The idea of backward light
In all natural materials, from air to water to glass, light bends in a predictable way. This property, called the refractive index, is always a positive number. But in 1967, Soviet physicist Victor Veselago published a theoretical paper exploring what would happen if a material had a negative refractive index. He predicted that in such a medium, light would bend the "wrong" way at an interface, and many optical phenomena would be reversed. For over three decades, this concept remained a theoretical curiosity because no such materials were known to exist.
The situation changed at the turn of the century, largely due to theoretical work conducted here at Imperial College London's Blackett Laboratory. Professor Sir John Pendry, a condensed matter theorist, began investigating artificial structures whose electromagnetic properties arise from their physical design rather than their chemical composition. These structures are called metamaterials. Pendry theorized that by arranging specific micro-structures in a repeating pattern, it would be possible to create a bulk material that interacts with electromagnetic waves in ways nature forbids. This work provided the theoretical basis for creating the first negative index material.
Building an impossible material
To achieve a negative refractive index, a material needs both negative electrical permittivity and negative magnetic permeability. While some materials have negative permittivity, negative permeability was the real challenge. In 1999, Pendry's group proposed a design that could produce this effect: an array of non-magnetic, conductive rings with small gaps, called split-ring resonators (SRRs). When an electromagnetic wave passes through, the SRRs create a resonant magnetic response that results in an effective negative permeability over a specific frequency range.
Combining Pendry's theoretical SRRs for negative permeability with an array of thin wires to produce negative permittivity, a team at the University of California, San Diego, led by David R. Smith, experimentally demonstrated the first negative index metamaterial in 2000. The following year, they built a prism from this new material and confirmed Veselago's prediction: it bent a beam of microwaves in the negative direction.
In a landmark 2000 paper, Pendry also proposed that a flat slab of negative-index material could function as a "perfect lens". Such a device could focus light, including the near-field waves that decay quickly and are lost in conventional lenses. This would allow imaging with a resolution far beyond the diffraction limit, which normally restricts detail to about half the wavelength of the light used.