Negative Refraction Confirmed
Daniel S. Burgess
The latest salvo in the battle over negative refractive index materials has been fired, and it has proponents of their existence declaring a victory. Independent groups at the Massachusetts Institute of Technology's Media Labor-atory in Cambridge and at Boeing Phantom Works in Seattle have presented the results of Snell's law experiments that demonstrate that some substances do refract through a negative angle.
"Negative refraction is a real phenomenon," said Andrew A. Houck, a researcher at the lab and at Harvard University, also in Cambridge. He acknowledged, however, that it remains unknown whether it will be possible to fabricate so-called "superlenses" that enable subwavelength focusing, perhaps the most amazing hypothetical consequences of the effect that could revolutionize applications such as photolithography and optical memory storage.
Nearly 40 years ago, V.G. Veselago posited that materials with a negative dielectric constant and permeability would have a negative refractive index, refracting an incident ray by the same magnitude but in the opposite direction from the normal line as a typical material. If they existed, these "left-handed substances" would enable the design of novel optical elements, such as flat-surfaced lenses, Veselago argued.
Then, in 2000, John B. Pendry of Imperial College of Science, Technology and Medicine in London took the concept further, hypothesizing that a slab of left-handed material would amplify the evanescent components of the wave field, thereby acting as a lens with a diffraction limit of zero. Following this work and a report of the experimental confirmation of the phenomenon by a group at the University of California, San Diego, negative refraction faced greater scrutiny, with theorists suggesting that the existence of the materials would threaten fundamental principles such as causality and finite signal speed. It was noted that more research would be required to settle the matter, and the new work appears to do just that.
Figure 1. Experimental measurements of the transmission of microwaves through composite materials such as this from the Massachusetts Institute of Technology Media Laboratory have confirmed the existence of negative refraction. Images courtesy of Andrew A. Houck.
Houck's team monitored the transmission of 10.5-GHz microwaves through prisms of right-handed Teflon and of a left-handed composite of tin-plated copper split rings and wires in a circuit board substrate (Figure 1). Using a translation stage to generate two-dimensional profiles of the propagating electric field, the team measured output angles of 6.4° and 9° for the left-handed prisms with an 18° or 26° exit face, respectively, corresponding to a refractive index of -0.35. The research group at Boeing measured transmitted 12.6- to 13.2-GHz microwaves at distances of 33 and 66 cm from the entrance face of their composite material, which displayed a refractive index of 1.03.
In a further confirmation of the Veselago predictions, a 6-cm-thick flat slab of the MIT material exhibited a focusing effect, concentrating the transmitted power to a point approximately 2.5 cm from the exit face (Figure 2). Houck noted that the material was rather lossy, attributable in part to the presence of a thin carbon film left on the surfaces by the high-power computer-controlled laser cutter that the researchers use to fabricate the arrays.
Figure 2. The MIT group has verified predictions that a flat slab of left-handed material would focus radiation like a lens. The microwave source is at the bottom center of the figure, and the blue region above the sample was not scanned. Note the high degree of attenuation through the material, one of the challenges to further development.
Neat trick, or more?
Attenuation in these materials is an immediate challenge, he said, and their complicated design continues to make it difficult to predict precisely what their refractive index will be for a particular wavelength. He is confident that both issues are solvable and said that he would not be surprised to see left-handed materials for optical wavelengths in the near future.
Nevertheless, it is uncertain where this work will lead. "The field seems to be at a crucial position right now," Houck said. "It seems clear that the phenomenon exists, and it will be seen over the next few years whether this can be harnessed and improved upon, or whether this is simply a flash-in-the-pan, 'neat' trick of physics."
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