"Left-Handed" Materials Could Make Perfect Lenses

UC San Diego Physicists have devised a composite material in which the effective electrical permittivity, and the magnetic permeability are both negative. This leads to bizarre optical properties. a) A top view of the experimental setup. Microwaves enter a cavity from the left. They fall on the composite material, consisting of alternation rows of rods and thin copper patterned disks (1 cm in diameter). b) A sideview of the microwaves hitting the composite material.

The controversy over whether left-handed materials (LHMs) can be realized appears to be resolved, thanks to new experimental results reported by speakers at the APS March Meeting in Austin, Texas. LHMs are defined as materials with a negative index of refraction. LHMs bend microwave and light beams the opposite way to ordinary lenses, and because they can, in principle, focus light without the need for curved surfaces, they have the potential of making the "perfect" lens and entirely new classes of electronic and optical devices.

In 1968 Victor Veselago of the Lebedev Physics Institute in Moscow argued that a material with both a negative electric permittivity and magnetic permeability would result in novel optical phenomena when light passed through it, including a reverse Doppler shift (wherein the light from a source coming toward you would be reddened and the light from a receding source would be blue-shifted), reverse Cerenkov radiation, and an inverse Snell's law-the index of refraction of the material is negative. Permittivity (epsilon) is a measure of a material's response to an applied electric field, while permeability (mu) is a measure of the material's response to an applied magnetic field.

It is rare for a material to have either negative permittivity or negative permeability, much less both, and until a few years ago, no such materials were known nor thought likely to exist. They certainly do not occur naturally. But in 1999, John Pendry of Imperial College showed how negative-epsilon materials could be built from rows of wires and negative-mu materials from arrays of tiny resonant rings. His material consisted of alternating layers of metal rods and "C" shaped rings lodged on a honeycomb array of printed circuit boards. Following his prescriptions, Sheldon Schultz and David Smith of the University of California, San Diego, succeeded in in constructing such a material at microwave frequencies, using copper wires and rings.

But then a group at the University of Texas, Austin, contended that the earlier studies of negative refraction failed to account for both the group and phase characteristics of electromagnetic waves, while another group at the Consejo Superior de Investigaciones Cientificas in Madrid believed that reports of perfect lensing made false assumptions about the behavior of radiation in LHMs. In response, Pendry insisted that both the Spanish and US studies were "seriously in error."

Now scientists at several labs have reported the experiments that have verified Pendry's original finding, effectively putting to rest at least that aspect of the ongoing work on LHMs. At the March meeting, two labs reported devising LHMs of their own and demonstrating a negative-index behavior when microwaves were sent into a wedge-shaped LHM "prism." A group from MIT, represented at the meeting by Andrew Houck, said that microwaves entering an LHM sample were, sure enough, refracted according to Snell's law, but with a negative sign.

The MIT experiment also provides evidence that light from a point source can be focused with a flat rectangular slab of LHM material.

Patanjali Parimi (Northeastern University) also reported at the meeting that his team of scientists had observed negative-index propagation on microwaves through a LHM sample. [For background and some simple movies, See http://sagar.physics.neu.edu/].

Two theorists present at the meeting, Clifford Krowne (Naval Research Laboratory) and Alexandre Pokrovski (University of Utah), affirmed that the experimental results had indeed established the existence of working left handed meta-materials but that an earlier criterion thought necessary for LHM behavior—namely that the material's permittivity and its permeability both had to be negative—was not strictly required.

Potential applications in the cell-phone industry alone are many: LHM devices would be handy for filtering, steering, and focusing microwaves.