Left-Handed Materials Go from Fact to Fiction and Back Again
Lenses that bend light the wrong way exist after all, according to the latest experiments
Microwaves refract negatively in left-handed materials. In the top image, showing light transmitted through a 26-degree prism of ordinary refracting material, the radiation (red indicates areas with more radiation, blue less) is refracted toward the right of a line perpendicular to the surface of the prism. In the second, showing refraction through a wedge of left-handed material, the waves bend toward the left side of the line, indicating a negative refractive index.
1 July 2003—Last week, when George Eleftheriades presented his latest research findings at a meeting of engineers and physicists in Philadelphia, he gave proof of a phenomenon that seems, on the face of it, impossible. With a planar slab of so-called left-handed material, a construct of conductors and inductors, he focused microwaves with a resolution better than a half wavelength—beyond the diffraction limit. Eleftheriades, associate professor of electrical and computer engineering at the University of Toronto (Canada), was using a material with a property that, until recently, was unknown in nature: if microwaves strike such negative-index-of-refraction materials at an angle, they bend inward, relative to a plane perpendicular to the refracting medium, rather than outward.
Left-handed materials initially were trumpeted as the key to radical new antenna designs and even ideal optical lenses with perfect powers of resolution. But, with the publication of critical papers arguing that their behavior had been misinterpreted, claims made for left-handed materials got bent badly out of shape last year. Some critics contended that negative refraction simply did not exist. ”I believe that left-handed materials will turn out to be another ’cold fusion,’ ” said veteran antenna designer and IEEE Fellow Robert Hansen, in a communication to IEEE Spectrum. (Hansen was reacting to a report the magazine carried, ”Left-handed Material Reacts to 3-D Light,” which appeared in the October 2002 issue, pp. 2425.)
In recent months, however, expert opinion has been turning back in favor of the materials and their strange properties. A number of experiments, among them the one Eleftheriades reported at the IEEE International Microwave Symposium in Philadelphia in June, have laid to rest some of the objections raised by critics. Several months before the Philadelphia symposium, at a meeting of the American Physical Society in Austin, Texas, last March, teams reported credibly that they had observed negative refraction in microwaves beamed into a prism-like wedge of left-handed material.
The experiments also seem to have opened the way to interesting applications in microwave communications, even if notions of a perfect optical lens capable of resolving features smaller than a wavelength ultimately turn out to be but a mirage. Commenting on the implications of the work he reported on last week, Eleftheriades predicted: ”Antennas, couplers, power dividers, multiplexers, phase shifters, and filters will be smaller, and will have more bandwidth and functionality than their traditional counterparts.”
Results vindicating negative refraction
One of the significant experiments supporting left-handedness was carried out by Andrew Houck, at the Massachusetts Institute of Technology (MIT) Media Lab (Cambridge), who published his findings in the 4 April issue of Physical Review Letters. Houck’s team used wedges made of metamaterials, composed of devices called split-ring resonators, and measured electric fields around the wedges in three dimensions. The result: a complete profile of the electric field. That profile was important because much of the doubt about negative refraction centered on the issue of what exactly was being detected where.
Skeptics had claimed that experimenters were really detecting evanescent waves, or ”near field” waves—waves generated in the metamaterial itself by internal reflection—rather than real refracted waves. Because of the strong absorption inside the metamaterials in the early experiments, it was difficult to distinguish between far-field and near-field waves, which diminish in amplitude and vanish almost immediately upon leaving the surface of a metamaterial.
Houck’s three-dimensional characterization of the external field [see images below] seemed to clearly address that objection, as did another finding reported in March, also in Applied Physics Letters. Claudio Parazzoli, an associate technical fellow at the Boeing Company’s Phantom Works (Seattle, Wash.), and his team said they detected negatively refracted waves at a remarkably long distance from a metamaterial containing split-ring resonating structures—far beyond the range of evanescent waves. Using a waveguide detector in open space at a distance of 3366 cm—several dozens of wavelengths from the metamaterial—they detected negatively refracted waves.
”This was the proof we were waiting for,” commented Olivier Martin, of the Swiss Federal Institute of Technology in Zurich. ”A near field would disappear almost completely at a distance of two wavelengths,” he told Spectrum.
A key test ahead
A stubborn critic of left-handed experiments, Manuel Nieto-Vesperinas, of the National Research Council of Spain in Madrid, agrees now that the work by Houck and Parazzoli do not have the problems associated with strong absorption by metamaterial in earlier experiments. Though he still has other concerns about the experiments, he believes the near and far fields have been successfully disentangled.
Not all the skeptics, though, are ready yet to concede even that. Nicolas Garcia, a colleague of Nieto-Vesperinas’ at Spain’s National Research Council, believes that in Houck’s experiment, absorption in the metamaterial is still strong enough so that more radiation passes through the thinner region, distorting the radiation pattern behind the wedge in such a way that it may mimic negative refraction.
Garcia supports that argument with optical experiments using a glass wedge and a very thin wedge made of gold to refract light waves, resulting in radiation patterns that are indeed confusing. Those experiments show, he argues, that the microwave experiments still fail to prove conclusively the existence of negative refraction.
In a paper soon to be published by Physical Review B, Garcia and several colleagues propose a different experiment that they believe will resolve the question about the existence of negative refraction once and for all. In the experiment that Garcia now is attempting, a microwave beam passing through a flat slab of left-handed material at an angle will split and emerge into two beams. If negative refraction really takes place, the beams will be much more separated from each other than they would if there were no negative refraction.
”This experiment will unequivocally close the discussion,” says Garcia.
As for Parazzoli’s work at Boeing, Garcia argues that calculations in his paper based on negative permittivity and negative permeability—at least until recently considered the necessary conditions of negative refraction—would result in a negative absorption as well. That is, the intensity of the beam would increase while it passes through the material, an obvious absurdity. ”This paper [by Parazolli] violates the laws of electrodynamics and thermodynamics,” Garcia says.
For nonspecialists in optics or microwave communications, the job of sorting out experimental findings is not helped by disagreements among the top experts on left-handed materials. The idea of a flat optical lens that escapes the diffraction limit, considered by some like Imperial College’s (London) John Pendry to be the Holy Grail of negative refraction, is considered by others to be flatly absurd. By the same token, the notion that negative permeability and permittivity are what’s needed to make negative refraction possible—a truism since the 1960s, when the Russian physicist Victor Veselago postulated the phenomenon—now are said to be not absolutely necessary after all.
Clifford Krowne of the Naval Research Laboratory in Washington, D.C., a theorist investigating the physical basis of negative refraction, feels that a new physics is involved. ”We are beaming waves around active devices that are quantum-sized [the structures have sizes in the same range as the wavelengths of the radiation],” he says. ”We are looking for basic physics solutions that haven’t been studied before.”
Despite the extraordinary properties of metamaterials, Rodger Walser, of the University of Texas (Austin), argues that they may turn out to be of merely academic interest, with little or no applicability to radio frequency engineering. Though he agrees that the carrier waves of microwave beams can be negatively refracted, he thinks that the modulation of the beams—which carries their information—will be lost because of the inhomogeneous nature and low intensity of the refracted wave.
”We have no problem with the carrier waves refracting. Our argument is that the information cannot negatively refract,” says Walser. If he’s right, the story of left-handed materials could end the same way cold fusion did, even if essentially different. While the materials and the phenomena do exist, whereas cold fusion does not, they may still have no practical use.