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Abstract
Metamaterials show us that nature's laws might not always be as fixed as they seem to be. One of the laws of optics, for example, states that a light ray passing from one transparent medium to another—air to water or glass, say—is bent at the interface by an amount that depends on the so-called refractive indices of the two media. And all transparent materials have a refractive index greater than that of a vacuum (or, roughly speaking, of air), which is set equal to 1. Or do they? In the 1960s, the Russian physicist Victor Veselago explained what would be needed, in theory, for a material to have a refractive index that is not only less than 1 but in fact negative, so that it bends light the ‘wrong way’.
Veselago's idea was all but forgotten until it was unearthed in the late 1990s by electrical engineer David Smith. He was wondering if it might be possible to make a ‘scaled-up’ version of such a material, built from ‘artificial atoms’ and later called a metamaterial, which could show this effect at longer wavelengths than those of light, in the microwave part of the spectrum. By happy coincidence, physicist Sir John Pendry of Imperial College London, UK, had already come up with a prescription for making a similar structure from loops of wire. Smith and his colleagues created a negative-refractive-index metamaterial in 1999—and it was only in the course of publishing this work that they rediscovered Veselago's prescient paper from 1967. Smith later moved to Duke University in North Carolina and teamed up with Pendry to develop a general theory—transformation optics—that described how such substances can manipulate the paths of light in new and unexpected ways. In 2006 they designed and built a metamaterial device with a far more remarkable property: it could bend light (rather, microwave) rays around an object to render it invisible.
With that dramatic innovation, the field of metamaterials was well and truly launched. NSR spoke with John Pendry about the history and prospects of this intersection of physics, materials science and engineering.
NSR: What are metamaterials and how did the notion arise? Where did the term itself come from?
Pendry: Metamaterials derive their properties from internal microstructure, rather than chemical composition. The microstructure must be finer that the electromagnetic wavelengths concerned, so that the metamaterial can be described by an effective permittivity ε [the resistance to creating an internal electric field] and magnetic permeability μ [the ability to support an internal magnetic field; these two quantities describe the material's response to electromagnetic radiation]. The notion arose from work I was doing for the Marconi company on radar-absorbing materials consisting of overlapping, very fine carbon fibres. It soon became apparent that the fibrous structure was key to understanding its broadband-absorbing properties. This led us to ask what other structures could give valuable new materials. Our thin-wire structures, creating an artificial plasma, were the direct descendants of the carbon-fibre work and provided access to negative ε at microwave frequencies. We then moved on to magnetism and the ‘split-ring resonators’, which provided novel magnetic properties and gave access to negative μ. These structures provided the ingredients for a metamaterial with a negative refractive index. David Smith and colleagues in San Diego combined the two negatives and rediscovered negative refraction, as prescribed by Veselago long ago. Later metamaterials were the ingredients of cloaks of invisibility, again by David Smith's group, now based at Duke University in North Carolina. The prefix ‘meta’ is Greek for ‘beyond’ and so describes materials with properties beyond what can be achieved naturally.
Metamaterials pioneer Sir John Pendry of Imperial College London. (Courtesy of John Pendry)
NSR: You have looked in particular at optical metamaterials, which are capable of manipulating light in unusual ways, sometimes running counter to our normal intuition of the laws of optics. What led you to want to do that?
Pendry: Pure serendipity. It was the San Diego team who unearthed Victor Veselago's original paper on negative refraction and made it a reality, demonstrating a negative angle of refraction for microwaves. Another of Victor's ideas was that negative refraction could be used to focus light in a most unusual way. My curiosity led me to calculate the resolving power of this device and I showed—to my astonishment and that of everyone else—that, for certain values of the negative refractive index, the focus was perfect, unaffected by the Abbe limit [which says that a microscope cannot resolve objects smaller than about half the wavelength of the illuminating light]. This has led to a whole series of experiments, chiefly with plasmonic systems [involving excitations of surface electrons, called plasmons], in which light is concentrated into sub-nanometre dimensions.
NSR: When did you start to realize that optical metamaterials might be an experimental and not just a theoretical possibility? And how did the collaboration with the Duke team arise?
Pendry: In fact the Marconi team with whom I was working in the late 1990s had already built and measured a system of split-ring resonators, as well as several other designs for metamaterials. Sadly this work was terminated with the demise of Marconi in the dotcom crash. However, David Smith, then working with Sheldon Schultz at the University of California at San Diego, was present when I gave a talk at the first Photonic and Electromagnetic Crystal Structures (PECS) conference in Laguna Beach, California. He immediately volunteered to do some experiments, from which the first realization of negative refraction emerged. David's team has been one of the leading lights in the field ever since, and we have had many fruitful collaborations.
NSR: Who came up with the notion of an ‘invisibility shield’—and, in particular, who decided to frame it within the context of ‘invisibility’ at all?
Pendry: Again it was serendipity. In April 2005 Valerie Browning, a DARPA [US Defense Advanced Research Projects Agency] official, was organizing a meeting in San Antonio on metamaterials and asked me to speak, with the request that I ‘ginger things up’. At that time I was working on the theory of transformation optics, a very powerful design tool in electromagnetics that I had developed, and thought it would be a good joke to show how to make objects invisible to electromagnetic radiation. My wife suggested that I made reference to someone called Harry Potter, of whom I had never heard but who apparently had something to do with cloaks. However, the joke was taken extremely seriously, and cloaking has since become a major theme in the metamaterials community. Again, David Smith's team played a key role. Although David was not present at the meeting due to a dose of flu, he did hear about my talk and volunteered to make the cloak. Despite the sound theoretical base, this was a major experimental challenge, completed in a few months. So the theory paper [Pendry JB, Schurig D and Smith DR. Science 2006; 312: 1780–2] was rapidly followed by the experiment [Schurig D, Mock JJ and Justice BJ et al. Science 2006; 314: 977–80].
