Scientists have tried this with sophisticated metamaterials,
but at the Vienna University of Technology (TU Vienna) it has now been done
with simple metals; materials with a negative refractive index bend light the
“wrong” way.
The effect can be seen just by poking a stick into the
water; at the water surface, the light changes its direction, the stick appears
to be bent. This tilt is described by the refractive index. For years,
scientists have been trying to create special materials with a negative
refractive index—their optical properties are quite different from those of
normal materials. Researchers at the TU Vienna could now show that even common
metals can have a negative refractive index, if they are placed in a magnetic
field.
Different kind of diffraction for better optics
When we drive a car into the snow at the edge of the road, the wheels on
the road may turn faster than the wheels on the snow. This changes the
direction of the car and it starts skidding. Something quite similar happens to
beams of light that travel through the interface between two materials, in
which light travels at different speeds—such as air and glass. “The
refractive index measures, how strongly the light is deflected”, explains
Andrei Pimenov, Professor at the Institute for Solid State Physics at the TU
Vienna. For years there have been speculations about the properties of possible
materials with a negative refractive index. Entering such a material, light
would bend in the opposite direction. Scientists believe that this could lead
to completely new optical effects and technologies.
Metal bends light
It was believed that these effects can only be achieved using so called
“metamaterials”. Such materials are constructed from small intricate structures,
which diffract the light in special ways on a microscopic level. At the TU
Vienna, scientists found out that with simple tricks even quite common metals
such as cobalt or iron can exhibit a negative refractive index. “We place
the metal in a strong magnetic field and irradiate it with light of precisely
the correct wavelength”, Andrei Pimenov explains. He uses microwave
radiation, which can penetrate thin foils of metal. Due to magnetic resonance
effects in the metal, the light is bent drastically at the surface. Within the
metal, it turns into the other direction, as if there was a mirror inside the
metal.
The perfect lens
Recently, materials with a negative diffractive index have attracted a
great deal of attention, because their peculiar behavior could allow for
completely new kinds of optical lenses. The resolution of regular lenses is
limited by the wave length of light. With long radar waves, it is impossible to
take a picture of a butterfly, with visible light, nobody can depict an atom.
“But using a material with a negative refractive index, one could
theoretically get infinitely high resolution”, says Andrei Pimenov. Being
able to use simple metals instead of complicated meta-materials makes things a
lot easier. However, before optical lenses with a negative refractive index can
be built, scientists have to find ways to compensate for the absorption of the
light in the material.
