Our roundtable panelists are the 2014 Kavli Prize in Nanoscience laureates:
- THOMAS W. EBBESEN – Physical chemist and director of the University of Strasbourg Institute for Advanced Study, Strasbourg, France. Upending accepted theory, he discovered that light could be transmitted very efficiently through holes smaller than its own wavelength with the involvement of surface plasmons, the interaction of light with electron waves on metal surfaces. This enables the imaging of objects smaller than the light’s own wavelength as well as ultrafast photodetectors.
- STEFAN W. HELL – Physicist and director of the Max Planck Institute for Biophysical Chemistry in Göttingen, Germany. His discovery of a way to overcome the resolution limits of fluorescence microscopy has given research laboratories around the world the ability to discern the nanoscale features of tissues and cells.
- SIR JOHN B. PENDRY – Physicist and chair of theoretical solid-state physics, Imperial College London. He formulated the concept of metamaterials, whose precisely designed nanostructures give them properties not found in nature. Pendry showed how metamaterials could create perfect lenses with unlimited resolution, as well as bend light to create an ‘invisibility cloak’ that hides objects from detection.
THE KAVLI FOUNDATION: The Kavli Prize in Nanoscience cites your breakthrough improvements in optical microscopy and imaging resolution. There are many ways to image small objects, such as electron beam microscopy and x-rays. What makes visible light so important?
STEFAN HELL: Visible light is the only way we can look into living tissue and cells noninvasively. If you tried to do this with other types of beams, living things would not last long. X-rays are usually lethal, and electron microscopy usually requires you to dehydrate cells or put them in a vacuum. So if you want to see how biomolecules interact with each other in living cells, you need to work with visible light.
SIR JOHN PENDRY: Absolutely right. And you can add to that the many technologies based on light sources, such as lasers. The application of lasers to things like spectroscopy means that we can use light to do such things as detecting molecules, making printed circuits, communicating information, and perhaps even computing.
Light has vast capabilities. But until recently, we have been unable to control light on a length scale of less than the wavelength, or about half a micron. This has limited the contribution of optics to nanotechnology research. This is the resolution limit we three set out to solve.
THOMAS EBBESEN: To understand resolution, imagine a wavelength as being akin to the size of a photon, or particle of light. That photon cannot provide details about objects that are less than half its size, just as you cannot use a large beach ball to count the number of ping-pong balls in a pile. So there appears to be an inherent limit for how finely we can resolve an image, and it is roughly half the size of the smallest wavelength of visible light.
PENDRY: Right, though there are really two types of light. The first sort we are familiar with, light that escapes from a surface and propagates freely through space. We use it in microscopes, though its resolution is limited to about one-quarter of a micron, or about 250 nanometers. Using it to create an image is like writing with a fat pencil that doesn't have the finesse to draw thin lines.
But there's another sort of light that ordinarily hunkers down near the surface. We call this near-field light, and it contains much more information about the structure of the surface and nearby objects. Professor Ebbesen and I have been developing techniques to capture and control this light and concentrate it into very, very small volumes.
TKF: For more than one hundred years, scientists believed the resolution image limit was a fundamental principle of nature. What made you think you could get around it, and how did you do it?
HELL: My work involves light that propagates freely. While examining the capabilities of optical microscopes to inspect semiconductors during manufacturing, I started to wonder if there was a way to get images with much higher spatial resolution than 250 nanometers using conventional lenses. Back then, we all thought it was impossible to do anything about either the lenses or the light. But after a while, I realized that we might be able to harness the properties of the materials we were imaging to overcome the resolution limits.
In fact, biologists were already doing something like that by tagging biomolecules with small molecules that that emit fluorescent light when illuminated. This let them locate tagged molecules inside cells and see what they are doing. But if there are a lot of molecules in a small space, all they could see through a microscope was one big clump.
So I wondered if I could turn the fluorescence of some molecules off, just briefly. Then I would turn the bright ones off and turn the dark ones on. This way, I would make sure that molecules next to one another would not emit light simultaneously. By separating their emission, I could tell densely packed features apart. This improved our ability to see details by an order of magnitude.
TKF: Professor Ebbesen, unlike Professor Hell, who modified the objects he was viewing, you worked directly with light, didn't you?
