29 Feb 2008
Marie Freebody speaks to metamaterial pioneer John Pendry about potential new developments in this exciting field, as well as the hurdles that still stand in the way.
John Pendry is a condensed matter theorist at Imperial College London, UK. In 1999, Pendry pioneered research into metamaterials and then the following year calculated that a negative-index medium could be used to make a perfect lens. Such a lens would focus an image with a resolution not restricted by the wavelength of light.
How do metamaterials work?
The way in which ordinary materials respond to light depends on the constituent atoms and molecules. For example, to make a wine glass sparkle, lead is added to the glass to make it more refractive. There are billions of glass molecules in one cubic wavelength so light sees a response that is averaged over these many molecules. In fact, the sub-units need not be as small as a molecule: they can perhaps be a few tens of nanometres in diameter and the light would still respond to the average response. The secret of a metamaterial is that we exploit the sub-wavelength structure as well as the chemistry to produce the properties that we desire. This is easily done at microwave frequencies where the wavelength is a few centimetres and sub-wavelength structures are a few millimetres across. Typical structures are thin metallic wires or rings.
Why is it important to develop this technology?
Combining structure with chemistry increases the range of electric and magnetic properties that can be engineered enormously. Before the advent of metamaterials, properties such as negative refraction could not be accessed using conventional materials.
What are the main applications of metamaterials and when do you see them occurring?
There are many applications that have yet to be invented and that is why they are so exciting. Firstly they will be deployed at microwave frequencies to improve existing inventions. For example, better, lighter lenses can be made for radar applications. Later, the technology will spread to terahertz frequencies and finally, in 5–10 years, to optical frequencies. Of course, when the wavelength is less than a micron, nanometre-structured metamaterials are required and that is quite a challenge.
What would you say is the most important recent advance?
The most important recent advance is the exploitation of coordinate transformation theory to prescribe the material properties needed to control electromagnetic radiation. This enabled the design and realization of an electromagnetic cloak, which we achieved in collaboration with David Smith's team at Duke University, US.
What are the key challenges that are left to overcome?
Metamaterials provide huge design flexibility, which in turn presents exciting challenges for new inventions. We need to extend the technology from the relatively easy-to-make millimetric-scale structures that work at microwave frequencies to much finer-scale structures for operation at shorter wavelengths. Another challenge at short wavelengths (high frequencies) is finding low-loss materials.
What do you think the next big breakthrough will be?
If I knew that then it would not be a breakthrough. There is a common misconception that breakthroughs can be ordered up at will, created in committees or by bureaucrats. Real breakthroughs as opposed to developments are products of human imagination that, thank goodness, is not to be predicted or commanded.
Who are the main research groups
currently focused on metamaterials?
In the early stages just a few groups worked in the field. In addition to my group at Imperial College, Smith's work has been extremely influential, but we were soon joined by many gifted teams: George Eleftheriades, Costas Soukoulis, Xiang Zhang, Vlad Shalaev, Roberto Merlin and Nikolay Zheludev.
• This article originally appeared in the February 2008 issue of Optics & Laser Europe magazine.