Metamaterial bends microwaves into beam
Foam-packed copper grid
broadcasts focused radiation for telecommunications.
23 November 2002
PHILIP BALL
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| Radiation can escape
from the material only as a beam perpendicular
to the surface. |
| © S.
Enoch |
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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.
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