A Negative Index of Refraction

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 permeability (Update 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, 858-534-1510, drs@sdss.ucsd.edu) 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.)