Engineered Structures with Bizarre Optical Properties Are Set to Migrate out of the Laboratory and Into the Marketplace

NEWS FEATURE

BY LEE BILLINGS

Engineered structures with bizarre optical properties are set to migrate out of the laboratory and into the marketplace.

om Driscoll would be happy if he commercialization are of more interest to Driscoll, a

never heard the phrase ‘Harry Potter- physicist who oversees metamaterials commercializa- 3 REF. style invisibility cloak’ again. But he tion at Intellectual Ventures, a patent-aggregation firm knows he will. The media can’t seem in Bellevue, Washington. Applications such as cheaper to resist using it when they report satellite communications, thinner smartphones and the latest advances in metamaterials ultrafast optical data processing are “where metama- — arrays of minuscule ‘elements’ that bend, scat- terials are poised to make a huge impact”, he says. Tter, transmit or otherwise shape electromagnetic Researchers still face some daunting challenges, radiation in ways that no natural material can. It is he adds — notably, finding cheap ways to fabricate true that metamaterials could, in principle, route and manipulate metamaterial elements on a scale light around objects and render them invisible, not of nanometres. But the first metamaterial-based unlike the cloak of a certain fictional wizard. And products are expected to come onto the market in a many metamaterials researchers are trying to make year or so. And, not long after that, Driscoll expects cloaking a reality, not least because the military has that average consumers will start to enjoy the ben- eagerly funded the development of such capabilities. efits, such as faster, cheaper Internet connectivity on However, if such applications ever come to pass board planes and from mobile phones. Such appli- it will be decades from now. Technologies closer to cations, he says, will move from being the stuff of

138 | NATURE | VOL 500 | 8 AUGUST 2013 © 2013 Macmillan Publishers Limited. All rights reserved FEATURE NEWS peoples’ fantasies “to becoming things they signals if they are coming from the satellite. can’t contemplate living without”. This technology is more compact than Wave The first laboratory demonstration of a meta alternatives such as dish antennas, says Smith. material was announced in 2000 by physicist It offers “significant savings in terms of cost, engineering David Smith and his colleagues at the University weight and power draw”. Kymeta has already Metamaterial elements scatter incoming 1 radiation in very precise ways. They can be any of California, San Diego . Following up on theo- performed demonstrations of this technology shape; common examples include spheres, rings, retical work done in the 1990s by John Pendry for investors and potential development part- crosses and chevrons. Their electromagnetic of Imperial College London, these researchers ners. But Smith cautions that the company has properties can often be changed by software. showed that an array of tiny copper wires and yet to set a price for the antenna and that it rings had a negative refractive index for micro- must still work to bring production costs down The spacing waves — meaning that microwave radiation while maintaining the strict performance between the elements can vary, flowing into the material is deflected in a direc- standards that regulatory agencies demand but is always less tion opposite to that normally observed (see for any device communicating with satellites. than the wavelength ‘Wave engineering’). That triggered intense Kymeta has shared so few details of its of the radiation. interest in metamaterials, in part because the antenna that researchers say it is hard to offer ability to bend radiation in such a way had an evaluation. But Smith is highly regarded potential for creating invisibility cloaks. in the field. If Kymeta brings the product to Since then, Smith and others have explored market, it may first offer its antenna for use a host of variations on the metamaterial idea, on private jets and passenger planes. If buyers often looking to manipulate radiation in ways respond well, the company hopes to incorpo- that have nothing to do with a negative refrac- rate the technology into other product lines, tive index. They have also moved beyond static such as portable, energy-efficient satellite- arrays, devising techniques to change the way communication units for rescue workers or the elements are arranged, how they are shaped researchers in the field. and how they respond to radiation. The result- In January, Smith’s group turned heads when ing materials can do things such as turn from it announced its demonstration of another opaque to transparent or from red to blue — all metamaterial device: a camera that can create Collectively, the array of elements at the flick of a switch. compressed microwave images without a lens functions similarly 2 or any moving parts . One important applica- to a hologram, Market movers tion of the device might be to reduce the cost shaping the radiation in ways no In January, Smith, now at Duke University in and complexity of airport security scanners. natural material can. Durham, North Carolina, took on a concur- In their current form, these scanners have to rent role as director of metamaterials com- physically sweep a microwave sensor over and mercialization efforts at Intellectual Ventures. around the subject. This produces an unwieldy Example: Negative index “I felt that the time was right, and we didn’t amount of data that has to be stored before it of refraction need to do any more science for some of these is processed into an image. The Duke group’s Metamaterials can be things,” he says. device requires very little data storage. It takes engineered to bend radiation in a direction A test case may come as early as next year. numerous snapshots by sending beams of opposite to that observed Kymeta of Redmond, Washington, a spin-off microwaves of multiple wavelengths across the in ordinary materials. from Intellectual Ventures, hopes to market a target at about ten times per second. When the compact antenna that would be one of the first microwaves are reflected back by the subject, consumer-oriented products based on meta- they fall on a thin strip of square copper meta- Positive Negative materials. The relatively inexpensive device material elements, each of which can be tuned refraction refraction would carry broadband satellite communi- to block or let through reflected radiation. The cations to and from planes, trains, ships, cars resulting pattern of opaque and transparent ele- and any other platform required to function ments can be varied very rapidly, with each con- in remote locations far from mobile networks. figuration transmitting a simplified snapshot of Application: At the heart of the antenna — the details of a scanned object into a single sensor. The sensor Invisibility cloak Radiation A cloak made of a negative- source which are confidential — is a flat circuit board measures the total intensity of radiation from index metamaterial can containing thousands of electronic metamat each snapshot, then outputs a stream of num- bend radiation around an object inside it, making that erial elements, each of which can have its bers that can be digitally processed to recon- object seem invisible. properties changed in an instant by the device’s struct a highly compressed image of the subject. internal software. This allows the antenna to This is admittedly just a first step: demon- track a satellite across the sky without having strations carried out so far have been crude to maintain a specific orientation towards it, affairs restricted to two-dimensional images the way a standard dish antenna does. Instead, of simple metallic objects. Expanding it to the antenna remains still while the software three-dimensional images of complex objects constantly adjusts the electrical properties of remains a challenge. But if that challenge can each individual metamaterial element. When be overcome, says Driscoll, airports could retire this is done correctly, waves emitted from the bulky, expensive, slow booths that currently the elements will reinforce one another and constitute security checkpoints, and instead use propagate skywards only in the direction of the a larger number of thin, inexpensive metamate- satellite; waves emitted in any other direction rial cameras hooked up to computers. Such a will cancel one another out and go nowhere. shift, Driscoll says, could extend security scan- Metamaterial At the same time — and for much the same ning to rooms, hallways, and corridors through- cloak reason — the array will most readily pick up out airports and other sensitive facilities.

