- Radical Lens Theory Repeals Diffraction Limit
Daniel C. McCarthy
LONDON -- Thinking positive doesn't always produce the best results. For centuries, lens makers so relied on materials with a positive refractive index to fabricate optics that they never questioned the intrinsic trade-off of these materials. No matter how efficiently conventional optics are formed or polished, they have a diffraction limit, which demarcates the limit of their accuracy. This intrinsic limit on the resolution that optics can provide is in the nature of a law. However, a series of equations formulated by John B. Pendry, a physicist at Imperial College, have found a loophole in that law.
Conventional lenses deflect incident light at an angle measurable as the refractive index. A material with an index of zero, for example, doesn't bend light at all. The index of glass is about 1.4, and lens makers compensate for this deflection by curving the surface of the lens to focus light rays at a point beyond the lens.
Pendry's work extends from established theories that postulate the behavior of materials with negative refractive indices. Such materials, if they existed, would also deflect light, but at an angle inverse to conventional lenses. This attribute would enable flat-surfaced optics that focus light rays into a mirror image of the source.
with a negative refractive index have an angle of deflection inverse to conventional media with positive refractive indices. If scientists succeed in developing negative refractive index materials, they will
realize flat-surfaced "superlenses" that focus all components of light -- including the near field -- into a mirror image of the source.
But Pendry goes further, arguing that negative refraction allows not only flat lenses, but also "superlenses" that have zero diffraction limit and that transmit images with perfect resolution. The diffraction limit of standard lenses arises from their ability to transmit only the radiant components of incident light. Other components, called the near field -- or evanescent waves, by Pendry -- decay a short distance beyond the lens, limiting information in focused images to the longer wavelengths of the source.
Pendry's calculations, which were published in the Oct. 30, 2000, issue of Physical Review Letters, demonstrate how a material with a refraction index of -1 actually amplifies near-field light enough to extend its range. This hints at several potential applications, such as optical lithography and data storage, where short wavelengths are in demand. If Pendry's theories translate into real-world materials, either sector could be less dependent on shorter-wavelength light sources and, instead, apply the short wavelengths of the near field.
But Pendry admits it is easy for a theoretician to speculate on such goals. "I think we need a proof of concept," he said. "I realize there's a lot of research to do before this can happen."
Researchers at the University of San Diego have demonstrated negative refractive index materials that amplify the near field for microwaves, but little has happened in the optical region. Thomas W. Ebbesen, a physical chemist at Louis Pasteur University in Strasbourg, France, plans to test Pendry's models in the optical range using thin-structured metal films.
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