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Metamaterial bends microwaves into beam

Foam-packed copper grid broadcasts focused radiation for telecommunications.
23 November 2002


Radiation can escape from the material only as a beam perpendicular to the surface.
© S. Enoch

A material that emits a focused beam of microwaves could form the basis of a new kind of directional antenna for telecommunication and satellite signals.

Microwave signals are currently marshalled into a narrow beam by broadcasting them from a parabolic reflector. A flat slab of the new 'metamaterial' would offer a "very compact and easy to manufacture" alternative, comments physicist John Pendry of Imperial College in London. It might also work as an interference-free receiver, picking up signals from one direction only.

The new metamaterial, developed by Stephan Enoch and colleagues at the Institut Fresnel in Marseille, consists of copper wires arrayed in grids of 5-millimetre squares 1 . These grids are stacked in layers, separated by 6-millimetre-thick slabs of foam; the wires focus the microwaves emitted from a cable embedded between the layers.

Because the metamaterial has a tiny refractive index, less than that of air, it bends radiation passing from it into air. The radiation leaves the surface at close to a right angle - no matter which way the ray was initially travelling. In other words, the only way that the radiation can escape from a slice of the metamaterial is as a beam perpendicular to the surface.

The trick only works for radiation of the same wavelength as the spacing between the components of the metamaterial. A few millimetres corresponds to the wavelength of microwave radiation. For a material to focus visible light in the same way, the components would have to be much closer together. Such a metamaterial could improve fibre-optic telecommunications and display technology.

Round the bend

Over the past few years, several groups have found that metamaterials made from regular arrays of metal wires and plates do bizarre things to electromagnetic radiation - microwaves, light and radio waves. The electrons in the metal components interact with the electromagnetic waves, altering the waves' course.

Several metamaterials bend light the 'wrong' way - they have a negative refractive index. Normal materials such as glass and water have a positive refractive index: they bend light in a certain way when it passes from air into the material. This makes a swimming pool, for example, look shallower than it really is when viewed from above the water. If water had a negative refractive index, the pool would look deeper.

Pendry and others are exploring the possibility of using negative-refracting metamaterials to create new types of lens.

  1. Enoch, S., Tayeb, G., Sabouroux, P., Guérin, N. & Vincent, P. A metamaterial for directive emission. Physical Review Letters, 89, 213902, (2002). |Article|
  2. Temelkuran, B. et al. Photonic crystal-based resonant antenna with a very high directivity. Journal of Applied Physics, 87, 603 - 605 , (2000). |Article|

© Nature News Service / Macmillan Magazines Ltd 2002

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