EBBESEN: Yes, though I was not planning to do anything in optics. I'm a physical chemist, and I was interested in trying to control how molecules emit light by placing them in confined spaces. I was working at NEC Corporation in Japan, sharing an office with nanofabrication specialists, so I asked them if they could make me a thin metal film with nanoscale holes that would hold the molecules.
They returned with a glass slide containing a gold film. In the middle square centimeter was 100 million holes. To my surprise, it looked very translucent. Based on my physics training, I expected it to have been much darker, because the diameter of those holes was much smaller than the wavelength of the light trying to pass through it. At first, I thought there was something wrong with the sample, but it was perfectly fine. Then I measured the light, and the light transmitted by the holes was greater than the amount of light reaching the holes.
This, of course, seemed impossible, and I had no explanation. Optics specialists were mostly skeptical of my results, but I knew this was real, even if I didn't know why. Seven years later, at NEC's Princeton facility, I met Peter Wolff, who thought the enhanced light transmission was coming from surface plasmons, light trapped on the surface of the metal grid. Together, we and others showed that the holes acted like an antenna to capture and transmit optical light. Light would pile up on top of the holes, and at certain wavelengths, the holes transmitted more of that light than actually reached the hole.
Since then, we learned to create nanoscale structures in the metal around single holes. This produces sub-wavelength bright spots that let us see objects that are smaller than the wavelength of the light.
TKF: A great story. Professor Pendry, you also developed a way to use light to see objects smaller than a half-wavelength, but you came at very differently, right?
Pendry: I was working on radar waves with a company that had a radar absorbing material that worked really well, but they didn’t understand why. I discovered that it was not the material's composition, but its internal structure -- very fine carbon needles -- that gave it such good properties. From that came the idea of metamaterials, materials with artificial structures designed to improve their properties.
I suggested a way to make metamaterials with a refractive index -- which describes how a material bends light -- that is negative. So instead of light bending normally as it does when entering water or glass, it bends the wrong way. When David Smith of Duke University made the first negative-refracting metamaterial, it created a huge controversy because it appeared to break many laws of physics. It doesn't really, of course, but there was a great deal of controversy about it.
I realized that we could use this material to make a lens. But instead of having a curved surface, this lens would be flat and unusually perfect. Intuitively, it seemed there was no limit to how finely it could resolve things, so one wet Sunday morning, I did the math. I expected to bump into a limit, but what the maths said was the lens is perfect if you make it perfectly.
TKF: No limits on resolution? What was the reaction?
PENDRY: I thought, "I'm going to get into big trouble with this." So I put it in a drawer. I took it out a week later, and it still seemed okay. I talked to my colleagues, who laughed a lot, and said it was ridiculous. When I asked them why, they couldn't tell me. My paper had a terrible time getting through the referee system before it was published, and it really caused a firestorm.
That was about 14 years ago, and nobody has found a flaw in it since. That paper showed that this so-called fundamental limit of resolution was not fundamental at all, and that you could focus light way down if you used the right devices.
TKF: So we have this new toolkit that lets us manipulate light and images in unexpected ways. How does that change things?
PENDRY: These ideas open people's minds to new possibilities, but the people who have been building devices using the old technology are not the ones who will move things forward.
We have to bring new people into the game. This may take a while, but think of the possibilities. You know, if you shine a laser beam across a sunlit room, it gets to where it's going because the photons from the laser and sun do not interact with one another. But if we squeeze them down into a very, very small volume, we could make light interact with light, just as electrons interact with electrons. So we could start doing things with light that we now do with electrons, only much, much faster and more subtly, and that might revolutionize computing.
"At the end of the day, all diseases involve malfunctioning cells. Nanoscale imaging should have a fundamental impact on our understanding of cells and diseases and thus benefit all of us." – Stefan Hell
We could also have photons interact with atoms. Ordinarily, photons are thousands of times larger than atoms, so they do not have very good conversations. But if you could squash photons down to the size of a molecule, that conversation is much more productive and it happens quicker.
EBBESEN: I would like to add to that. It takes an enormous amount of electrical energy to power the vast computer infrastructure that makes up the web. Companies that operate networking centers are already using optical fibers to send and route information. This saves energy. The next step is to do computing with light and save even more energy. This is going to be increasingly important in our data-rich society.
