Metamaterials | History
The Story of Metamaterials and Negative Index: A Personal Perspective
One of the questions I am often asked is how metamaterials first started. Arguably, metamaterials is now a billion dollar enterprise, if you add up all of the funding that has poured into metamaterials research and development over the years. In addition, metamaterials has become one of the most exposedif not overexposedareas of research, with the term "metamaterial" reaching well into the popular media.
So, how did it all start?
Just as a matter of full disclosure, what I'm writing here is not an official or comprehensive history. It's a history from my point-of-view, about what was on my mind at the time and the events that happened to and around me. The field of metamaterials has a lot of threads; I don't think anyone at this point could recount a complete story of the development of the field, which by now contains the contributions from thousands of researchers.
But, for me, the beginnings of metamaterials were very modest. I had no grand visions, or particular end goals. Metamaterials, or artificial materials, were for me just a hobby, really - a means of trying to understand an entirely unrelated phenomenon. I never anticipated that negative index materials, or cloaking, or any of the other almost magical properties of metamaterials would exist. All of the really big discoveries made in this field, by our group or by other groups, were all pleasant surprises!
I don't know how a discovery is "typically" made. What I find interesting about the events that led to the development of the metamaterials field is the seeming randomness of events and, maybe, the luck that was involved. If there is any message here, it is to learn as much as you can about everything, as deeply as possible; and, to play around with ideas until you obtain an almost intuitive knowledge of them. As you build an internal mental picture of a subject, you also want to allow that picture to evolve and adapt as you gain more information. Being intellectually honest is also important - never let your desired outcome interfere with reality, however tempting! I suspect these very generic and simple rules are prerequisites for progress and discovery in any field, but I can only give a firsthand account of what happened for the case of metamaterials. By definition, a discovery is something you don't anticipate, so all you can do to make a discovery is be very aware; be opportunistic; and gain as much knowledge as possible.
In the following, I provide my personal background and perspective on the events surrounding metamaterials, but I also try to include conversations and discussions and arguments I had with my friends and colleagues. People assess and react to the unknown very differently and with very different states of mind. I think, for the case of metamaterials, the controversy that erupted and the back-and-forth scientific discussions really highlight the excitement that resulted as our collective intuition was challenged by some of the notions that emerged. It was great to be at the center of the metamaterials explosion; metamaterials continues to be an endlessly fascinating field with seemingly endless possibilities. Although some of the arguments and debates were frustrating at the time, I never blamed anyone for expressing an alternative point-of-view; I have come away from the entire experience with the belief that the scientific community as a collective is dedicated, sincere and robust. I'm always amazed at the willingness that others have to deeply evaluate and investigate any idea that arises, for no reason other than pure scientific curiosity. It's a healthy impulse that guides us all to the truth and makes possible the intense progress that benefits all of our lives!
The World Discovers Negative Index
It was in mid-December of 1999, during the time our manuscript on negative index materials was under submission to Physical Review Letters. While doing some background research, Willie Padilla and I had run across a paper by Russian physicist Victor Veselago, published in 1968, that had theoretically predicted the existence of the artificial material we had just succeeded in creating and had demonstrated in the lab - a material with a negative index of refraction. Because our material was actually a construction of little copper wires and rings rather than a conventional material, we were calling it a "metamaterial," a term that had just recently been coined to describe artificial materials. Shelly Schultz, my PhD and postdoctoral advisor, burst into my office one day, really excited - practically breathless.
"I've been in this business for over forty years, but now I'm going to do something I've never done in my life." As often with Shelly, he delivered the news with a level of seriousness that bordered on the dramatic.
"What's that? I asked.
"I'm going to organize a press conference on the discovery of negative index metamaterials at the American Physical Society March Meeting in Minneapolis."
While initially skeptical about the artificial material work, over time Shelly had become truly inspired, especially once we had discovered the Veselago paper; now he was brimming with enthusiasm. Shelly held a deep and profound passion for science and especially loved to challenge himself and others with intellectual puzzles and conundrums. This passion made him a dynamic and dazzling lecturer who could inspire and transfix his audience, as he rattled out physics concepts with a rapid-fire, machine gun like tempo. Though most of his life had been spent in San Diego, Shelly had never lost the fast-paced, New York delivery that he was well known for. Shelly didn't just present the facts; he liked to build a story, usually dangling some mystery in front of the audience at the beginning of a lecture, then building up to the resolution for a grand finale. His lectures were thus engaging, delivered in an inimitable style that left his listeners excited and wanting more. He was one of the very few professors at UCSD who routinely had a 100% approval rating from his classes - a nearly impossible feat.
In Veselago's paper about negative index materials, Shelly had found one of the greatest mysteries that he had ever come across. His intuition had been knocked for a loop, and Shelly was reveling in the endless puzzles that a negative index-of-refraction brought. Veselago suggested that nearly all of electromagnetics would have to be rethought if negative index materials were ever foundand we had found them! Everything that Shelly had taught for forty years - even basic concepts, such as Snell's law of refraction - now required a second look. For Shelly, this was big news, and he wanted to share it with the world.
