The magic of metamaterials

Function through structure” is something of a mantra for researchers of metamaterials, a truly twenty-first century field which aims to create materials with bizarre and fascinating optical and magnetic properties, invented here at Imperial by Professor Sir John Pendry.

“It was serendipity, really,” he told me. His office is warm and inviting, a far cry from the laser and steel-filled lab one might expect of the inventor of the invisibility cloak. “I was doing consultancy work with the Marconi Company in the 90s. They were interested in producing stealth cladding for warships which consisted of carbon fibres. These worked very well, but they didn’t know why.” “The carbon fibres strongly absorbed in the middle of the typical radar frequency band, but in a very narrow range; however, they found that if they spread them very thinly on paper, it becomes a broad band absorber over nearly the entire radar frequency range, but why?"

“Obviously, they were interacting with each other. My first impression was that they were touching, but my second take was that if you have a wire, there’s a magnetic field associated with it that’s inversely proportional to the radius. If the wires are very thin, you have a very powerful magnetic field. There’s a magnetic loop connecting the wires, so it’s magnetic induction. You wouldn’t think that, would you, just from putting a conductor down? That gave me the idea of function through structure.”

Marconi had unwittingly invented the first metamaterial. “They were so pleased with that result that they said, ‘go away and do something interesting that you like!’ So I went away, and what I did involved a series of structures whose properties were dependent on the structure rather than what they were made of.”

John Pendry’s publication of his theory in 1999 gave rise to the new topic of metamaterials, in which the elements or molecules that make up the material are secondary to the arrangement of its larger parts: while some metamaterials are discernible only on the nanoscale, some have structures that are easily visible, like magnetic split-rings.

“The next thing was to see if we could get some unusual magnetic properties, so we devised these little so-called split rings. They’re about 5mm across. If you put a magnetic field along here, the current tries to go round the loop but can’t because we’ve deliberately cut it, but there is some capacitance between the two loops, so the current can flow through that. What you’ve got with loops with an inductance and a gap with capacitance; t’s a little resonant circuit. This material has a very peculiar property: it behaves like an ordinary magnetic material below the resonant frequency, but if you tune the frequency above that, the response flips, so the applied magnetic field is trying to push one way, and the response is in the opposite direction!”

This was the first step towards one of the greatest early breakthroughs in the field: materials with a negative refractive index, where light bends the opposite way to normal materials. In 1967, Russian physicist Victor Veselago theorised that a substance with both a negative permittivity and permeability – the properties which define its response to electric and magnetic fields – would also have a negative refractive index.

“You never find this in nature.” Professor Pendry continues, “it was the realisation by my friend and collaborator David Smith in San Diego that with this new metamaterial which I announced in 1999, they could make in the laboratory a negative refractive material. His group made the first; that set off a lot of interest and a lot of controversy. It also stimulated me to think of what else you could do with negative refraction: if you build a lens out of this negative refractory material and you do it right – you have to do it exactly right – then that would break the so-called diffraction limit.

“It’s hard to explain how controversial these ideas were at the time, people got really upset and I really had to handle some very violent criticism. I think those people would have to agree with me now that negative refraction, sub-wavelength resolution is a valid concept.”

From this theory came the perfect lens, in which light can be focussed down to points far smaller than its wavelength, at the nanometre scale. This could lead to microscopes able to directly see large molecules, such as DNA. At such scales, the usual laws used to describe light rays break down: ‘transformation optics’ is the name given to manipulating light waves at this level.

“Have you heard of DARPA?” The USA’s Defense Advanced Research Projects Agency is most famous for inventing what would later become the Internet. “They had some funding problem, and invited me to a meeting. I wanted to show off this transformation optics, so I had to think of something that would surprise them – that’s how the invisibility cloak came into being. I realised I could use this transformation technique to hide an object, and oh boy, did they go wild on this! I didn’t expect them to take it quite as seriously as they did.”

The invisibility cloak is what Professor Pendry is most famous for in the public eye. “That led to, largely due to JK Rowling, a huge amount of popular interest. I think it was a great opportunity to sell science to people: everybody knows what light is; thanks to Harry Potter, everyone knows what a cloak is and that it’s something magical. If you say you can make one the public are immediately interested and say, ‘it’s magic! How can that be?!’ But you explain to them that it isn’t magic, it’s really just hard science.”

Small-scale, single-wavelength invisibility cloaks have been made, but we’re still very far away from creating one of the Deathly Hallows. More modest metamaterial products, however, are already being manufactured. “It’s a typical trajectory in engineering when you have a very new idea: the engineers want to do what they’re doing already, but just a little bit better, so they’re making lenses which are lighter and focus a bit better.”

Toyota are funding the development of metamaterial lenses to focus terahertz rays, which are found in the radars of cars with active cruise control systems, and in projects to make self-driving cars. Similar lenses can be used to track the motion of satellites without costly mechanically-moving dishes – essential for telecommunications in remote areas.

In Imperial’s Department of Electrical and Electronic engineering, Professor Richard Syms is developing metamaterial-based detectors for MRI machines, which allow safe and detailed imaging of the heart and brain.

Professor Pendry is keen to point out these beneficial areas of research: “people say it’s all about radar and stealth, killing people and so on, but it’s not: it’s about safety in cars, and health. In any place where you can use electromagnetism, you can probably find an application for metamaterials.”