Metamaterials | History
WECS, Laguna Beach, and Meeting John Pendry
David Vier, Willie, Syrus and I completed the poster maybe a day before the Workshop on Electromagnetic Crystal Structures (WECS) in Laguna Beach that Eli Yablonovitch had invited me to. From the agenda, I saw that John Pendry was going to be there, and I was really interested to see what he would think about our approach to the wire medium. Shelly, who was also going up to the meeting, suggested we meet with John together, so we agreed on a time to meet at the poster session.
It was the first time I'd ever met John; he was incredibly good natured and extremely approachable. He graciously agreed to look over our poster, and after just a few minutes confirmed that we had an equivalent way of arriving at a microwave plasmonic medium. He was actually amused by the loop-wires that we had made.
"Who made all of those loops?" John wondered.
"Syrus and Willie, a couple of graduate students in our lab."
"Oh dear!" John kind of laughed. "Seems like a lot of work. I think I'll stick with my thin wires, thank you very much," he added, jokingly.
Thin wires, loop wires, it didn't matter to me; we had done it! We now had a plasmonic medium that was easy for us to make and measure, and which also had John Pendry's endorsement. I was completely satisfied.
"By the way, are you coming to my talk?" John asked. "I think youll find it very interesting as well."
Shelly and I, of course, attended all the talks at the meeting, and John's talk was top on our list. And the timing could not have been better: John was about to give his first talk on the split-ring medium.
I had started playing around with the wire medium to create something that would have a negative ? at microwave frequencies, with a goal of emulating the nanoparticles we were working with in our microscopy experiments. I had not thought beyond that possibility.
But John's talk changed that. He was now interested in trying to create artificial magnetism, and proposed a series of new structures, from conductors rolled up into little tubes like "Swiss rolls," to little, flat, rings with gaps in them, he called "split-ring resonators." Though there were no inherently magnetic materials in these structures - they were pure conductors - John predicted they would have a magnetic resonance, with a regions where the effective permeability, μ could be controlled, with large positive values or even negative values.
"A negative μ?! That's impossible! What does that even mean?" Shelly, who was sitting next to me during John's talk, was leaning over with a shocked look on his face. "John's crazy! There's no such thing as negative μ. I've never heard of it."
I thought about it. We all accepted that ? could be negative. Why should a negative μ be impossible? What was really exciting was the fact that John was now proposing an artificial magnetic structure that could control μ. With the wires, that could control ?, and the rings that could control μ, you could create materials with completely arbitrary electromagnetic properties. That's what was really amazing.
I returned to UCSD, fired up about demonstrating artificial magnetism. Shelly was also now somewhat interested, because he had believed that a negative μ was impossible, and was curious to see if this impossible material could be made. So, we were both motivated in our own ways to continue building microwave structures, at least for a little while longer.
Also, I'm pretty sure, Shelly and I were the only ones interested in building the artificial magnetic structures at that point.
You see, the WECS was a conference about photonic crystals. And, the vast majority of people working on photonic crystals are, naturally, interested in photonics, which usually is about visible light or light at near-infrared wavelengths. Probably ninety-nine percent of the people at the workshop had no interest in microwaves. When it came time for questions, there was an uncomfortable silence, except for one person. That person was George Merkel, a researcher from the Army Research Laboratory.
George came across as a little annoyed. "Everything you're talking about has been done before," George announced. "These ring structures, the wire structures - they were all looked at in the 40s and 50s, mostly by Army researchers. It's interesting stuff, but it didn't go anywhere. There was some thought of using the wire medium as a way to model wave propagation in the ionsphere, but it didn't catch on. Some people made lenses and other quasi-optical devices."
It turned out that George Merkel was right! He forwarded a big stack of old papers to Eli, who didn't have time to read them all and who then sent them on to me. The wire medium had indeed been discovered by Rotman, and the split-ring resonator as a magnetic material had been described by Scheulnekoff. In fact, their approach was an engineering one that was much closer to the way I was now thinking about these things.
One of the difficulties of all of those papers, though, was that the effective medium theories were rough and approximate, having been done before there were full-wave numerical simulators that would be able to give precise results. The formulas, even if approximate, were difficult to derive and cumbersome, sometimes expressed as infinite sums that would have to be truncated. Every time you would change the geometry, even a little, you might need to derive entirely different formulas. It was daunting! I could see how difficult it might have been to arrive at a well-designed artificial electromagnetic material using the approximate analytical formulas that gave the only viable design path at the time.
John Pendry's work, while an independent rediscovery of sorts, had cast artificial materials in a new light. It was the perspective of a physicist rather than an engineer, and that opened up new opportunities.
How Do You Measure Artificial Magnetism?
A wonderful coincidence happened after the WECS workshop. One of the IEEE journals was going to publish a special issue with papers relating to the talks given at the workshop. As is typical for scientific journals, the editor was tasked with sending out the various manuscripts to be reviewed, and used the conference participants as the primary reviewers. I received John Pendrys manuscript on artificial magnetism to review, which nicely contained all of the various structures he had described in his talk at Laguna Beach.
It seemed like fate was directing us.
So, I sent John an email asking if he minded if we started working on demonstrating artificial magnetism in the lab, using his unpublished manuscript as a guide. Syrus, Willie and David were all now pretty excited about artificial electromagnetic structures, since everything seemed to be working so smoothly and quickly. We had just finished up a paper on the split-wire structure that was about to be published in Applied Physics Letters. It seemed like there was at least some appetite out there in the world for microwave plasmons.
