What are metamaterials?
Lipstick, credit-card chips, carbon fibre and catalytic converters are used every day without a second thought about the complex science behind their creation.
But these items could not exist without putting atoms together in new ways to make metamaterials – fascinating structures that could also unlock the possibilities of invisibility and quantum computing.
Professor Roy Sambles, a professor in experimental physics and head of Exeter’s Electromagnetic and Acoustic Materials research group is giving a public lecture, as part of the Students’ Guild Research Uncovered Series, on how the materials of the future could be man-made creations.
Ahead of the lecture we asked Roy what are metamaterials? How they are created? And, what real-word applications they may have?
What are metamaterials?
We have all heard of the periodic table of elements – nature can put these elements together to make very different materials. Put sodium, a violent and reactive metal, with chlorine, a highly reactive gas, and you get something everyone loves and uses – salt.
But how does this relate to metamaterials?
Roy explained: “If I take a metal, such as copper [a good conductor of electricity], and I put it together with a dielectric [a poor conductor of electricity] such as glass all I have to start with is a piece of copper and a piece of glass – this is dull.
“But if when I put them together they are smaller than the wavelength of the wave with which I am interrogating them I can build synthetic atoms – they are not atoms of course – but I can make little structures that have properties I have designed in.”
Roy explained that this was accidently done by pre-Romans when they put tiny particles of gold in glass to create purple windows.
He said: “Gold in particle form behaves differently, and these particles were smaller than the wavelength of light. Light then interacts with the particles in a funny way and gives you bright colours – in this case purple. In pre-Roman times people working with glass knew this would happen but they didn’t know why!”
He added: “This is part of what we are doing. We are playing with small objects and designing them with specific properties, opening up a huge toolkit for materials scientists to play with.”
How do you create a metamaterial?
Making a metamaterial involves making something small then interrogating it with different wavelengths of light, radio, sound and microwaves.
Humans are fantastically good at decoding information collected in our eyes and ears. Structures that are on the scale of sound waves, which have wavelengths in the centimetre range, are easy to fabricate and can cleverly manipulate sound.
We can, in principle, do the same with light – but it is difficult to make something small enough to work in the visible domain because light waves are very short.
However you can make a lot of interesting materials with new and weird properties for use at microwave frequencies.
What metamaterials do we already have?
Credit-card chips, carbon fibre, catalytic converters and lipstick – a powder in a base sometimes with sheen affects created by tiny flat plates.
Also satellite TV dishes, Roy explained: “Your TV dish can be a mesh because the radiation wavelength is bigger than the size of the hole so it can’t get through. The structure collectively responds like a perfect mirror for microwaves even though to you and I it is quite transparent.”
What could we use metamaterials for?
One exciting and potentially life-saving application is protecting buildings from the damaging ground travelling sound waves of earthquakes. This could be done by directing sound waves around a building by coating the foundations in a metamaterial structured to deflect them.
We could also be moving towards a world where electronics are replaced by light (optical processing) because light is quicker – it would also avoid down conversion of optical fibre signals to electronic and back again. But light needs to be slowed down to be manipulated – and stored – which isn’t easy.
Roy explained: “We are talking about storing it not for a day or a week, but for a nanosecond. That is a long time in terms of light!
“Once you can store light you can use it as the core piece of computing. One of the possibilities is building a quantum computer.”
Metamaterials could have medical application such as helping detect and predict illnesses by telling us exactly what is in a blood or plasma sample molecule by molecule within seconds.
Synthetic metamaterials could also be used to develop bio-compatible materials, such as bone replacements, that are strong, robust, and won’t be rejected by the body.
At Exeter we are looking at the next generation of communication technology – 5G – and whether aerials and antennas can be made smaller and flatter.
Roy revealed: “There is world-wide excitement about this. We are doing the underpinning, looking at the basic physics of how they might work and what we can do with them.
Roy is also working with electrical systems company Thales to look at the positioning and tensioning of anchors for oil rigs.
Roy said: “You can’t do this with electromagnetic waves because the ocean conducts electricity, but you can do it with sound waves.
“We are developing improved ways of getting sound into the ocean out of an ultra-sound generator. If we can make new microphones and transducers for sound we can also improve ultra-sound imaging of us because the same problem, to some degree, arises in us.”
Could we make things invisible?
Despite a metamaterial being created that enables you to cloak, or hide, something Roy doesn’t believe Harry Potter style invisibility will ever be possible!
He said: “It might be possible to achieve somewhat better than has been achieved so far. It might be possible to hide a tank to some radar wavelengths. But it is impossible to hide a person to visible radiation because the spectral issues are colossal.”
Roy has been involved in work to make things invisible to radar.
He explained: “When planes land at Heathrow, air traffic control relies on their radar system, but if you are not careful this sees a reflection of the aircraft in adjacent buildings.
“This issue can be solved by cloaking the building in a metamaterial that is transparent to visible radiation, so it is not obvious to everybody, but absorbs or removes radar.”
Roy has worked with defence and aerospace security company QinetiQ to develop a similar approach to cloak the blades of wind turbines to stop them reflecting radar.
Work with QinetiQ is also developing anti-counterfeiting devices that may be incorporated into glass or labels to make things almost impossible to forge.
Roy will be talking about electromagnetic metamaterials for Exeter Guild’s Research Uncovered event on 24 February 2014.