Topsy Turvy: Researchers Announce First True "Left-Handed" Material

Scientists at the University of California, San Diego have devised the world's first truly "left-handed" material, they announced at the APS March Meeting in Minneapolis. In this medium, light waves are expected to exhibit a reverse Doppler effect. That is, the light from a source coming toward you would be reddened and the light from a receding source would be blue shifted. The UCSD composite material, consisting of an assembly of copper rings and wires, should eventually have important optics and telecommunications applications.

To understand how a reverse Doppler shift and other bizarre optical effects come about, consider that a light wave is a set of mutually reinforcing oscillating electric and magnetic fields. The relationship between the fields and the light motion is described picturesquely by what physicists call the "right hand rule": if the fingers of the right hand represent the wave's electric field, and if the fingers curl around to the base of the hand, representing the magnetic field, then the outstretched thumb indicates the direction of the flow of light energy. Customarily one can depict the light beam moving through a medium as an advancing plane of radiation, and this plane, in turn, is equivalent to the sum of many constituent wavelets, also moving in the same direction as the energy flow. But in the UCSD composite medium this is not the case. The velocity of the wavelets runs opposite to the energy flow, and this makes the UCSD composite a "left handed substance," the first of its kind.

Such a material was first envisioned in the 1960's by the Russian physicist Victor Veselago of the Lebedev Physics Institute, who argued that a material with both a negative electric permittivity and a negative magnetic permeability would, when light passed through it, result in novel optical phenomena, including a reverse Doppler shift, an inverse Snell effect (the optical illusion which makes a pencil dipped into water seem to bend), and reverse Cerenkov radiation. Permittivity (denoted by the Greek letter epsilon) is a measure of a material's response to an applied electric field, while permeability (denoted by the letter mu) is a measure of the material's response to an applied magnetic field. In Veselago's day, no negative-mu materials were known, nor thought likely to exist. More recently, however, John Pendry of Imperial College has shown how negative-epsilon materials could be built from rows of wires and negative-mu materials from arrays of tiny resonant rings.

Now, Sheldon Schultz and David Smith of UCSD reported that they had followed Pendry's prescriptions and succeeded in constructing a material with both a negative mu and a negative epsilon, at least at microwave frequencies. The raw materials used, copper wires and copper rings, do not have unusual properties of their own and indeed are non-magnetic. But when incoming microwaves fall upon alternating rows of the rings and wires mounted on a playing-card-sized platform and set in a cavity, then a resonant reaction between the light and the whole of the ring-and-wire array sets up tiny induced currents, which contribute fields of their own. The net result is a set of fields moving to the left even as electromagnetic energy is moving to the right. This effective medium is an example of a "meta-material." Another example is a photonic crystal (consisting of stacks of tiny rods or solid material bored out with a honeycomb pattern of voids) which excludes light at certain frequencies.

At a late-breaking press conference in Minneapolis, Schultz and Smith said that having demonstrated that their medium possessed a negative mu and epsilon, they were now proceeding to explore the novel optical effects predicted by Veselago. Furthermore, they hope to adapt their design to accommodate shorter wavelengths. As for applications in microwave communications, a medium which focuses waves when other materials would disperse them (and vice versa) ought to be useful in improving existing delay lines, antennas, and filters.

Outside commentators at the press conference showed interest and curiosity. Marvin Cohen of UC Berkeley said that until he read the UCSD paper he had not thought a negative-mu material was possible. Walter Kohn of UC Santa Barbara, winner of the 1998 Nobel Prize in chemistry, considered the UCSD work ".an extremely interesting result. I would be surprised if there weren't interesting applications."

-Phillip F. Schewe; AIP Public Information