Fraunhofer in Japan: towards a nanotech future
This innovation is invisible – to the naked eye, at least. It takes a scanning electron microscope to discern the tiny, no more than nanometer-scale structures in the novel type of crystal. Yet although it is smaller than a speck of dust, this structure yields tremendous performance. “Given the right conditions, a photonic crystal will enable one to direct light signals wherever we choose, in all three spatial directions. You might call it a miniaturized optical switching center,“ explains Michael Popall of the Fraunhofer Institute for Silicate Research ISC. “Photonic crystal structures like these will control the signals in the optical connections and computers of the future.“ It takes extremely fine dexterity to produce these crystals. The amazing physical properties are the result of minute variations in structure: Depending on the frequency of the light, just a tiny displacement in the crystal lattice can affect photon transport. Teams of researchers all round the world are working on the development of photonic crystals. Scientists from the Fraunhofer Institute in Würzburg, working in cooperation with researchers at the Laser Center in Hanover, have already made significant progress: At nano tech in Tokyo, they will be showing video recordings that document the structure of such a crystal. As starting material they use a honey-like liquid made of inorganic-oxidic nano-units with organically modified surface. This ORMOCER® resin hardens when it is touched by a ray of light: “We can control polymerization with pinpoint accuracy: As soon as a laser beam hits the resin, organic bridges form between the inorganic units, and this creates stability.“ So far it sounds quite easy. But in fact, a great deal of meticulous research work has gone into the creation of the new photonic crystal structures. Last year, Popall and his team won the Japanese 'nano tech award 2005' for their two-dimensional structures. They shared the prize with colleagues from the Fraunhofer Institute for Applied Optics and Precision Engineering IOF, who had developed a nano-embossing technology for the production of ORMOCER®-based micro-optical systems. Popall will be back in Tokyo this year: At nano tech 2006, he demonstrates how the ISC and Laser Center researchers have now mastered even the third dimension. “To do this, we focus a femtosecond laser into the resin which provides sufficient energy to cross-link the organically modified inorganic-oxidic nano-units precisely at the desired spot, to the best performance in the size of the laser focus. Outside the focus, the material is exposed to the beam but without producing any reaction since the energy provided is too low. In this way – point by point, layer by layer – we are able to create a crystal with very specific optical properties. Our technique is precise, adaptable and cost-effective, and is eminently suitable for producing the kind of photonic crystals that will be needed in tomorrow's optical data processing.“ ORMOCER®s are already found in many practical micro-optical applications such as micro-lens arrays for lasers or high-resolution optical arrays such as beamers. “Applications of this kind are of great interest to the Japanese market. The Japanese have a strong affinity with all things practical and a particular affection for nanotechnology,“ reports Dr. Lorenz Granrath of the Fraunhofer Representative Office in Tokyo. “It is no coincidence that nano tech, established as a scientific forum four years ago, has evolved into the world's biggest trade fair in this sector.“ Another example of practical application is going to be presented by the Fraunhofer Polymer Surfaces Alliance POLO as a part of the joint Fraunhofer exhibit in Tokyo. “Flexible high-barrier or ultra-barrier films are our topic this year. Films of this type, designed to protect packaged goods from permeation by vapors such as steam or gases like oxygen, can be used in the food industry today to prevent fresh produce from perishing and dry goods from absorbing moisture,“ explains the Alliance spokeswoman Dr. Sabine Amberg-Schwab. The laminated barrier films consist of a plastic substrate film bearing a thin layer of silicate or aluminum oxide which is covered with a hybrid polymer coating – and here, too, the substance in question is the nanotech material ORMOCER®. Because they are thin and flexible, these coatings are suited to numerous applications. Fraunhofer researchers have developed a film that exhibits barrier properties superior to those of the food-protection films, and hope to use it for encapsulating solar cells. “We plan to develop even more efficient coatings in the future – ultra-barrier coatings,“ continues Amberg-Schwab. “Materials for encapsulating flexible OLEDs, for instance, have to meet extremely high standards: only one millionth of a gram of steam is permitted to permeate a square meter of the film in one day. Such ambitious targets pose a real challenge to researchers.“
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