'Perfect lens'debate continues
The discussion of the metal lens or artificial dielectric lens was reported in "Spanish researchers question claim of 'perfect lens' " (August, p. 50). This discovery was made more than 50 years ago. W. E. Koch did much of the original work that was published in "Metal-lens antennas" (IRE proc. 34, 828; November 1946) and "Metallic delay lenses" (Bell Sys. Tech. J. 27, 58; January 1948).
The book Antennas Theory and Practice by Sergei A. Schelkunoff and Harald T. Friis (1952) devotes nearly a whole chapter to the subject including a "loop loaded with capacitance" (quite similar to Smith's split-ring resonator). Patent law calls this "prior art." More recent antenna texts spend only a few pages on the topic.
Some years ago I designed and built a metallic horn lens antenna. The lens had a dielectric constant of less than one. The antenna had gain over an isotropic radiator but it does not amplify or create energy. After all, it is a passive device with some small amount of loss.
Eric V. Berger
[email protected]
The researcher replies . . .
Work on artificial materials has certainly gone on for at least 50 years, primarily in the defense and government research labs. However, to our knowledge, no artificial material, or "metamaterial" was ever introduced that had the material property of a negative index of refraction, which appears to have been first hypothesized by Veselago in 1964.
In terms of lenses, there are indeed many types of lenses used in microwave and RF technology that are composed of artificial materials—however, the sub-diffraction-limited focusing that Pendry noted in describing the "perfect lens"
is an effect entirely related to negative refraction. While the effect may be ultimately difficult to observe in the types of metamaterials we are currently working with, the concept has not been introduced in prior work, and is a significant motivating factor in the current development of metamaterials.
It is important to clarify that Pendry's "perfect lens" is a near-field effect, in which an image very close (within a wavelength) to a slab with permittivity and permeability both equal to -1, is transferred to the opposite side of the slab, with resolution well beyond any conventional (diffraction-limited) lens. While this may seem unusual from a traditional optics point of view, it is certainly consistent with the notion of near-field optics (often termed NSOM or SNOM).
David R. Smith
Associate Adjunct Professor
University of California, San Diego
[email protected]
Corrections
"Laser-based breath analyzer diagnoses asthma, schizophrenia" (June, p. 22) incorrectly noted that a breath analyzer made by Oridion Medical Systems (Jerusalem, Israel).requires the use of ozone tanks. According to Oridion, the system is a free-standing device that meets all safety and environmental applications. The system uses a proprietary technology called Molecular Correlation Spectrometry to monitor carbon dioxide levels in breath and analyze it for numerous medical applications.
"Fresnel lens yields three-dimensional measurements" (August, p. 28), referred to "Fresnel lens." It should have said "Fresnel zone plate." A zone plate works by interference, and a Fresnel lens by refraction (and in the case of lighthouse Fresnel lenses, by reflection, too). The focal length of a Fresnel lens is not proportional to the wavelength of light, but for a zone plate, it is.
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