I, on the other hand, was shocked and almost immediately paralyzed with fear. Why did we need a press conference? A press conference would mostly target non-scientists; why would the general public be the least bit interested in something as arcane as negative index? What immediately jumped into my mind was all the press that had been generated by the cold fusion experiments years before, in which a group of scientists from the University of Utah had claimed to have accomplished a nuclear fusion process in a material. The images of the Utah scientists prematurely celebrating their presumed breakthrough with Champaign suddenly came to mind. Similar to what Shelly was proposing, the Utah group had decided to announce their results before their paper was published, before the rest of the scientific community even had a chance to confirm their findings. In the end, when the results could not be reproduced, the Utah scientists - perhaps unfairly - suffered enormously.
A press conference could seriously backfire. We had done a very interesting experiment, for sure, but we had no idea if negative index was actually useful for anything; moreover, all of our interpretations had not been really tested. We were drawing our conclusions by inferring a lot from a combination of experiments and simulations; there would need to be much more work done to really substantiate the results.
But Shelly was adamant. He saw negative index as an important breakthrough and was convinced the rest of the world would be as fascinated as he was. Brushing aside my concerns, he wanted to organize a press conference, and he wanted me to go with him to Minneapolis to participate. My only consolation was that, unlike cold fusion, which stood to solve the worlds energy needs, no one would understand or care about negative index. If we were somehow wrong or had misinterpreted our results, maybe we could still go unnoticed.
As the time of the APS meeting and press conference drew near, reporters started calling. And calling. And calling. I had thought no one would be interested, but it seemed like everyone was interested.
"I was on the phone for more than an hour with the science reporter from the New York Times, Shelly announced one day. "I tried to explain negative refraction to the reporter, but I'm a little disappointed that he had so much trouble getting it. Shelly seemed genuinely surprised.
The New York Times? Seriously? That was coverage I hadn't expected in my wildest thoughts. And many other major newspapers were also calling and considering running the story. There wasn't much I could do. I could only answer questions if any reporters or journalists decided to call, to the best of my ability. The cat was out of the bag, and there was no turning back.
For the press conference, the staff at APS wanted a couple of distinguished physicists to be on a panel to support and discuss the results. Shelly asked Walter Kohn, a physicist at UC Santa Barbara who had won the Nobel Prize in Chemistry in 1998, and Marvin Cohen from UC Berkeley. They were both intrigued with the negative index after Shelly explained to them the experiments, and graciously agreed to be on the panel.
The press conference was held in March of 2000, about a month before the paper would be published in early May. While reporters and possibly some researchers would have copies of our manuscript to evaluate, most other scientists would not. I was incredibly uncomfortable with it all! The night before the press conference, I couldnt sleep, distraught and almost sick with the thought that we were making a big deal about our work when we might have missed something technical, or were overstating the significance. It was frightening.
The concept of a negative index of refraction is pretty technical. Shelly had a talent for explaining complex ideas and making people feel like they understood them. He was at a stage in his career where he knew how to convey the big picture, and not get bogged down in the technical details. I didn't have that talent. Moreover, Shelly was not shy; he was outgoing and filled with self-confidence. I was far more the introvert, and also riddled with doubts that we hadnt been self-critical enough with respect to our theory or experiments. I was terrified about every statement I made, and tried to qualify every sentence to the point that everything I said became useless. As I anticipated the day of the press conference, I imagined trying to explain negative index to journalists, or - even worse - trying to answer what negative index might be good for. The thought of telling a reporter "negative index is good for reversing Cerenkov radiation" made me queasy. Even trying to explain what a "magnetic permeability" and an "electric permittivity," which were absolutely necessary to understand the structure we had build, was next to impossible. Those concepts are difficult enough for physicists to get straight!
Shelly's enthusiasm, though, was unbridled and unstoppable. "Negative index is a major, major discovery!" He'd say, "What is it good for? I don't know. But when the laser was invented, no one knew what to do with that either, and now lasers are in supermarkets. No one could have predicted that!"
Negative index was pretty neat, but, in the end, we were scattering microwaves off of metal. To my mind, while we had found a really interesting effect and fulfilled a thirty year old conjecture, it wasn't the same as demonstrating a laser, which had produced non-classical, coherent light for the first time based on a quantum mechanical effect. I would never have compared our metamaterial structure to the laser.
But, despite all of my reservations and concerns, the press conference went forward. And, instead of no one being interested, the science journalists were really intrigued and negative index looked like it would become the story for the 2000 APS Meeting.
