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Light Gives New Material Magnetic Personality

16 November 2005 (All day)
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Nature

Antimagnet.
The pairs of nanopillars act as bar magnets that cancel out (in blue) the light's magnetic field (red and yellow).

If visible light caused magnetism, paperclips would clump together every time you flicked on your office lamp. Paperclips may be safe for now, but researchers have sculpted gold into a material that becomes magnetic in a cockeyed way when exposed to light. Their innovation could lead to nanolasers, ultra-sensitive chemical detectors, or even a lens that could focus on details far tinier than the best lenses used today.

A ray of light has two parts: an electric field and a magnetic field. When most objects are hit with visible light, their responses to the electric field determines whether light can pass through them, and if so, which direction it bends. Metals have a “negative” response to the electric field, making them opaque. Materials such as glass or water, on the other hand, have a "positive" response to the electric field, causing a light ray to refract through them and making them appear transparent.

If, however, a material could be coaxed into having negative responses to both the electric and magnetic fields of a light ray, marvelous things could happen. Work with microwaves suggest the light wave would refract through the material, but bend the opposite way than it does normally. A lens made out of this "left-handed" stuff would be able to focus the tiniest details of an image, something a regular right-handed lens cannot do. (The tiny details decay too quickly for a regular lens to pick up.) But such a lens would require doing the same thing with visible light that researchers have done with microwaves--a feat that has proved elusive thus far.

Now, researchers have developed a material that appears to respond to visible light in this unique manner. Alexander Grigorenko, a physicist at the University of Manchester in the United Kingdom, and colleagues arranged tapered gold pillars, just about 80 nanometers tall and 100 nanometers wide at the base, in pairs on a glass slab. The nano-size and spacing were just right to interact with light waves, which are a few hundred nanometers long. When red or green light shone on the pillars, it excited the electrons in the metal, which began to vibrate in opposite directions--the electrons on one pillar would go up while the electrons on the other pillar would go down. Each pair of pillars acted like a tiny circuit with its own magnetic field. Together, the pairs produced a magnetic field strong enough to make the whole slab respond negatively to the light’s magnetic field, That is, within the material the magnetic field points in the opposite direction of the field applied by the light, the researchers report 17 November in Nature.

"This work represents a major step forward," for left-handed materials, says George Eleftheriades, a physicist at University of Toronto in Canada. He calls the results "truly extraordinary." Grigorenko notes, though, that the researchers have not yet made a left-handed lens from the material--it takes so much energy to make the magnetism that not much light gets transmitted.

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