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Home , Bending, Illusion optics, John Pendry, Refraction, Victor Veselago

National Science Review INTERVIEW 5: 200–202, 2018 doi: 10.1093/nsr/nwx118 Advance access publication 20 September 2017

Special Topic: the laws of with metamaterials: an interview with John Pendry By Philip Ball Downloaded from https://academic.oup.com/nsr/article/5/2/200/4209243 by guest on 23 September 2021 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, statesthat a passing from one transparent medium to another—air to or , 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 greater thanthatofa (or, roughly speaking, of air), which is set equal to 1. Or do they? In the 1960s, the Russian 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 itbends 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 ifit might be possible to make a ‘scaled-up’ version of such a material, built from ‘artificial atoms’ and later called a , which could show this effect at longer than those of light, in the part of the spectrum. By happy coincidence, physicist SirJohn Pendry of , 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 workthat they rediscovered Veselago’s prescient paper from 1967. Smith later moved to in North Carolina and teamed upwith Pendry to develop a general theory——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 thehistory and prospects of this intersection of , 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 mi- crostructure must be finer that the electromagnetic wavelengths concerned, so that the metamaterial can be described by an ef- fective permittivity ε [the resistance to creating an internal elec- tric field] and magnetic permeability μ [the ability to support an internal magnetic field; these two quantities describe the material’s response to electromagnetic ]. The notion arose from work I was doing for the Marconi company on - absorbing materials consisting of overlapping, very fine carbon fibres.Itsoonbecameapparentthatthefibrousstructurewaskey to understanding its broadband-absorbing properties. This led ustoaskwhatotherstructurescouldgivevaluablenewmaterials. Our thin-wire structures, creating an artificial , were the direct descendants of the carbon-fibre work and provided access to negative ε at microwave . We then moved on to and the ‘split-ring resonators’, which provided novel magnetic properties and gave access to negative μ. These struc- tures provided the ingredients for a metamaterial with a negative refractive index. David Smith and colleagues in San Diego com- Metamaterials pioneer Sir John Pendry of Imperial College London. bined the two negatives and rediscovered , (Courtesy of John Pendry)

C The Author(s) 2017. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For permissions,please e-mail: [email protected] INTERVIEW Ball 201 as prescribed by Veselago long ago. Later metamaterials were the ingredients of cloaks of , again by David  Smith’s group, now based at Duke University in North Car- I thought it would be a good joke to show how to make olina. The prefix ‘meta’ is Greek for ‘beyond’ and sode- objects invisible to electromagnetic radiation. scribes materials with properties beyond what can be achieved —John Pendry naturally. NSR: You have looked in particular at optical metamaterials,  which are capable of manipulating light in unusual ways, some- and volunteered to make the cloak. Despite the theoreti- times running counter to our normal intuition of the laws of op- cal base, this was a major experimental challenge, completed in tics. What led you to want to do that? a few months. So the theory paper [Pendry JB, Schurig D and Pendry: Pure serendipity. It was the San Diego team who un- Smith DR. Science 2006; 312: 1780–2] was rapidly followed by earthed Victor Veselago’s original paper on negative refraction the experiment [Schurig D, Mock JJ and Justice BJ et al. Science Downloaded from https://academic.oup.com/nsr/article/5/2/200/4209243 by guest on 23 September 2021 and made it a reality, demonstrating a negative angle of refrac- 2006; 314: 977–80]. tion for . Another of Victor’s ideas was that negative NSR: Talk of invisibility obviously has a big resonance with the refraction could be used to focus light in a most unusual way. general public. Do you or did you worry that it might invoke un- My curiosity led me to calculate the resolving power of this de- realisticexpectations? Whatwas the response to your firstpapers vice and I showed—to my astonishment and that of everyone on the topic, the first theoretical and the second experimental? else—that, for certain values of the negative refractive index, the Pendry: The first paper got a huge response from the press. Here focus was perfect, unaffected by the Abbe limit [which says that was something that was created from very technical considera- a cannot resolve objects smaller than about half the tions, but led an object to which the general public could relate. of the illuminating light]. This has led to a whole se- It has been a gift to science communicators. On the paper’s first ries of experiments, chiefly with plasmonic systems [involving day I appeared on the BBC’s major news programme at 8:00 am excitations of surface , called plasmons], in which light and spent the entire day answering telephone calls from jour- is concentrated into sub-nanometre dimensions. nalists, finally taking the receiver off the hook at 10:00 pm.For NSR: When did you start to realize that optical metamaterials a scientist who had led most of his life well sheltered from the might be an experimental and not just a theoretical possibility? popular press this was an unnerving experience! And yes, there And how did the collaboration with the Duke team arise? are unrealistic expectations. For a start, the so-called cloak must Pendry: In fact the Marconi team with whom I was working in have a finite thickness: it is definitely not wearable. I believe that the late 1990s had already built and measured a system of split- we are on a learning curve as to what can be achieved, and what ring resonators, as well as several other designs for metamateri- is unlikely to be realized. Also the lure of the media has to be als. Sadly this work was terminated with the demise of Marconi tempered with a determination to show that we are engaged in in the dotcom crash. However, David Smith, then working with very serious ground-breaking science and, despite my initial in- Sheldon Schultz at the University of California at San Diego, tentions in the San Antonio talk, are not a bunch of jokers. was present when I gave a talk at the first Photonic and Elec- NSR: What kinds of applications do you think optical metama- tromagnetic Crystal Structures (PECS) conference in Laguna terials might realistically be expected to find? Beach, California. He immediately volunteered to do some ex- Pendry: In optics the great challenge is to control light on the periments, from which the first realization of negative refraction nanoscale. This can be achieved by creating structures—that is, emerged. David’s team has been one of the leading in the metamaterials—that can guide the light to a nanoscale destina- field ever since, and we have had many fruitful collaborations. tion. In its simplest realization this might involve concentrat- NSR: Who came up with the notion of an ‘invisibility shield’— ing light onto a molecule to give single-molecule sensitivity to and, in particular, who decided to frame it within the context of . A more complex application would be to use the ‘invisibility’ at all? concentrated energy to enhance interaction between so Pendry: Again it was serendipity. In April 2005 Valerie Brown- that one light source could control another, but with relatively ing, a DARPA [US Defense Advanced Research Projects modest power input. Switchable metamaterials are now coming Agency] official, was organizing a meeting in San Antonio on into play and can be used to scan a of light at an incredi- metamaterials and asked me to speak, with the request that I bly high rate in order to interrogate a scene. This is already being ‘ginger things up’. At that time I was working on the theory of done at terahertz frequencies. transformation optics, a very powerful design tool in electro- NSR: How easy is it to extend these concepts from microwave magnetics that I had developed, and thought it would be a good to optical wavelengths? What are the challenges? joke to show how to make objects invisible to electromagnetic Pendry: The challenges are mainly experimental. The sub- radiation. My wife suggested that I made reference to someone wavelength size requirements of metamaterials mean that engi- called Harry Potter, of whom I had never heard but whoap- neering must be on the nanoscale. This is expensive and time- parently had something to do with cloaks. However, the joke consuming. However, in the case of metasurfaces (structured was taken extremely seriously, and cloaking has since become 2D objects) there is well-developed technology that is being ex- a major theme in the metamaterials community. Again, David ploited to make novel and holographic devices. 3D nano- Smith’s team played a key role. Although David was not present engineering is more difficult, but even here [chemically based] at the meeting due to a dose of flu, he did hear about my talk self-assembly and related technologies offer a way forward. 202 Natl Sci Rev, 2018, Vol. 5, No. 2 INTERVIEW

