|May 25, 2006
Research News Editor Eric Smalley carried out an email conversation with
John Pendry, a condensed matter theorist who has worked at the Blackett
Laboratory, Imperial College London, since 1981. He began his career at
the Cavendish Laboratory, Cambridge, followed by six years at the Daresbury
Laboratory where he headed the theoretical group.
Pendry has conducted extensive research into the interaction of electrons and photons with surfaces. Early in his career he developed theories of low energy diffraction, electronic surface states, and statistics of transport in one-dimensional systems.
In 1992 he turned his attention to photonic materials -- structures punctuated with holes or plugs of material that promise to control light the way wires and circuits control electricity -- and developed some of the first computer codes capable of handling them.
His current research concerns materials whose normal response to electromagnetic fields is reversed, leading to negative refractive index values. The refractive index is the property that creates the illusion of a straw bending at the intersection of air and water. Materials that bend light the wrong way are not found in nature and harbor potentially useful properties. Pendry has worked out a scheme for a perfect lens made from negative refractive index materials whose resolution is unlimited by wavelength. He has also developed a theoretical framework for bending electromagnetic waves around objects, in other words for building invisibility cloaks.
Pendry is the author of Low Energy Electron Diffraction, co-author of Surface Crystallographic Information Service and has published more than 200 scientific papers. He was elected a Fellow of the Royal Society and the Institute of Physics in 1984. He was Dean of the Royal College of Science at Imperial College London from 1993 to 1996, head of the Physics department from 1998 to 2001, and first Principal of the Faculty of Physical Sciences from 2001 to 2002. Pendry was knighted in the Queen's Birthday honors list in June 2004 for services to science.
TRN: What got you interested in science and technology?
Pendry: From an early age I was always interested in science. Having scientifically literate relatives helped: they gave me books to read and answered questions.
Also I grew up in the 1940’s and 1950’s when ‘the endless frontier’ was the hot topic of the day and there was lots of positive press coverage. In those times science was seen as the answer to all our problems, not the cause of them!
TRN: Taking a historical perspective, can you give me some key examples of how science has been used to solve problems, and how it has been used to cause them?
Pendry: I have a nice example from my own department: Harry Harold Hopkins, a former professor of physics at Imperial College, developed the endoscope, an optical device for peering inside the body, which has been described as ‘transformed virtually every surgical speciality making him without doubt one of the most important figures in 20th century surgery’. It illustrates a point I often like to make, that physics is to the rest of science what machine tools are to engineering. A corollary is that science places power in our hands which can be used for good or ill. Technology has been abused in this way throughout the ages from gunpowder to atomic bombs.
TRN: What are the important or significant trends you see in science and technology research?
Pendry: In the wider context the most significant trends are the questioning of the scientific method and the hostile reactions of the public to technology, especially in Europe. We need another TE Huxley to fight Darwin’s and all of our battles over again.
Within science there is a change in emphasis from physical to medical sciences. Physical scientists like myself have to come to terms with this and ask where we fit into the picture. I think we do, and that our contribution is vital to the health of science as a whole. There are positive ways in which this is being recognised, such as the recent emphasis on interdisciplinary activities.
TRN: What are metamaterials?
Pendry: Whenever we need to make something, at some stage we need to decide on the material from which it will be constructed. Materials are central to the advance of technology. This is particularly true of electromagnetism: we seek high quality glass for lenses, low loss conductors for transmission of power, and so on.
Unfortunately the traditional route to discovery of new materials, that of seeking a new chemical composition, gives a limited range of properties: there are electromagnetic properties known to be allowed in principle, but not found in the current range of materials.
The answer we have come up with is to experiment with the physical structure instead of the chemical composition. A array of tiny nanostructures, each one practically invisible to light, serves just as well as a lattice of molecules when it comes to defining the electrical and magnetic response of a system. In fact the nanostructures are much better because we have enormous flexibility in their design.
Using this concept materials can be made with most unusual properties, including a negative refractive index which leads to some extraordinary effects.
TRN: Tell me about the trends in metamaterials research. What do you see as the most important issues to investigate?