In the meantime, a key research goal for of gold metamaterial elements carved out of a The team has since been working to refine the Smith and his group is the development of 60-nanometre-thick silicon wafer using elec- superlens concept; in 2007 the researchers robust and marketable metamaterial devices tron-beam lithography techniques developed advanced the idea by developing ‘hyperlenses’ that are not restricted to radio, microwave for the microchip industry. The elements are from curved, nested layers of compounds such or infrared wavelengths. If the technologies fixed, so cannot be tuned after fabrication. But as silver, aluminium and quartz6. The lenses not could be made to work with visible light, they by selecting a specific size and spacing during only capture evanescent waves, but can also feed would become much more useful for applica- the manufacturing process, physicists can shape them into a conventional optical system. Ulti- tions such as fibre-optic communications or light of a chosen wavelength in exactly the right mately, this could allow sub-wavelength details consumer-oriented cameras and displays. way to make it come to a point. to be viewed through the eyepiece of a stand- “It won’t be easy,” cautions Stephane Capasso warns that commercial applications ard microscope. But the complex structure and Larouche, a member of Smith’s research team of such flat lenses are probably still a decade behaviour of hyperlenses makes them difficult at Duke. For any given type of radiation, he away. This is partly because silicon is a rigid to manufacture and use in this way. explains, metamaterials can wield their exotic and fragile substrate for etching the elements; powers only if the elements are smaller and researchers are looking at more robust and Reversible focus more closely spaced than the wavelength of flexible alternatives that would be easier to By pairing conventional optics with super- that radiation. “So the shorter and shorter the handle on the production line. They are also lenses and hyperlenses based on meta- wavelength we wish to use, the smaller each looking for better ways to control the carving materials, Zhang hopes eventually to find metamaterial element must be,” says Larouche. of the nanoscale elements, which has to be applications far beyond microscopy. Just as In the microwave and radio regions of the done very precisely. these constructs can magnify sub-wavelength spectrum, this is relatively easy: wavelengths But once the technology is mastered, says detail, they can also be run in reverse, direct- are measured in centimetres to metres. But an Capasso, one obvious application is in smart- ing beams of light into sub-wavelength focal optical metamaterial’s elements would have to phone cameras. Lenses, along with batteries, points — a property of potentially revolution- measure considerably less than a micrometre. are among the most stubborn limiting factors ary importance for fabricating minuscule That is not impossible: today’s high- structures using photolithography. performance microchips contain If superlenses and hyperlenses can features only a few tens of nanome- “Metamaterials are be harnessed for this purpose, the tres across. But unlike those essen- ultra-fine beams of light could be tially static features, says Larouche, poised to make a huge used to etch much smaller features the metamaterial elements in many than is possible today. This could applications would need to incorpo- impact.” greatly increase the density of data rate ways for software to change their proper- in smartphone thickness, he says, speculating storage on optical drives, as well as the num- ties dynamically as needed. “Too often we have that a smartphone incorporating a flat camera ber of components that can be crammed onto gorgeous ideas,” he says, “but we have no way lens could potentially be made “as thin as a computer chips. of fabricating them.” credit card”. The flat lens also avoids aberra- Smith is cautious on that score, pointing out tions that plague glass lenses, such as the col- that hyperlenses and superlenses tend to dis- Flat focus oured ‘fringes’ created by the inability to focus sipate substantially more of the light energy Despite these difficulties, workable designs for all wavelengths to the same point. This means passing through them than other advanced optical metamaterials have begun to emerge. that Capasso’s flat lens could also be used to lithographic techniques now in development. One was published in March3 by a group make better, aberration-free microscopes. This, he says, makes them prime examples of working under Nikolay Zheludev, a physicist As good as they might ultimately be, the flat “strong and compelling science that is not yet at the University of Southampton, UK, who lenses would still be subject to the diffraction practical for any sort of product path” at optical directs a research centre focused on metama- limit, which dictates that no conventional lens wavelengths. But, he adds, Zhang’s efforts are terials at Nanyang Technological University can resolve details much smaller than the wave- “heroic experiments that illustrate the poten- in Singapore. The team’s device can greatly length of the light that illuminates its target. tial of metamaterials in a fundamental way”. alter its ability to transmit or reflect optical This limit averages about 200 nanometres for Zhang concedes that hyperlenses and super- wavelengths by means of nanometre-scale, visible light. But metamaterials offer a means of lenses are not yet ready for prime time, but electrically controlled metamaterial elements fabricating ‘superlenses’ that could surpass such believes there is plenty of room for ongoing etched from gold film; it could one day serve limits, allowing researchers to see sub-wave- research to change that situation in the coming as a switch in high-speed fibre-optic commu- length details of target objects such as viruses years. “The economic impact could be huge,” he nications networks. or the ever-changing structures in living cells. says. “I am cautiously optimistic that metama- Meanwhile, because it is so hard to make and The key is to recognize that the missing terials, superlenses and lithography will prove control three-dimensional metamaterial arrays details are still there, carried in ‘evanescent’ truly revolutionary. If people aren’t too short- at optical scales, some researchers are focusing waves of reflected light that die away very rap- sighted, what we can do with metamaterials will on two-dimensional ‘metasurfaces’. In August idly with distance from the illuminated object. be limited only by our imaginations.” ■ 2012, a group led by Federico Capasso at Har- Normally, these waves have effectively vanished vard University in Cambridge, Massachusetts, before they can be captured and focused by a Lee Billings is a freelance writer based in New unveiled a flat metamaterial lens that can focus lens. But a metamaterial superlens designed York. infrared light to a point in much the same way to be placed within tens of nanometres of an 4 1. Smith, D. R., Padilla, W. J., Vier, D. C., Nemat-Nasser, as a glass lens . “I don’t want to claim absolute object can pick up and magnify these waves. S. C. & Schultz, S. Phys. Rev. Lett. 84, 4184–4187 novelty in this,” Capasso says, “but I believe we An early proof-of-concept superlens was (2000). are the first group to so clearly put flat optics on demonstrated in 2005 by a group working 2. Hunt, J. et al. Science 339, 310–313 (2013). 3. Ou, J.-Y., Plum, E. Zhang, J. & Zheludev, N. I. Nature the agenda for commercial applications.” under Xiang Zhang, a physicist at the Univer- Nanotechnol. 8, 252–255 (2013). A conventional lens relies on refraction sity of California, Berkeley5. Zhang’s group 4. Aieta, F. et al. Nano Lett. 12, 4932–4936 (2012). to bend light to a point by passing it through produced a simple metamaterial consisting of a 5. Fang, N., Lee, H., Sun, C. & Zhang, X. Science 308, 534–537 (2005). varying thicknesses of glass. Capasso’s lens 35-nanometre-thick layer of silver in a sandwich 6. Liu, Z., Lee, H., Xiong, Y., Sun, C. & Zhang, X. Science passes light through a two-dimensional array with nanoscale layers of chromium and plastic. 315, 1686 (2007).