The Perfect Lens: Resolution Beyond the Limits of Wavelength
published: Jan. 6, 2014, recorded: March 2007, views: 88
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According to 1 Corinthians, “For now, we see through a glass, darkly.” But according to Sir John Pendry, now we can actually see through perfectly – not through glass, though. The perfect view is a product of materials science married to theoretical genius, Pendry’s insights into the physics of light and the surprising concept of “negative refraction.”
We have all observed refraction, in the deep end of a swimming pool, for example, where the water looks shallower than it really is. In fact, you can easily calculate the refractive index of water: it’s the actual depth divided by the apparent depth. Unfortunately, that is the only simple mathematical idea in this lecture. The index of refraction in nature is always greater than zero. Building on ideas from Fermat and Maxwell – whose equations, especially the parameters magnetic permeability and electric permittivity, are central to the argument – Pendry uses geometry to persuade us that refraction can in principle be negative. His argument was sharply disputed in the physics literature, and Pendry jokes that he earned his knighthood in combat, using equations as lances.
Why was this controversy so heated? The theory points to a marvelous conclusion: in a material with a refractive index of exactly negative one (-1), the optical distance from an object to its image exactly cancels out. In other words, the image is the object, and therefore we can say that such material functions as a perfect lens. This would enable scientists to defeat a limitation previously considered fundamental: that no lens can resolve more finely than the wavelength of light. Thanks to negative refraction, Sir John claimed, a perfect lens could resolve produce a sharp image of objects smaller than the light used to illuminate them.
Proof came from experiments. Because no natural material demonstrates negative refraction, scientists harnessed metamaterials (a term coined only in 1999). A metamaterial is defined not by its composition but by its structure – a manmade, three-dimensional, periodic cellular architecture designed to produce an optimized response to specific excitation. Researchers at UC San Diego, Boeing, Berkeley, and elsewhere have now produced clear, sharply focused images of text and gratings inscribed at sub-nanometer scales but illuminated at much longer wavelengths.
Pendry emphasizes that negative refraction is still a radically new concept in optics, so truly breakthrough industrial applications have yet to be imagined. But just as the laser developed from a laboratory curiosity to revolutionize medicine and communications, Sir John expects that negative refraction may have enormous potential. “Maybe one of the young grad students in the audience,” he concludes, “will go out and build this stuff, and hopefully make a few billions that he gives back to MIT.”
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