John wrote back that he would be very happy to see us work on the artificial magnetic structures, but also cautioned us that he was working with Mike Wiltshire at Marconi, a company in the UK. Mike was also trying to confirm John's predictions, and had quite a head start. It would be very likely they would beat us to publishing. But, since this was a hobby for us and we weren't trying to compete with anyone, I thought it was wonderful to have potentially others who were interested in the same structures.
After looking through the artificial magnetic structures John had proposed, I thought it would be easiest to use what John described as the Swiss roll structure. To make a Swiss roll, we just needed to take some foil and roll it up into a tube, like a cigar. Of course, I was busy with our main projects, as usual, and couldn't devote too much time to the experiments. But Willie Padilla had gotten really excited with the magnetic structures, and took over the lead in putting the structure together. He made a little apparatus to roll the foil into tubes, and diligently got to work making a bunch of them.
One problem, though. The Swiss rolls resonated at megahertz frequencies, which are radio waves, and have wavelengths of tens of meters or more. So making a medium that is several wavelengths in size would be a real challenge. Distracted enough with other things, I hadnt really thought that part through.
So, a few days later, I walked into the lab and Willie showed me about twenty Swiss rolls he had rolled. Each one had the right resonance, but how could we actually prove the resonance was magnetic? And, without putting the resonators into a medium, how could you actually prove an effective magnetic permeability, μ, which is a collective property of a material?
"That's great," I told Willie, admiring his work, "but I think we need more to do a measurement."
"Ok. I can make more," Willie responded. "How many do you think we need?"
"Oh, maybe two, three hundred. Maybe more."
Willie looked unhappy. He'd have to think of a faster way to roll those Swiss rolls. The current method would take far too long and would be way too laborious. And, there was limited time we could spend on the project. Shelly was already concerned that too much time was being wasted on all of this microwave stuff.
A few days later, Willie came back with a new idea.
"I think I can make the split-ring resonators, and I think theyre going to be easier. I checked with the electronics shop, and I think we can make the rings using lithography, like the way they make circuit boards. It's easy and I can make tons of them."
"I like the Swiss rolls." I countered. "The theory for the Swiss rolls is way more accurate. I don't trust the John's theory to be accurate for the split rings at all, aside from the qualitative features." The split rings were flat, disk-like loops of metal. To get an analytical expression for their effective permeability, John assumed that the sheets of split rings would be stacked so close together that they could be approximated as tubes. But, anything we could make would not be nearly so dense. In addition, John assumed the wavelength to be much larger than the split ring, but it was unlikely wed be able to make the rings smaller than about a tenth of the wavelength. The Swiss rolls, even though they were physically larger, were still hundreds of times smaller than the wavelengths at which they operated, so they were a better match for Johns theory.
"But this is so easy," Willie protested. "I can make them quickly, they're all the identical, and we can make as many as we need."
"How will we measure them?" I pondered.
"Can't we use the parallel plate waveguide, like we did for the wires?"
The parallel plate waveguide was two flat pieces of metal that could confine microwaves to propagate in a plane, making things two-dimensional. The parallel plate waveguide was really convenient because you needed a lot less material to do a measurement, since you forced all the energy to be in the plane. Willie was right: This was the way to go. And, he was doing all the work!
"Ok, let's do that then" I agreed.
Now the question became what kind of measurement we could do in a short amount of time. The entire project was unfunded and definitely not of any practical interest. I was only getting help from Willie because Willie and I were both just really intrigued with the possibilities of artificial materials. Syrus, though enthusiastic as well, had already cut his time on our project way back, needing to focus on his dissertation and graduating. With resources dropping, we wouldnt be able to carry on much longer. We had to work fast.
The split ring medium had a magnetic resonance, with a resonance frequency set by the geometry. Just below the resonance, there was a band of frequencies where the permeability was negative. In a very rough sense, a material with positive μ will be transparent, while a material with negative μ will be opaque. This property of a material could be used as a signature: Design the negative μ frequency band of the split ring medium to be right in the range of our measurement. All we would have to do would be to send a beam of microwaves onto the sample and see what made it through! If we saw a gap in the transmission - a range of frequencies where the transmission suddenly dropped - and if that gap coincided with our designed and simulated region of negative μ, then we could declare success. That was the plan.
Again, I worked with David Vier on the simulations to design the ring. I knew we couldn't use John's formulas directly, and that wed have to rely on numerical computation. Since I had worked on computing the properties of periodic structures for my dissertation, computing the properties of the split ring medium came naturally. Very quickly, David and I had a design for a split ring that had a lattice constant of one centimeter (perfect for the planar waveguide) and which resonated at around 5.8 GHz.
We gave the design to Willie, who made the samples within a day. The samples consisted of strips of circuit board with split rings of copper patterned on them. To hold them together, he inserted them into foam, which consists mostly of air and is thus nearly invisible to microwaves. The whole sample was inserted into the parallel plate waveguide, and then we measured the transmission.
It worked perfectly!
Right where we expected the artificial magnetic material to be opaque, the transmission dropped as expected, by orders of magnitude. We had succeeded in making an artificial magnetic structure. It's kind of amazing to think back on that moment, since there have been thousands and thousands of metamaterial papers since that time, with thousands and thousands of researchers simulating and analyzing these little split ring elements and their variants. But, at that moment, we were the first. (I did learn later that Mike Wiltshire at Marconi had actually tried the same technique and had lithographically produced very similar patterns of split rings and tried to measure them; he was also very close, but because he was not doing simulations, he could not form a conclusive argument about their properties.)
