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Negative refraction gets natural

23 May 2007

Physicists in Germany claim to have found the first naturally occurring material that has a negative, rather than a positive, refractive index.

The material -- a metallic ferromagnet -- is very different from all other negative-refractive-index materials known to date, which have had structures that have been artificially engineered in the laboratory. The ferromagnets, which have been shown to exhibit negative refraction up to gigahertz frequencies, could be used in novel devices such as superlenses (Phys. Rev. Lett. 98 197401).

Nearly all materials have a positive refractive index, but Russian physicist Victor Veselago showed in 1968 showed that refraction could be negative if both the permeability and permittivity of the material were negative at the same time. Scientists have since fabricated devices that exploit negative refraction, such as high-resolution "superlenses", but these have all used negative-refraction materials that been created artificially in the lab, such as arrangements of copper rings or rods.

Now, however, Andrei Pimenov of the Universität Würzburg, along with colleagues at other German institutions, has shown that negative refraction can occur in metal ferromagnets – in other words, in natural materials.

Pimenov's team first suspected that ferromagnets might have a negative refractive index after testing materials consisting of layers of ferromagnets and superconductors last year. They found to their surprise that the materials exhibited weak negative refraction if the superconducting layers were in the normally-conducting phase and if an applied magnetic field was used to keep the ferromagnetic layers in a "resonant" state – which is when the magnetic moments rotate at the same frequency as the incident light.

These results prompted the research team to find out whether pure ferromagnets could behave as negative-refraction materials in their own right. Pimenov and colleagues shone light on thin films of the metallic ferromagnet La2/3Ca1/3MnO3, and then measured how the amplitude and phase of the transmitted light changed using an interferometer.

From these values they calculated the permittivity and the permeability, and hence the refractive index, and found that for frequencies of light up to 150 GHz the refractive index was negative. At higher frequencies, however, the effect began to peter out.

Pimenov told Physics Web that metallic ferromagnets should, in principle, be able to have a negative refractive index up to frequencies of 1 THz. However, he added that such materials are unlikely to have negative refractive index at optical frequencies (above 450 THz), which would rule them out as optical superlenses.

Nevertheless, Pimenov said that his team is now going to start testing other materials including iron. These could demonstrate negative refraction while in a resonant state at frequencies slightly above 150 GHz, which could be useful for telecommunications applications.

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