Pendry: The first challenge is to develop the concept itself. What sort of nanostructures give the best metamaterials? How do we design them? How can we construct them?
The latter is particularly difficult because ideally we are looking for fully three dimensional structures made to order.
The second challenge will be to get people to use them.
When the laser first appeared on the scene, so radical was the concept that it was generally derided as a solution in search of a problem. That is not a phrase you hear much today! We need to look again at many issues in science and technology and ask if by using the new metamaterials we can now solve problems that previously seemed impossible, or if existing solutions can be improved.
Fortunately many imaginative people have been attracted to the field and new ideas are flowing.
TRN: What are the most exciting new ideas? What is the next laser, or what technology is the next laser likely to emerge from?
Pendry: Subwavelength resolution is so revolutionary that I find it the most exciting idea to emerge from metamaterials.
TRN: Can you give me some background and context about why this is revolutionary?
Pendry: Because everyone thought that it was impossible before negative refraction came along
TRN: Can you give us a brief synopsis of the diffraction limit and explain the significance of "beating it"?
Pendry: Electricity and magnetism appear in several forms: there is the ordinary electricity that comes out of the mains socket or from a battery, there is the field holding a magnetic sticker to the refrigerator door, and there is the light that enables us to see objects, to name only a few.
If we peer down a microscope to see a tiny object we use only the light radiated by that object. Only part of the electric and magnetic fields surrounding a luminous object escape and reach your eye, the rest remain localised close to the object itself.
Conventionally the light is divided into the ‘near-field’ and ‘far field’.
Unfortunately the ‘near-field’ is missing from the information that reaches your eye and all the fine-resolution details are contained in the near-field component. Using only the far field is equivalent to writing with a blunt pencil whose width is defined by the wavelength.
TRN: What is negative refraction, or should I say a negative index of refraction?
Pendry: It can be defined using Snell’s law of refraction: imagine a ray of light entering a glass block. In air the ray makes an angle θ1 with the surface normal, and inside the glass an angle θ2, then from Snell’s law,
Or best expressed in pictorial form:
Where we can see the characteristic chevron shape of the refraction as the ray folds back on itself inside the negative medium. Although we can state the effect very simply, the consequences are subtle and far reaching.
TRN: Why was this thought to be impossible for so long, and what was the breakthrough that led to its demonstration?
Pendry: The effect was predicted some years ago in 1968 by Veselago. He gave us the recipe in terms of the electrical permittivity, ε, and the magnetic permeability, μ, that had to be realised to achieve negative refraction. Maxwell's equations tell us that the refractive index is given by,
and by convention the positive sign for the square root is always assumed. However if both ε < 0 and μ < 0, then we have to choose the negative sign and n < 0. This requirement of negative values for raises the issue mentioned above: no such property has ever been found in a naturally occurring material. Only with the advent of metamaterials did we have the power to create by design an entirely new substance with the required properties.
TRN: Can you give me quick definitions of electrical permittivity and magnetic permeability?
Pendry: Electrical permittivity describes how a material responds to an electric field. In an applied field atoms are polarised: their electrons and nuclei move slightly in opposite directions. This increases the strength of the field inside the material by a factor of ε, the permittivity. A similar definition holds for the magnetic permeability, μ, except that the magnetic effect is produced by the alignment of electron spins.
TRN: What is a perfect lens and what would you use it for?
Pendry: First I should mention that another thing Veselago taught us was that a slab of negatively refracting material can act as a lens as can easily be seen by applying the law of negative refraction to rays. See the figure below.
This lens has some curious properties: when the condition is realised, it is singularly free of aberrations.
Some years ago I discovered yet another astonishing property. Unlike any other lens every devised it is capable of capturing and focussing all the components of the electromagnetic field including the elusive near-field. In other words its performance is not restricted by the wavelength limit.
In order to do this it must somehow amplify the near-field to compensate for the very rapidly decaying nature which the field normally has. This is achieved by the fields exciting a resonance in the negative index material.
Let me explain: imagine a wine glass made of excellent crystal. It has an acoustic resonance which you hear if you tap the glass. Sing at the glass at the same frequency as the resonance and the glass will capture the sound and ring loudly. In fact if the crystal is extremely perfect and the singer powerful enough, sufficient energy may be captured to break the glass.