As I had expected, it was a difficult topic for the reporters to cover, and equally difficult to convey the importance. I also had my first experience of dealing with science reporters, which turned out to be fantastic. The journalists who covered the science stories truly loved science and tried very hard to get the story right, while making it palatable for their audience. I was deeply impressed with their care in writing the story, fact checking and allowing us to review drafts for accuracy. They worked hard trying to explain our own discovery for us, suggesting analogies and even postulating possible uses for our arcane materials.
I also got a sense of where accuracy has to be sacrificed for the sake of generating interest or conveying to non-specialists quickly. At one point I was working with a reporter from the Washington Post, who was covering the APS press conference. He was on the phone with his editor, and we were making last minute changes to his story just before his deadline. The editor wanted a title.
The reporter was listening to something the editor was saying. Then, he cupped his hand over the phone and leaned over to me: "My editor likes the title 'Left-Handed Material Said to Reverse Energy.' Is that ok?
I thought about it. "No, our material really doesn't reverse energy-I'm not even sure what that means exactly. Our material just scatters waves, and inside the material waves appear to propagate the wrong way, back toward the source. But it's just an illusion; the flow of energy is still the same. The title is definitely misleading, if not entirely wrong."
The reporter got back on the phone to the editor. "He says your title isn't right. Energy doesn't go backwards. Yeah. Yeah. Ok."
The reporter leaned over again, hand over the phone and declared "we're gonna run with that title anyway. My editor likes it."
Not much I could do! The article was great, and, wrong as it was, the short, catchy title would unquestionably attract readers' attention.
To my surprise, the press release from the APS (which can be viewed here) and the press conference caught fire and within a week everyone was talking about negative index. The story was global, and reporters even found and obtained comments from Victor Veselago, who had postulated about negative index more than three decades before.
The Beginnings
In 1997, I was into my third year of a postdoc in the group of Sheldon ("Shelly") Schultz, in the Physics Department of the University of California, San Diego (UCSD). I had completed my PhD in the same group in 1994, on the topic of microwave scattering in photonic crystals. After graduating and becoming a postdoc, I had switched research topics, and was now investigating the optical microscopy of metal nanoparticles. While the two topics may seem unrelated, they both involved the interaction of light with materials - structured materials, in the former case; and metals, in the latter. My postdoc advisor, Shelly, had formed a company called Seashell Technologies to try and commercialize the metal nanoparticles for use as super bright labels in biological assays. As part of my postdoc research, I was to develop methods to simulate and better understand how light interacted with the metal nanoparticles and how we might optimize them.
Why metal nanoparticles?
As it turns out, certain metals scatter visible light like crazy. The incident electric fields of the light cause the electrons in the metal to oscillate, scattering the light with enormous efficiency. If you look at a bunch of silver nanoparticles under the right kind of microscope, they look like bright, colorful beacons, or colorful stars in the night sky. Physicists have longer understood the basics of why the metal nanoparticles are so bright: Light impinging on a metal sphere, for example, is a classical physics problem that can be solved exactly; all that is needed to describe the problem is a certain property of the metal called the "electric permittivity,"" which is denoted by the symbol ε. No matter how complicated are the microscopic details of a material, its response to electromagnetic waves (like light or radio waves) can be mostly understood by just two parameters: The electric permittivity, ε, and the magnetic permeability, μ. For most non-magnetic materials, like metals, you can ignore μ entirely, and pretty much describe all the optical properties of the material just by ε.
For certain metals at optical wavelengths, ε is a negative number. A nanoparticle that has a negative ε can actually bind light on its surface, squeezing the light into nanosized regions that are much smaller than the wavelength of light. This tight, localization of light is one of the main features of metal nanoparticles and what makes them so fascinating to study.
It also makes them really tough to study.
Because light can vary at a scale much smaller than the wavelength on a metal nanoparticle, it is hard to compute light scattering from a metal nanoparticle that has an arbitrary shape. Most electromagnetic simulators solve for the electric and magnetic fields at a discrete number of points within a volume. Normally, the density of points needed is a few times smaller than the wavelength. For metal nanoparticles, though, a much larger density is required, meaning way more points to capture the physics accurately. And so, the computational time shoots up as does the required memory. While the computational difficulties are much more tractable now, the "plasmon" problem was a major challenge in 1997!
Since our group was more on the experimental and conceptual side, it didn't seem like developing and debugging numerical methods was the most efficient task for me. Luckily, at a near-field conference I attended in the Czech Republic, I saw an inspiring talk by Olivier Martin (then a postdoc at the ETH in Zurich), who presented what looked like the ideal method for computing the properties of nanoparticles. His was the first talk of the conference, and I knew immediately that his method was the one we should be using! At the conference I was able to discuss the plasmon problem with Olivier, and he also became very intrigued with the challenge. He was very happy to visit our group in San Diego and help us get a more quantitative understanding of nanoparticles with different shapes. His method was great, and we collaborated closely for many years.
Still, it was a struggle to figure out exactly what was going on with these nanoparticles, especially when trying to match up the simulation predictions with what we were seeing under the microscope.
