constraints in mind, the scientific community wasn’t initially sure what to make of the split-ring resonator demonstration. Many still scoffed at the theory and the experimental results, unsure about the basic idea of negative refraction and whether Veselago had been right. Perhaps Pendry and Smith were mistaken.
“The basic concepts you learn in early physics classes need to be modified to understand how waves behave in negative media,” Smith said. “It was a bit of a learning experience for the entire community.”
But everyone took notice with they started making things disappear.
Smith, Pendry and their colleagues realized that for metamaterials to really take off, they’d have to demonstrate what their invention could do. Because metamaterials offer such a large amount of freedom, however, they weren’t sure where to start.
Pendry took to his archives to find an answer, dusting off a computational tool he created in 1996 for transforming coordinate systems. Think of drawing a grid on a sheet of rubber, stretching it out and needing to know exactly how the squares are going to look afterward.
Pendry realized that this conceptual trick of changing the coordinate system could be used as a design tool: You imagine controlling the trajectory of light by simply changing the coordinate system. Once you have things the way you want them, you can use that transformation to obtain the exact material properties needed to make the device.
“The idea was elegant and conceptually simple,” Smith said. “The material properties are extremely challenging, but they were a perfect match for metamaterials.”
The transformation calculation allowed the researchers to design metamaterials where each cell was not identical. By subtly changing each cell’s properties, they could steer electromagnetic waves along a gradient — a concept called transformation optics — which led to an even crazier idea: A sheet of metamaterial cells could be tailored in this way to move far beyond the way a simple glass lens bends light; they might actually bend electromagnetic waves around an object to rejoin on the far side as if the object were never there.
“Somewhere along the way John decided we could make an invisibility cloak out of this,” said Smith. “He said it partially as a joke, I think. But the math held up, and you could write down the prescription for a cloak that was actually fairly simple.”
“David said ‘We have to build this!’” Pendry recalled. “We wrote the theory paper in 2006 and it came out pretty fast in Science. Now, normally there might be some interest in such a paper, but it doesn’t change your life. The response to this was simply astounding.”
Media worldwide stampeded to Smith and Pendry, eager to draw comparisons between their somewhat crude theoretical proposal and the famous fictional invisibility cloaks of Harry Potter and Star Trek. With the level of attention the theory paper received — including some genuine surprise from the scientific community — the team knew they’d have to act quickly to be the first to build a working device. As with most things in science, however, this proved easier said than done.
While the calculations were not terribly difficult to make, the level of precision required to actually build the device made the process challenging. Joined by their colleague at Duke University, Professor Steve Cummer, Smith and Pendry began a more systematic analysis of the prospective cloak, and discovered the original Pendry design would require a significant effort and many months — if not years — of work. So they did what any good scientists would in such a situation — they cut a few corners. With small compromises in the quality of cloak, they produced a working prototype in just four months.
The world’s first invisibility cloak is made with concentric rings of flat metamaterials that look a bit like old filmstrips. And it worked; when an electromagnetic wave encountered the device, it was ushered around the rings, avoiding an object placed in the center, and returned to its original form on the other side. To an observer or detector on the far side of the array, the waves came through as if nothing had been in the way.
The success, however, came with caveats. It only worked in two dimensions — any wave coming at an angle from above or below would spoil the effect. It had efficiency losses and only worked in a very narrow frequency range. And because smaller waves require smaller designs, it only worked with microwaves, which, despite their name, can be up to 3 feet long.
“Clearly this had its limitations,” said Smith. “We thought that if we showed just how limited this technology was at the time, it would calm everyone down. But of course it again turned into a gigantic media bonanza.”
“It was a huge success for metamaterials,” Smith said.
While the crude and limited 2006 microwave invisibility cloak sparked a lot of imagination, Smith knew that momentum is a finite resource. For this new field to persist, someone would have to figure out a useful purpose for metamaterials.
The looming question was, once again, where to start.
“When a technology comes along that is truly transformational, it takes a long time before it’s used in practical devices,” said Pendry. “Take the laser. When it was first built in the 1950s, it was called a ‘maser,’ which people joked actually stood for ‘Money Acquisition Scheme for Expensive Research.’ Everybody thought that it was interesting but ultimately useless.”
To set a course toward developing market-viable technologies, Smith and his colleagues at Duke turned to Intellectual Ventures, co-founded by Nathan Myhrvold, former chief technology officer for Microsoft. The Bellevue, Washington company acquires licenses to interesting technologies and works to develop ideas into products.
“At a university, you miss all the pieces needed to be successful at commercialization,” said Smith. “You need to think about markets, manufacturability and supply chains. That’s not standard, at least not in a physics or engineering department.”
The metamaterials team chose to pursue antennas first because one of the largest driving forces in global development over the past 50 years has been the sweeping improvements in communications.
And people are never satisfied with the speed of their internet.
Looking at the landscape, the team recognized one glaring need for new and improved communications technology — satellite connections on moving objects. When satellite TV was new, heavy, car-sized dishes were posted unattractively in backyards and on rooftops to connect to a signal coming from somewhere above the Earth’s equator. With the advent of greater bandwidth and better electronics, those behemoths gave way to plate-sized dishes — still ugly, and requiring a technician to aim properly.
But platforms without the luxury of being stationary, such as ocean freighters, still carry equipment more akin to the original technology. Perched on top of many ocean-faring vessels is what amounts to a giant, cumbersome satellite dish on an omni-directional pivot called a gimbal.
