Materials with a negative refractive index are not just limited to artificial structures created in the lab as previously assumed. German physicists have shown such materials can also occur in nature. The researchers at the Universities of Würzburg, Augsburg, and Göttingen observed negative refraction in a ferromagnetic metal at gigahertz frequencies [Pimenov et al., Phys. Rev. Lett. (2007) 98, 197401].
Negative refraction materials (NRMs) tend to reverse all our understanding of optics. When light enters an NRM at an angle, it is refracted on the opposite side of the normal to standard positive index materials. This means lenses can be constructed from flat NRM blocks with resolutions beyond the diffraction limit. The superlensing ability of NRMs could lead to applications in microscopy and lithography.
The response of materials to electromagnetic waves is described by their permittivity e and permeability μ. NRMs require both these values to be negative. Fabricated arrays of Cu wires and rings have been used to gain negative e and μ at microwave frequencies and above, with developments progressing toward optical frequencies.
The permittivity of metals is negative below their plasma frequency. The German group led by Konrad Samwer of the University of Göttingen have now shown that, if the metal is simultaneously ferromagnetic, negative permeability can also be achieved close to its ferromagnetic resonance.
The team used La2/3Ca1/3MnO3 as an ideal test case because it shows a strong ferromagnetic resonance. Using 150 GHz radiation, e and μ were determined separately for the material. A negative refraction index was verified under external magnetic fields close to the ferromagnetic resonance.
“[This] is a valuable contribution to the field,” says John Pendry of Imperial College London. “The huge potential of negative refraction is attracting many to look again at the possibilities for realizing this effect, and I do not doubt that more possibilities will emerge in time.”
The researchers do point out that there are problems to be solved. Negative refraction in ferromagnets is also limited to the millimeter wavelength range, fading away toward higher frequencies.