Physicists in the US have demonstrated negative refraction at optical wavelengths for the first time. Vladimir Shalaev and colleagues at Purdue University obtained the result in a material consisting of an array of pairs of parallel gold nanorods (arXiv.org/abs/physics/0504091). The team says the structure is relatively easy to fabricate and could lead to optical "superlenses" that reflect no light and operate with sub-wavelength resolution. Such lenses could be used to make better DVD players and improved biomedical sensors.
The existence of materials with a negative refractive index was first predicted by the Russian physicist Victor Veselago in 1967. He speculated that materials in which the electric permittivity and the magnetic permeability are both less than zero would refract light in the opposite direction to conventional materials. In 2000, John Pendry, a theorist at Imperial College in London, showed that such materials could also behave as perfect lenses.
Although negative-refractive-index materials do not occur naturally, several groups have successfully made them. But so far these materials have only operated at microwave frequencies.
The new negative-refraction material made by the Purdue team consists of an array of closely spaced pairs of parallel gold nanorods, measuring 2-mm by 2-mm (figure 1). Two rods form a “tuning fork”, which has a pronounced resonance at a certain frequency of light. This resonance occurs for both the electric and magnetic components of light and it can result in negative refraction at frequencies higher than the resonance frequency — an observation that agrees with previous calculations performed by the group.
“This work is an imaginative solution to obtaining both a negative electric and negative magnetic response — and hence a negative refractive index — at optical frequencies in a material,” Pendry told PhysicsWeb. “It exploits the two resonances, symmetric and asymmetric, of two parallel rods to achieve the effect. Further work needs to be done to apply the ideas to make a working device, but the approach is promising.”
A transmission medium with a negative index of refraction would enable a flat planar lens to focus light to a precision that is smaller than the wavelength of light itself, says the team. Such a superlens could therefore overcome the so-called diffraction limit – the fact that the resolution of an object can be no smaller than half a wavelength of the light used to illuminate it.
“Portable and versatile, the new lens would have the potential to revolutionize the market for most technology areas where light is used,” says team member Alexander Kildishev. “These include optical recording (for enhanced DVDs), nanofabrication and optical lithography, enhanced sensing, such as in biomedical sensors and implants.”
The team now plans on fabricating new optical materials and building its superlens.