Resonances act as very powerful amplifiers by capturing the incident energy and storing it. In the negative index material there exists an electromagnetic surface resonance, a surface plasmon, which does the same job for electromagnetic radiation and hence can deliver an amplified near-field to the image. The missing components are restored and high resolution is achieved.
In fact the resolution is now limited not by the properties of the rather limited far field but by how perfectly we can manufacture the lens to achieve the n = -1 condition exactly.
TRN: So this can be used to capture more visual information. Can you quantify this somehow?
Pendry: Yes. The resolution determines the density of information. In a DVD the information is written as a series of dots, something like Morse code. A lens with good resolution can write and read dots that are more closely spaced and therefore more densely packed. In the shortest distance we can resolve is d, we can expect to write 1/d2 dots per unit area. Thus increasing the resolution of a lens by a factor of 10 increases the information content of a DVD by a factor of 100.
TRN: In lay terms, what are plasmons and surface plasmon polaritons?
Pendry: In a metal the electrons are no longer bound to individual atoms as they are in, say, glass. Instead they are free to range over the whole body of the metal.
This freedom is responsible for the electrical conductivity of metals. It has another consequence if we try to shake the metal violently -- perhaps by hitting it with an energetic particle from an accelerator. Then the electrons will wobble to and fro like a liquid in a bucket. This liquid state of charges is called a plasma. It is also seen in ionised gases which have similar properties.
Normally you have to be inside a plasma to stimulate plasmon resonances, but there is a bit of the plasma that pokes out of the surface and is referred to as a ‘surface plasmon polariton’, SPP for short. These can capture energy from outside the metal, such as the weak tails of the object’s near-field. We use the SPPs to amplify the near-field and add the missing resolution to our image.
TRN: How do the technologies you're working on relate to business, culture, and social life?
Pendry: There are currently projects under way in various laboratories, not just in London, to make DVD players with higher capacity by writing better focussed dots on the platter; to obtain better magnetic resonance images in hospitals by controlling the magnetic fields more precisely (all the MRI signal information is contained in the near-field components); and to increase the performance of radar antennae and mobile phones.
So, yes, I suppose that covers business, culture and social life.
TRN: Can you give me a sense of how much more information can be stored on such higher capacity DVDs, and what types of information can be observed with such higher resolution MRIs?
Pendry: Even the latest DVDs lag behind magnetic recording technology. The new blue-ray DVDs hold about 50Gbytes, but magnetic hard drives can be bought with 500Gbyte capacity.
To rival and beat magnetic technology we need an order of magnitude increase in the density of information on the DVD. From the formula above that means we have to exceed the wavelength limit by at least a factor of four.
Regarding magnetic resonance imaging: the objective there is not to increase the resolution which is already excellent and is achieved not by using a lens but through tomography.
Instead we want to control the information collecting process more effectively which would lead to faster scan times and hence less discomfort to the patient, and greater throughput of a very expensive facility.
TRN: How would radar and mobile phone performance be improved?
Pendry: By making them more efficient, more directional, or giving them greater functionality
TRN: What are the important social questions related to today's cutting-edge technologies?
Pendry: These days most of the social questions come from medical science and technology.
Physical sciences have in the past been strongly associated with nuclear and weapons technology but in peaceful times these technologies are less prominent and less of an issue.
These days physical science as used by the general public is largely about enablement. For example the determination of the structure of DNA was enabled by the techniques of Xray diffraction. So physics does not get the blame for genetic manipulation, but nor does it get any of the credit for the benefits.
Socially speaking physical scientists are in something of a quiet backwater, which is a good place to get on with your work. The social questions that are raised are in my opinion largely chimeras such as the nonsense spoken about nano-goo and the noise made about possible radiation hazards from mobile phones.
There is one social question on which I do feel fairly strongly and that is risk perception. It is really a question for mathematics but still relevant to technology.
As social beings we cope very well with events which happen often. We see a lot of them so we understand them well and appreciate the risks. As a result our motorways are well populated with cars even though the risk of death or injury is much greater than travel by train.
