New optics with artificial atoms

If anyone, dwarf Mime from the Nibelungen saga used to be the only one to know how to forge a cloak of invisibility. Magical fabrics like these would have had to be able to steer light around the object and so refract light in exactly the opposite direction to what conventional materials do. And to do this, "invisible materials" would have had to respond to both the magnetic and the electrical fields of the radiated light waves. Such materials do not exist in nature, but a start has been made in the lab: Dr. Stefan Linden from the Institute of Nanotechnology  at the Research Center Karlsruhe researches metamaterials with unusual optical properties. He does this by building structures made of artificial "atoms" that look much like horseshoes or intricate nets under the electron microscope.

Linden started as a postdoc under Prof. Dr. Martin Wegener of the University of Karlsruhe (TH). He wanted to try out what the theoretician Costa Soukoulis of Iowa State University had proposed, namely to use tiny electromagnetic oscillating circuits to manipulate light in such a way that the effective refractive index becomes negative. This had already worked using microwaves whose wavelengths lie in the centimetre range. To use this effect for much shorter light wavelengths as well, Linden had to generate oscillating circuit structures on a substrate that repeat once every few undred nanometres. "I had already gained some experience with electron beam lithography, which was no problem - and later I was astonished to find that no-one had done this before us," remembers Linden. He still collaborates closely with Wegener, but meanwhile, as the leader of a Helmholtz-University Young Investigators Group, with three doctoral students and three Diplom students. A polymer film is first applied to a glass substrate into which the "artificial atoms" are inscribed with an electron beam. After developing these, layers of gold or silver and magnesium fluoride are deposited. These "artificial atoms" affect the light like tiny electromagnetic oscillating circuits. The light stimulates them and reradiates an electromagnetic wave, after a time-lapse. This can lead to a very strange phenomenon. When a light pulse passes through the sample, physicists observe that the maximum of the passing pulse appears behind the sample before the peak of the entering pulse has reached the front. "The pulse deforms in the sample itself, because the various wavelengths pass through the material at different speeds. At the same time, the passing pulse is still causally connected to the entering pulse," explains Linden. The negative refraction effect is still strongest in a wavelength of 1500 nanometres, i.e. in the infrared range. The present record lies at the red end of the visible spectrum, at 780 nanometres. To get light with even shorter wavelengths to "reverse" refract, the structures would have to be substantially scaled down once more. In addition, only individual layers have been studied so far. To identify the "right" materials, it would be necessary to apply several layers one after the other. However, this would involve extremely high losses, because only around 70 per cent of the light is transmitted through a layer, and in the case of sequentially switched layers this multiplies quickly. "It might be possible to balance this out with an optical amplifier, but that still needs to be developed first," believes Linden. "All this is currently pure basic research," emphasises the physicist, but he can already see some applications on the horizon, for example, perfect lenses. Because optical lenses made of conventional materials can only resolve objects that are larger than a light wavelength, while lenses made of metamaterials could be "perfect". This means a light microscope with 10,000 fold magnification would be possible in principle. And invisible cloaks? Linden can't help a smile. "In principle, it's conceivable," he admits. Of course, this would call for other complex problems to be solved. In addition, a cloak such as Harry Potter wore to secretly quarry through the library cannot be produced. Because the optical properties are the same in both directions, users would also be cut off optically from the outside world, i.e. would be blind to all intents and purposes.