I thought it would be a good joke to show how to make objects invisible to electromagnetic radiation.
—John Pendry
NSR: Talk of invisibility obviously has a big resonance with the general public. Do you or did you worry that it might invoke unrealistic expectations? What was the response to your first papers on the topic, the first theoretical and the second experimental?
Pendry: The first paper got a huge response from the press. Here was something that was created from very technical considerations, but led an object to which the general public could relate. It has been a gift to science communicators. On the paper's first day I appeared on the BBC’s major news programme at 8:00 am and spent the entire day answering telephone calls from journalists, finally taking the receiver off the hook at 10:00 pm. For a scientist who had led most of his life well sheltered from the popular press this was an unnerving experience! And yes, there are unrealistic expectations. For a start, the so-called cloak must have a finite thickness: it is definitely not wearable. I believe that we are on a learning curve as to what can be achieved, and what is unlikely to be realized. Also the lure of the media has to be tempered with a determination to show that we are engaged in very serious ground-breaking science and, despite my initial intentions in the San Antonio talk, are not a bunch of jokers.
NSR: What kinds of applications do you think optical metamaterials might realistically be expected to find?
Pendry: In optics the great challenge is to control light on the nanoscale. This can be achieved by creating structures—that is, metamaterials—that can guide the light to a nanoscale destination. In its simplest realization this might involve concentrating light onto a molecule to give single-molecule sensitivity to spectroscopy. A more complex application would be to use the concentrated energy to enhance interaction between photons so that one light source could control another, but with relatively modest power input. Switchable metamaterials are now coming into play and can be used to scan a beam of light at an incredibly high rate in order to interrogate a scene. This is already being done at terahertz frequencies.
NSR: How easy is it to extend these concepts from microwave to optical wavelengths? What are the challenges?
Pendry: The challenges are mainly experimental. The sub-wavelength size requirements of metamaterials mean that engineering must be on the nanoscale. This is expensive and time-consuming. However, in the case of metasurfaces (structured 2D objects) there is well-developed technology that is being exploited to make novel lenses and holographic devices. 3D nano-engineering is more difficult, but even here [chemically based] self-assembly and related technologies offer a way forward.
NSR: You subsequently realized that, provided you are ready to make some compromises, manipulation of visible light using this general notion of transformation optics is entirely possible, for example with the ‘carpet cloak’. Can you explain what that is and how the idea arose?
Pendry: Jensen Li was working with me as a post doc at the time. He and I realized that the cloak could be thought of as a lens that made an object appear so small that it was in practice invisible. In fact the object could be made to appear as a small point, as a line, or appear to be flattened into an infinitely thin sheet. Of these, the point is hardest to achieve, and the flat sheet the easiest, at least as regards the material parameters. There is the problem that a flat sheet will act as a mirror, so will only be invisible if you can find a way of hiding a flat mirror, which we did by placing the cloak on another mirror; hence the ‘carpet’. Materials lose a lot of their response in the visible region, and the original design of the cloak has so far proved impossible to realize at optical frequencies. However, we figured the carpet cloak could be built, and indeed has been by two separate groups.
NSR: It sometimes seems as if the possibilities for transformation optics are limited only by our imaginations—there is talk, for example, of ‘illusion optics’ (where any object is made to resemble any other by manipulating the light scattering) and ‘spacetime cloaks’ (where events are hidden in space and time). What in your view are some of the most inventive ideas here, both in theory and in practice?
Pendry: There have already been exciting applications in electromagnetism, but further progress can be expected in the field of plasmonics, where structure plays an absolutely crucial role in determining the plasmon spectrum.
NSR: There are many proposals to extend much the same ideas to other wave phenomena: acoustic and sonar, and elastic deformations too—and even for seismological engineering, perhaps ‘hiding’ buildings or entire cities from earthquakes. Where do you see this field heading next?
Pendry: Acoustics has been a productive area: although there are thorny issues in solids, where invariance is a problem, liquids present opportunities. Whereas in electromagnetism losses [a ‘dimming’ of the scattered waves] are often a problem for some of the more esoteric designs, in acoustics there are many instances of materials with extremely low loss waiting to be exploited.
I believe that we have a good balance between speculation and practical applications.
—John Pendry
NSR: This field has a particularly strong profile in China and among Chinese researchers. Do you have any thoughts on why that might be so? Which are, in your view, the leading research centres or groups in this area in China?
Pendry: Yes, China has been early into the game of metamaterials and transformation optics and has a flourishing school of researchers. In particular I could mention Che Ting Chan (Hong Kong), Tie Jun Cui (Nanjing), Sailing He (Hangzhou), Lei Zhou (Shanghai), Shining Zhu (Nanjing), and many others.
NSR: What do you think experience with metamaterials and transformation optics has taught us about how our ingenuity can potentially ‘subvert’ what are sometimes imagined to be fundamental principles (such as optical laws)? Is there enough space for such innovative and perhaps high-risk thinking in scientific research today? How do we find the right balance between speculative thinking and practically realistic objectives?
Pendry: I believe that we have a good balance between speculation and practical applications. The latter are essential if the former is not to be dismissed as mere day-dreaming. It is one of the strengths of the transformation optics/metamaterials field that even the more exotic ideas have been translated into experiment and that some are emerging as commercial products.
