About Negative Index of Refraction and the Superlens

What limits the sharpness of an image? The answer has to do with the wave nature of light.  If you look at a bright light from a distance at night, and you bring your fingertips together so you see the light through the tiny space between them, you'll see bands.  These bands are a diffraction pattern, caused by the interference of light at different parts of the opening between your fingertips. The closer you bring your fingers together, the more the bands spread out. (see photos).

This photograph shows the diffraction of red laser light as it passes through slits of different widths.  Notice that the narrower the slit, the wider the diffraction pattern. (photo credit: Jaime Lush, Harvard University Science Center)

Diffraction pattern of a point source made by circular opening; a star imaged by a telescope makes a pattern like this. (photo credit: C.C. Jones, Union College, Schenectady, N.Y)

These drawings compare the path of light rays through a prism of glass (above) and of a negative-index-of-refraction material (below), both in air.

This drawing shows a metamaterial, composed of an array of wires and split metal rings. When illuminated by radiation with a wavelength much longer in the size of the wires and rings, this material has a negative index of refraction. (image courtesy of NASA)

Diffraction is caused when light strikes an edge and bends around it.  When you take a photograph, the light passes through the circular opening of the lens, and the edge of the lens produces diffraction bands in the image.  The photo shows diffraction rings—called Airy rings—produced by a circular aperture limits the fineness of detail, the resolution, that a lens can capture.

But not all the light that strikes a small object winds up in the diffraction pattern.  Some of the light, called "near-field" or "evanescent," decays away so quickly with distance that it has all but disappeared within a wavelength or two, but this light contains information about the sub-wavelength detail of the object. In 2000, the English physicist John Pendry predicted that this "near-field" light could be imaged by a lens made of a material with a negative index of refraction.

The index of refraction specifies how a material bends light.  For example, as light leaves a glass prism and enters air, the light beam bends as shown in the upper drawing, which corresponds to a positive index of refraction.  But if the prism material has a negative index of refraction, the light would bend as shown in the lower drawing (see sidebar).

Negative index of refraction materials had been predicted back in the 1960s. In a theoretical study, the Soviet physicist Victor Veselago concluded that these materials would be consistent with the laws of physics, and would exhibit quite interesting behavior, including:

• reversal of the Doppler effect (light emitted by a receding object would be blue-shifted rather than red-shifted)
• backwards emission of Cherenkov radiation (radiation emitted by objects moving in a material at a speed faster than the speed of light in that material)
• backwards energy flow, compared with the direction that the light waves move.

Much later, in the 1990s, the English physicist John Pendry recognized that a lens need not be homogeneous, like glass or water, but rather could be a construction of different materials. The result, dubbed a “metamaterial,” refracted light, provided the constituents were all much smaller than the wavelength of the light. Typically, metamaterials are made from an array of wires and split metal rings, as shown in the photo.  When light strikes this lens, the electrons move back and forth in the wires and rings, producing electric and magnetic fields.

In 2000, David Smith and a group at the University of California San Diego used millimeter-size constitutents to create a metamaterial lens that refracted a radar beam in a direction that corresponded to negative index of refraction. After some further theoretical work, the existence of these new materials was generally accepted.