Metamaterial - Rewriting the Rules for Optics

Posted on Wednesday 2nd April 2014

invisible101Following his Inaugural Lecture, Ros Dodd met Professor Shuang Zhang to find out more about the internationally leading Birmingham research addressing the challenges of metamaterials and invisibility.

It is the stuff of science-fiction movies, but the idea of an ‘invisibility cloak’ is not as fantastical as it sounds.

In fact, groundbreaking research being done by Professor Shuang Zhang offers the tantalising possibility that – in perhaps little more than a decade – soldiers will be able to make themselves invisible to the enemy in a war zone.

Shuang, Professor of Metamaterials in the School of Physics and Astronomy, and his research group have already successfully ‘disappeared’ small objects such as a pinhead and a paper clip and are now working on making invisible large objects, including human beings.

The way this is done is through the manipulation of light waves by developing materials capable of hiding from view an object by ‘splitting’ light and conformally bending the waves around it. These novel materials – known as metamaterials, which are materials engineered to have exotic optical properties, such as negative refractive indices, which may not be found in Nature – diffuse light so that even the target’s shadow is obscured.

This mind-blowing research has already won several plaudits: it was among the ‘top ten breakthroughs for 2010’ named by Physics World, the ‘top 100 stories of 2011’ by Discover magazine and Shuang won the 2010 IUPAP (International Union of Pure and Applied Physics) Young Scientist Prize in Optics award.

One type of invisibility cloak can be made using natural materials such as birefringent calcite crystal. But, explains Shuang, this gives an object only surface or ‘carpet’ invisibility. His aim is for complete invisibility.

‘For a cloak to work in free space – in other words, to make something completely invisible – you have to use metamaterials, and that is what we are working on at the moment.

‘We are also working on developing a switchable invisibility cloak. By applying electrical voltage to the liquid crystal, you can change the orientation of the molecules and by so doing can “switch off” visibility and then switch it back on again.

‘In principle, this research could make soldiers invisible in the field. But we are at least ten years away from that.’

The same science is being used to analogue certain astrophysics phenomena, such as using metamaterials to create artificial black holes.

‘We can construct a metamaterial that can mimic what happens in a photonic black hole – a region of space from which light cannot escape,’ says Shuang. ‘So the metamaterial is designed to suck light into the centre. We had a theoretical paper on this published in 2009, and are now only two to three years away from proving that it can be done.’

It is the breadth of work that can be done using metamaterials that so fascinates Shuang. ‘Creating an invisibility cloak and mimicking a black hole are just subsets of our research here.’

His group is also engaged in the emerging field of metasurfaces – which consist of a monolayer of photonic artificial atoms.

 ‘We are coming to it with a different approach: We are using the structured surface, which is already thin but can be more powerful than optical elements such as a lens, as a dual polarity lens that can switch between a concave or convex lens. The same technique can be used to make high-definition, three-dimensional holography with a wide field of view. This is very exciting and very practical, because it can be used for ultra-dense data storage and 3D display.’

What Shuang also finds fascinating is that metamaterials research – being relatively new – throws up novel physics along the way.

Born in China, Shuang came to Birmingham four years ago as a Reader and was promoted to Professor in March last year.

His first degree was in Physics, after which he left China for the US, where he completed his Masters at Northeastern University, Boston and his PhD – in Electrical Engineering – from the University of New Mexico, Albuquerque.

In 2009, while working as an Assistant Research Engineer at the University of California, Berkeley, he received an email from Birmingham.

‘It was from the then-Head of the School of Physics, who said that Birmingham was setting up a research centre in metamaterials and would I be interested in the initiative?’ says Shuang. ‘I thought it was fantastic, because the University was investing £1.5m in the centre. I visited and was really impressed with what I saw, so I agreed to come and help build the new centre.

‘It is progressing very well in terms of research. We have generated a lot of research articles and research grants and are attracting some of the best young theorists in the world. So I am glad I came to Birmingham – it was the right decision.’