Magnetic fields direct light

Physicists trick the law of refraction

D nnfilm Uni Augsburg
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When light strikes glass surfaces, lenses or prisms, the incident rays are refracted into the solder, depending on the material they penetrate. This law of positive refraction makes it possible to control or focus light rays with optical devices. Augsburg physicists have now experimentally proven in a novel material that there is also a negative refraction.

The phenomenon has been known for almost 40 years: The propagation of the light wave, contrary to the positive refraction, occurs opposite to the direction of the transported energy. What the Russian physicist Victor Veselago theoretically predicted in 1967, the scientists of the Collaborative Research Center 484 of the University of Augsburg have realized in a material of multilayers of ferromagnetic and superconducting thin films. With the help of thin films, to which they applied strong magnetic fields, they could switch between positive and negative refraction of the light.

The laws of classical optics, such as the positive refraction of light, are among the foundations of fundamental physics and are based on materials with positive refractive index, which break radiation towards the solder. The knowledge of the refractive dependence of light on the material that permeates it has enabled lens and optical device designs as universal tools for many fields of technology and research.

When the Russian physicist Victor Veselago pointed out in his theoretical considerations in 1967 that there must also be the phenomenon of negative refraction, he received little attention. Because in nature, no materials are known that have a negative refractive index. In the case of negative refraction, the physicist postulated, the propagation of the light wave takes place counter to the direction of the transported energy. What Veselago had theoretically predicted nearly 40 years ago is now proven: The phenomenon of negative refraction has revolutionized the laws of classical optics.

"Superlensing" possible

Negative refraction, for example, can overcome the resolution limit of optical devices, a technique described as "superlensing". With this technology, you can image objects that are smaller than a wavelength of light. In the visible range, this corresponds to a few hundred nanometers (one nanometer is one billionth of a meter). display

Only recently has this effect of negative refraction been demonstrated in metamaterials and photonic crystals. Metamaterials are a new class of materials that are artificially manufactured to technologically realize optical properties that are not found in nature. Photonic crystals are regular arrangements of scattering centers (two- and three-dimensional scattering lattices, nanostructures) that can be used to modify certain properties of light.

The law of refraction has been overcome with superconducting thin films

The Augsburg physicists Professor Alois Loidl and Andrei Pimenov from the Department of Experimental Physics V, in cooperation with Polish and US colleagues from the Polish Academy of Sciences, Warszawa and the Northern University of Illinois, succeeded in realizing negative refraction in another system in multilayers of ferromagnetic and superconducting thin films.

These multi-layer thin films have been assembled from a variety of materials and are around 300 nanometers thick. In these hybrid materials, the multilayers of the high-temperature superconductor YbaCuO, which loses its resistance at 90 degrees Kelvin (about -180 C), provide for negative dielectric permittivity and the ferromagnetic (La: Sr) MnO Layers of which the thin films are composed, for negative magnetic permeability. According to theoretical models, exactly those components that are necessary for the realization of negative refractive indices.

Negative refraction detected in external magnetic fields

In transmission experiments physicists finally proved the existence of negative refraction in external magnetic fields. They sent light through the thin film layers and were thus able to detect this unusual optical phenomenon.

As an interesting side effect, the scientists were able to switch between positive and negative refraction by means of a magnetic field and thus control the light waves by changing the magnetic field. In the metamaterials used negative refraction is initially limited to low temperatures of about minus 200 degrees Celsius and high magnetic fields of about three Tesla and therefore not very promising for applications (for comparison: the geomagnetic field is round 100, 000 times weaker than that in the tests).

However, the researchers see a concrete way out: they now want to replace the ferromagnetic layers with antiferromagnetic ones. Thus, negative refraction could already be realized without external magnetic fields that require too much technological effort. A lens made of the corresponding material would overcome the classical laws of optics. Such lenses could have a great future for applications in optoelectronic components.

(University of Augsburg, 24.05.2006 - DLO)