HELL: Today, all major manufacturers offer some variation of nanoscale resolution fluorescence microscopy. They are used widely in life science laboratories worldwide because they let us see things we couldn't see before, such as the molecular distribution in a neuron's synapses or how the protein composition of organelles changes over the cell cycle. Nanoscale microscopy is becoming a standard tool in any high-end lab, but again, it took a while.
TKF: Has your work changed how you think about the nature of light?
HELL: What I realized is that the resolution of optical microscopy is not about perfecting the art of focusing alone. It's critical to perceive resolution as a game that also involves the material that one is observing. We can resolve what we are looking by briefly changing a materials' reflectivity or fluorescence. This is a totally different view of the optical microscope than I had learned many years back, and it guides my thinking and the research of my group.
EBBESEN: I'm not a physicist, and I wasn't even thinking about optics, so I had a lot to learn. Yet my ignorance enabled me to look at the problem without preconceptions. One of the great challenges of being a scientist is not getting blocked by what you know. When you have new ideas, you don't want to fall into the trap of, "Oh, this is impossible because it's not written in the textbook."
I play with light today in ways that I never would have thought of 10 or 15 years ago, and it's a joy. The laws of nature are so beautiful. There's an aesthetic aspect to the type of science we do, as we conceptualize an idea and gradually unveil new facets.
PENDRY: Yes, I agree, especially when it comes to the laws of nature. We shouldn't forget, of course, that this year is the one-hundred- fiftieth anniversary of Maxwell's equations, which laid down the laws of electromagnetism that control all these things which we do. One of the things I have been working on is a new set of laws, derived from Maxwell's equations, to replace the old-fashioned laws of refraction.
The refractive index is good for describing really big things, but if we want to understand how we can see very, very small things with light, we've really got to go back to Maxwell's equations. The problem is that those equations are very sophisticated. From them, we've been trying to extract a new way of thinking about the movement of light.
We call it transformation optics. If you want to control how light moves, you might think that a stupid way of doing it would be to bend space, which would then bend light as well. But Einstein showed that we don't have to bend space. We can actually change the electromagnetic properties of a material, and as far as light is concerned, it looks as though we have bent space. Reconciling Einstein and Maxwell is a new way of looking at controlling light, and we've used it to design very, very curious lenses, harvest light, make objects invisible, and so on.
So I think that will become quite a useful tool, in electromagnetism in the future, as we increasingly concentrate on the nanoscale and not just the wavelength scale.
"That paper showed that this so-called fundamental limit of resolution was not fundamental at all, and that you could focus light way down if you used the right devices." – Sir John Pendry
TKF: Where do you see the next challenges in sub-wavelength optics? What should we tackle next and why?
PENDRY: I would say that the big challenges are for the experimentalists. If you're talking about manipulating light on the nanoscale, someone has to construct metamaterials with nanoscale precision, and that is very, very hard.
For we theorists, there's also a challenge. When we shrink light's electromagnetic fields to nearly atomic length scales, then we've got to worry about how electrons in an atom responds to light. We really need a new synthesis that describes this response on a sub-nanometer scale. Whether we can do that in a way that engineers can then use to design devices is not yet clear, but it is a challenge we are seriously trying to address.
EBBESEN: To reduce what we are doing to practice, we need to make very small structures in a very reproducible way. This is indeed a significant engineering challenge.
And there are so many directions we can take. For instance, I'm trying to manipulate material properties by playing with the electromagnetic environment in which light resonates to make hybrid light-matter states of matter. This can lead to new tools to manipulate chemical reactions or even change bulk properties.
HELL: In the past, the best biological microscopy was done by those who manufactured the ‘best’ lenses. Today, it is those who design the ‘best’ fluorescent molecules. Initially a tough physics problem, the optical nanoimaging of cells has become a modern chemistry topic. There is a huge incentive to design molecules that provide the sharpest pictures. But this is not just about nice pictures. At the end of the day, all diseases involve malfunctioning cells. Nanoscale imaging should have a fundamental impact on our understanding of cells and diseases and thus benefit all of us.
TKF: So far, we have talking primarily about imaging. But your work has broader applications, right?
PENDRY: My focus is theory, so applications are a little ways down the line. But I think theorists have an advantage because we can look over such a broad range of issues. So as well as nanooptics, I'm also interested in controlling the radiation used in radar and cellphone communications.
In fact, my longtime collaborator, David Smith, is involved with a company called Kymeta that is using metamaterials to build a satellite communications dish that is cheaper, lighter, and more energy efficient than the current generation of dishes.