There is one alternative, but only the Pentagon can afford it. A phased array is basically a satellite dish with many tiny antennas inside. Rather than moving the dish, the electric properties of the individual antennas can be reconfigured to “point” the dish in different directions to maintain satellite connectivity. While this technology is used on vehicles such as military aircraft, it’s hugely expensive and requires an unruly amount of power.
A cluster of small, tunable devices is also a pretty good description of a metamaterial.
“It turns out that a metamaterial architecture can create the same functionality as a dish or a phased array, but with a much simpler design,” said Smith. “Just by being a little bit more clever, you can make something that is much lower in cost, similar or even better in performance, and with a very low power draw.”
A metamaterial satellite antenna company called Kymeta was born out of this idea in August, 2012. With Nathan Kundtz, a recent Duke PhD graduate from Smith’s lab, leading the way, it did not take long for the company to produce a working prototype. In 2017, Kymeta launched its first commercial product, the mTenna — a flat-panel satellite antenna about the size of a stop sign that can replace maritime communications technology. It doesn’t need a gimbal, either. It’s mounted on any solid surface of the ship.
The price and maintenance costs of the mTenna are a steal compared to the current costs of maritime satellite connectivity, and it’s just the start. Because the Kymeta manufacturing process uses the same technology that makes LCD televisions, the infrastructure already exists to scale up production, drive down costs and expand to new markets. By expanding the ability to connect moving objects to high-bandwidth communications networks, the technology is expected to speed up the automation of freight trains and fleets of semi-trucks. Morning commutes on passenger railways could become more productive for those onboard.
Even your own car may soon be connected to satellites on nearly every curve of the road. In 2016, Toyota announced a partnership with Kymeta to explore adding a smaller version of the mTenna to their entire lineup of cars in the not-too-distant future.
“What we’re doing isn’t terribly ambitious — we’re just looking to revolutionize wireless communications,” Kundtz said dryly. “If you’ve seen the movie ‘The Martian,’ there’s a scene where Matt Damon is hit by a huge satellite dish during a windstorm. Clearly we have a different view of the future.”
Kymeta isn’t the only company making waves with metamaterials — three others have also been spun out of Duke Engineering labs to date by Intellectual Ventures. Pivotal Commware (their fourth company, founded in 2016) announced in 2017 that it had secured a $17 million funding round to focus on delivering ground-based communications, which includes delivering signals more efficiently as well as receiving them.
“We call that technology an electronically reconfigurable antenna,” said Smith. “It consists of a lot of little metamaterial elements, each of which can be slightly tuned to provide a piece of the overall beam. So if you want to change a signal’s direction, the elements change in a certain way to radiate different patterns.”
Although the details have remained secret thus far, Pivotal’s first customer is reported to be an air-to-ground communications competitor to companies like Gogo Inflight Internet that provide the much-derided internet service on airline flights. The technology could also be useful for ‘wireless backhaul,’ which is the data transmitted from an end user — such as your cellphone — to a node in a much larger network.
Their second company, called Evolv (founded in 2013), is working with Smith’s laboratory on security scanning applications.
“The Department of Homeland Security came to us and said they didn’t like that people have to stop and pose, nor that their machines are big monoliths with huge infrastructure costs,’” said Smith. “In the same way that we’d been successful miniaturizing and making antennas lightweight and low-cost, they asked if metamaterials could do the same thing for security imaging.”
As it turns out, the answer is probably yes.
The security scanning prototype being tested in Smith’s lab consists of almost two dozen metamaterial antennas mounted to a wall in a large grid. This wall of antennas broadcasts a variety of radiation patterns and frequencies, which bounce off objects and are then detected by receiving antennas in the same array.
While the resulting data from the randomness of the signals can be extremely difficult to interpret, Smith’s team is showing it is possible to reconstruct the object. And with the wide variety of frequencies being used, the researchers can do more than just tell if an object is being concealed by a stationary person — they can tell you what materials it’s made of while the subject is still walking past.
In much the same fashion, their third Intellectual Ventures spinout, called Echodyne (founded in 2014), builds radar systems suited for drones and connected cars. A thin strip of metamaterial antenna on the front and back bumper of a vehicle could replace backup sensors and do a whole lot more, Smith said.
And then there’s the wireless power transfer company that Smith is cooking up.
“Wouldn’t it be really nice if you never had to plug anything in to recharge, including toys, cellphones, Roombas, robots, whatever?” said Smith. “One way to efficiently power something is by using microwaves. Why not have one of these Kymeta-like panels that could actually be used to power something from a distance? There’s a lot of issues with achieving that, but it’s very, very possible to do.”
With such a disparate range of topics being tackled, who knows where the next must-have device will come from? There are several faculty members at Duke Engineering working to apply metamaterials to new inventions. While none of these projects are as close to commercialization as those pursued by the six Intellectual Ventures-backed companies, researchers within the field feel like it is only a matter of time.
“I think at Duke we have the highest density of metamaterials researchers at one institution,” said Smith. “It’s really been a place that has made unique contributions, especially through our connection with Intellectual Ventures, which is something that should be looked at as a model for incubation and translation. It’s usually very, very difficult to take ideas from the lab and get them to a commercial point. We’ve been successful not once, but now four times over, with another couple of companies on their way and many more downstream. That is something to really be excited about.”
“Metamaterials started in a very small way,” Pendry said. “But I think now, when asking where the center of metamaterials action is, I always look to Duke as the place where the exploitation of metamaterials is a fulcrum. Particularly as David and his team have been so energetic in driving forward metamaterials, Duke has had and will have a very important role. I always tell my students that however bright the ideas are that you have as a theorist, they will not live as ideas unless somebody picks up and runs with them and make something useful out of them. And the Duke team is doing that in spades.”
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