On the other hand very few of us have witnessed a train crash. All we know is that they are terrible events and many people are killed. How rare they are is not factored into our judgement. As a result national expenditure on safety is completely skewed by this perception. Per notional death avoided rail safety is about one hundred times more expensive than road safety.
This is just one example of how rare but terrible events drive public policy in the wrong direction.
TRN: What do you imagine you or your successor will be working on in 10 years? In 20 years?
Pendry: Recently Science magazine on the occasion of its 100th birthday asked this question of its reviewing editors. I did not answer that question either. It turns out that same question was asked on the 75th birthday and the predictions were nearly all wide of the mark.
The whole point of research is to find the unexpected. If we do not plan on doing that it ceases to be research.
TRN: What's the most important piece of advice you can give to a child who shows interest in science and technology?
Pendry: To do anything well you have to practice hard. That applies to music, painting, writing and science. To do that you have to have the raw enthusiasm because otherwise it is not fun and you stop doing it.
So lots of books around mainly to be dipped into rather than absorbed line by line, plenty of opportunities for simply messing around with science whether that means doing a few experiments in the kitchen or solving maths puzzles if you are more theoretical. And the support and interest of a parent or a favourite relative.
TRN: What's the most important piece of advice you can give to a college student who shows interest in science and technology?
Pendry: In contrast to early interest in science you have to get serious at College: time to read those books properly, time to work through the problems sheets. But also early childhood interest in science helps enormously because there lie the beginnings of independent thought and invention. What about working up a few topics outside the lecture courses stimulated by the desire to understand one of your early experiments?
TRN: What could be done to improve the pursuit of science and technology research in terms of business trends, politics, and/or social trends?
Pendry: In a modern economy we do not need huge numbers of technologists, but the few that we need have to be smart and well trained otherwise they are uncompetitive with the bright Chinese and Indian technologists with whom they will compete. I despair of our school system which presides over an environment which is all too often chaotic and unconducive to scholarly values.
TRN: What books that have some connection to science or technology have impressed you in some way, and why?
Pendry: In science I am a much greater reader of journals than of books. For an excellent briefing covering a wide range of science and technology one cannot do better that read the Economist regularly. For more specialist material I turn to Science and Nature.
Other than that I read quite a few biographies which give insight into how others solved the problems in their lives (or failed to do so). I admire greatly scientists who have contributed over a broad spectrum, especially if in their work they have enabled others to achieve. Two examples come to mind: Lawrence Bragg and John Randle.
TRN: Is there a particular image (or images) related to science or technology that you find particularly compelling or instructive? Why do you like it; why do you find it compelling or instructive? Can we get a copy or pointer we can include with the interview?
Pendry: A couple of years ago I spent a few months sabbatical at UCSD with Shelly Schultz and David Smith, two friends and collaborators also working on metamaterials.
One of the structures they had made was a tiny sphere no more that a few millimetres in diameter. It was one of the first metamaterials, designed to behave like a plasma of free electrons and I could not resist photographing it.
The warm San Diego sunshine used for illumination reminds me of that beautiful city, and the glittering wires which comprise the structure bring back all the excitement of our first discoveries in this new field.
TRN: What are your interests outside of work, and how do they inform how you understand and think about of science and technology?
Pendry: I take my music moderately seriously and try to practice regularly. It is an enormous challenge for an amateur to acquire even a fraction of the skills of a professional.
How does it affect my thinking about science and technology? Although there are some similarities as in the level of dedication needed to achieve anything useful, music is really quite different. It inhabits a another part of the brain from science and you look at the world in a different way after playing the piano.
The garden takes up the rest of my time together with wandering around the countryside with a camera both of which are very relaxing. I even manage to use a few of my butterfly pictures in lectures to illustrate diffraction effects.
TRN: What types of music do you play?
Pendry: Mainly the 19th century classics. I am working on some Grieg and some Schumann at the moment. I love the listen to Liszt but his work is generally too difficult for an amateur to play. Rachmaninov as a pianist is my ideal; I have not heard his technique exceeded.
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