So there are practical applications, but to find them, we have to look over the entire electromagnetic spectrum. That's actually an outcome of our invisibility work. Really, invisibility was a very difficult grand challenge, and we thought that if we could demonstrate it using metamaterials, then we could do almost anything. It certainly attracted a lot of attention, not least from Harry Potter fans, but also there are serious applications in satellite communications and other devices as well.
We are also looking at larger wavelengths, like the ones used by cellphones. Instead of building nanostructures to manipulate those waves, we can engineer millimeter structures, which are much easier to build. We can also engineer similar structures to pick up the very subtle signals emitted by magnetic resonance imaging of the heart.
"Yet my ignorance enabled me to look at the problem without preconceptions. One of the great challenges of being a scientist is not getting blocked by what you know." – Thomas Ebbesen
HELL: I believe that turning molecular optical properties on and off could help you create some of the metamaterials you are after. It can be used to control which molecules link with others to form polymers, so we can create very tightly packed three-dimensional nanostructures with metamaterial properties. Once we get the chemistry sorted out, I think you will get the metamaterials that can interact with very small wavelengths.
PENDRY: I have some exciting things I'd like to do experimentally. If you could write nanoholograms, for example, that would be something I'd like to see.
HELL: We'd like to work on that problem, so maybe at some point, we will be able to help you.
PENDRY: I look forward to that.
TKF: Professor Ebbesen, what about non-imaging spinoffs from your work?
EBBESEN: In our data-intensive society, photodetectors play an important role in transferring information carried by light into an electronic signals. They must do this at very high speeds. Typically, when you shrink photodetectors to very tiny dimensions, they become much faster but pick up very little light. The world's fastest silicon photodetector uses the type of hole structures that we make, and since they act like antennas, they capture and transmit lots of light. So you have the best of both worlds, very small and very sensitive.
At NEC in Japan, where I used to work, researchers have built these ultrafast photodetectors and tested them on computer processors. Many modern processors actually contain multiple smaller processors, and they use this photodetector to help them communicate by light with one another. Everything happens so fast in electronics -- billions of operations per second -- that only light is fast enough to keep the processors synchronized, so they can work on problems together instead of waiting for each processor to complete its task individually.
This is yet another example of something I would never have foreseen. But again, as Professor Pendry was saying earlier, people end up applying these ideas in areas that we are not likely to think of today. This is one of the exciting things about new technology and new science
TKF: I'm curious. All three of you seemed to have faced a great deal of resistance when you unveiled your initial results. Could you talk about that?
HELL: My initial idea, put forth in 1994, was met with great skepticism initially. People argued publicly that it would never work. I had to put a lot of effort into finding a laboratory to set up my experiments and to show that my approach was viable.
EBBESEN: I must confess that, like all scientists, I am very skeptical by nature. The first time I heard about Professor Hell's work, I was also very skeptical. Just like most people were when they heard my results.
HELL: Some things appeared counterintuitive. As Professor Pendry just mentioned, people thought that it wouldn't be possible, for fundamental reasons, to focus the light down to a submicron point. Or it wouldn't be possible to get light through such tiny holes. And it wouldn't be possible to see something in a conventional microscope that is smaller than the limit of how tightly you can focus light.
Our ideas were perceived as a kind of, well, exaggeration. We were going up against common knowledge, and everyone who challenged those notions was considered kind of unusual, or too courageous, or -- I don't know.
PENDRY: That's a polite way of putting it, yes.
TKF: You were considered cranks?
HELL: Yes, to some extent, absolutely. I remember very vividly the controversy surrounding Professor Pendry's and Professor Ebbesen's work, and some of the papers challenging their ideas. I had the same experience. In one highly regarded journal, the first paper relating to my work was a comment questioning the implications of my results.
PENDRY: When these negative comments appear, your first reaction is anger and upset. But after a while, you realize that the controversy actually helps you. It draws people's attention to what you have done. Although you may never reconcile your challengers, the controversy draws in many people who wouldn't be interested otherwise, who say, "Here's something interesting. Maybe it's wrong, maybe it's right. Let's try to find out." So although controversy can be painful, it's part of the birth process of a new idea.
EBBESEN: Absolutely. It also shows that your results are important and that you are challenging accepted notions.