NSR: You subsequently realized that, provided you are ready to make some compromises, manipulationof visible lightusing this  general notion of transformation optics is entirely possible, for I believe that we have a good balance between specu- example with the ‘carpet cloak’. Can you explain what that is and lation and practical applications. how the idea arose? —John Pendry 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 that made an object appear so small that it was in prac- present opportunities. Whereas in losses tice invisible. In fact the object could be made to appear as a [a ‘dimming’ of the scattered ] are often a problem for small point, as a line, or appear to be flattened into an infinitely some of the more esoteric designs, in there are many thin sheet. Of these, the point is hardest to achieve, and the instances of materials with extremely low loss waiting to be ex- flat sheet the easiest, at least as regards the material parameters. ploited. Downloaded from https://academic.oup.com/nsr/article/5/2/200/4209243 by guest on 23 September 2021 There is the problem that a flat sheet will act as a mirror, sowill NSR: This field has a particularly strong profile in China and only be invisible if you can find a way of hiding a flat mirror, among Chinese researchers. Do you have any thoughts on why which we did by placing the cloak on another mirror; hence the that might be so? Which are, in your view, the leading research ‘carpet’. Materials lose a lot of their response in the visible re- centres or groups in this area in China? gion, and the original design of the cloak has so far proved im- Pendry: Yes, China has been early into the game of metama- possible to realize at optical frequencies. However, we figured terials and transformation optics and has a flourishing school the carpet cloak could be built, and indeed has been by two of researchers. In particular I could mention Che Ting Chan separate groups. (Hong Kong), Tie Jun Cui (Nanjing), Sailing He (Hangzhou), NSR: It sometimes seems as if the possibilities for transforma- Lei Zhou (Shanghai), Shining Zhu (Nanjing), and many others. tion optics are limited only by our imaginations—there is talk, NSR: What do you think experience with metamaterials and for example, of ‘illusion optics’ (where any object is made to transformation optics has taught us about how our ingenuity can resemble any other by manipulating the light scattering) and potentially ‘subvert’ what are sometimes imagined to be funda- ‘spacetime cloaks’ (where events are hidden in space and time). mental principles (such as optical laws)? Is there enough space What in your view are some of the most inventive ideas here, for such innovative and perhaps high-risk thinking in scientific both in theory and in practice? research today? How do we find the right balance between spec- Pendry: There have already been exciting applications in elec- ulative thinking and practically realistic objectives? tromagnetism, but further progress can be expected in the field Pendry: I believe that we have a good balance between specula- of plasmonics, where structure plays an absolutely crucial role in tion and practical applications. The latter are essential if the for- determining the plasmon spectrum. mer is not to be dismissed as mere day-dreaming. It is one of the NSR: There are many proposals to extend much the same ideas strengths of the transformation optics/metamaterials field that to other phenomena: acoustic and , and elastic defor- even the more exotic ideas have been translated into experiment mations too—and even for seismological engineering, perhaps and that some are emerging as commercial products. ‘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 , where invariance is a problem, Philip Ball writes for NSR from London.