A negative index of refraction in a medium has been demonstrated experimentally
for the first time. When light or any other electromagnetic radiation
passes from one substance to another, the angle at which it travels
in the new medium is described by Snell's law, a relation that includes
the incoming angle as well as each substance's index of refraction,
the ratio of the speed of light in vacuum to its velocity in the medium.
Unusual refraction behavior has been expected in "left handed
materials," composites introduced last year which, when electromagnetic
radiation is incident upon them, produce electromagnetic responses with
negative values of properties known as electric permittivity and magnetic
476). However, the first left-handed composites, which consisted
of arrays of posts and rings arranged in a specific pattern, exhibited
left-handed effects in only one dimension.
With the aid of computer design, UC-San Diego researchers (David Smith,
have constructed a left-handed composite material that exhibits left-handed
effects in two dimensions, enabling demonstrations of the unusual refraction.
The new composites consist of wires and cut rings of copper inside a
fiberglass sheet and arranged in square boxes like the inserts in a
wine crate. Shining microwaves at frequencies similar to those used
in police radar, the researchers found that the microwaves refract in
an opposite direction from what one gets in all other known materials.
More specifically, the microwaves have a negative "phase"
velocity in the left-handed composite, giving it a negative index of
refraction. In this setting, negative phase velocity means that the
microwave appears to move backwards, in a direction opposed to the flow
of microwave energy. (A helpful
website illustrates this and many other points.
Such materials have many potential applications; for example, they
can be used create more highly directional antennas which would create
more beamlike radiation patterns; this would allow more antennas to
be placed in closely packed space, something that can benefit telecommunications
(Shelby et al., Science,
6 April 2001.)