Read more: “Instant Expert: Metamaterials”
Nature has given us materials with special properties, such as the reflecting metals we use in mirrors – now artificial structures are going further
How a material affects light falling upon it is dictated in part by its chemical composition, but its internal structure can have an even stronger influence. Silvered mirrors are highly reflecting, but black-and-white photographs also owe their blackness to silver – billions of nanometre-scale spheres of the metal embedded in the film. This dramatic difference arises because the silver spheres are much smaller than the wavelength of light.
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Metamaterials extend this concept with artificial structures that might be nanometres across for visible light, or as large as a few millimetres for microwave radiation. Their properties are engineered by manipulating their structure rather than their chemical composition.
The possibilities these materials open up are limited only by our imagination, and not by the number of elements in the periodic table. As a result, metamaterials research has exploded during the past decade. It has given us optical properties we once thought were impossible, including negative refraction never found in nature, and novel devices such as invisibility cloaks.
The road to metamaterials
The idea that a material’s internal structure could influence its response to light has been debated since before the time of James Clerk Maxwell, the 19th-century Scottish physicist who demonstrated how light, magnetism and electricity are all aspects the same phenomenon.
In the 1950s, researchers studying long-range radio communication built structures made from thin metallic wires to simulate how Earth’s ionosphere interacts with radio transmissions. But it wasn’t until the 1990s that we finally realised just how radically a material’s structure could influence its properties.
I helped the GEC-Marconi company to produce a so-called split-ring structure. Manufactured by etching a copper circuit board with rings a few millimetres across, it had a particularly strong response to radar signals, which produced electric currents in the copper that in turn produced an induced magnetic field (see image).
This structure had another interesting property. Most magnetic materials align in the same direction as the applied field, like a compass needle pointing north. In contrast, the new metamaterial aligned its magnetism in the opposite direction. In 2000, David Smith, then at the University of California, San Diego, took this split-ring structure and used it to make the first material capable of bending radiation in the opposite direction to normal materials such as glass. This was the negative refraction whose existence had been predicted decades earlier by Victor Veselago.
A decade on, and we now have metamaterials with exquisitely intricate structures, like the ones fashioned from microscopic helices of gold by Martin Wegener’s group at the Karlsruhe Institute of Technology in Germany (see image). When light shines on the material, electric currents are induced in the helices. Interestingly, the size of the currents depends strongly on whether the light is left or right-circularly polarised. The response is stronger than even a solution of sugar, one of the most “optically active” natural substances.
New optics, new theories
The Russian engineer Victor Veselago was one of the driving forces behind the early development of metamaterials. In 1967, he predicted the possibility of materials with a negative refractive index which would bend light the “wrong” way (see diagram).
All materials in nature have a positive refractive index. The refractive index is a measure of the property that makes submerged objects appear closer to the surface than they really are, and swimming pools shallow when they are not. The greater the refractive index of a material, the closer to the surface an object within it appears. The logical consequence of this law is that a fish in a pond filled with a negatively refracting material would appear to have jumped right out of it (left).
Veselago’s argument that negative refraction should be possible was complex. Light consists of both electric and magnetic fields, with energy shared equally between the two. Light propagates by tossing energy between the fields as if the electric and magnetic components were in a dance. Atoms in a material with a positive refractive index – water, glass or most other transparent materials – respond by aligning their own electric and magnetic fields to the applied fields so as to slow down the dance.
Veselago imagined a material that aligned its electric and magnetic fields in the opposite direction to those in the light beam. This, he reasoned, would reverse the dance and create the effect of negative refraction. His dream was only realised with the advent of metamaterials.
Read more: “Instant Expert: Metamaterials”
First invisibility cloak
David Smith was working at the University of California, San Diego, when he adapted the split-ring structure to create the first metamaterial with a negative refractive index (below, top). The copper loops change the magnetic response, while thin copper wires on the surface behind provide the required electrical response. Smith’s group has made extensive contributions to the development of metamaterials, including the first working invisibility cloak in 2006. It, too, operated at radar frequencies.