Light can transport much more information than electrical signals in the same amount of time. Three-dimensional photonic crystals will revolutionize telecommunications. But: at the network nodes there are still no competitive compact, fully optical control processors available – although work is progressing in this area.

People have always been fascinated by natural crystals due to their beauty and strict principle of arrangement. The spatial arrangement of the atoms in a crystal is based on the laws of the bonding forces which form a basic cell that is periodically continued to create a crystal lattice. This defines the outer shape and the properties of the crystal. For photonic applications the transparency, the refraction index and the lattice constants are important. Transparent optical materials with different refraction indices are extremely important for high-quality optics. Ernst Abbe recognized this back in 1883 when he extended the correction of lenses to three colors. Even with the combination of glasses with different refraction indices from Schott he still needed fluorite for the final correction. He recovered fluorite crystals himself from a quarry in Oltschiburg, to the southeast of Lake Brienz (Figure 1). In further developments by Carl Zeiss in Jena, high-resolution lenses were produced, which revolutionized lens development through to the present day. These days fluorite, with its special refractive properties for optics, is produced synthetically in a high purity grade.

Synthetic crystals are also the basis for laser development. Like the Nd:YAG crystal, which is the most commonly used laser medium in solid state lasers, they are grown in an ever-increasing number of new combinations for applications in the laser industry. A large range of high-purity synthetic crystals, which are optimized through the insertion of atoms – usually from rare earths – are offered commercially for special laser applications.

The periodic modulation of the refraction index that is typical for crystals also occurs in other materials. Wave propagation is possible here in the form of modes in allowed bands, which are separated from each other by band gaps. These crystals have come to be referred to as “photonic crystals”. They could be key components for compact optical semiconductors. For example, inside these crystals through laser radiation it is possible to generate self-focusing properties as a non-linear effect, which creates a channel that can, in turn, act as a waveguide and transport light. In this way optical elements are created that can directly control and process light at the same time – without the need for additional optical fibers, physical guides or conversion into electronic signals.

The advantage in using these crystals is the possibility of frequency selection through electric and also purely optical activation, which would allow the development of a compact, fully optical control processor. This is also the objective of a three-year project entitled 'NewTon' being carried out by BASF, who, together with partners like Laser Centre Hanover, Thales Aerospace Division, Photon Design Ltd., the Technical University of Denmark and Ecole Nationale Superieure des Telecommunications de Bretagne are carrying out research work into the development of photonic crystals. The starting materials for production of these crystals could be aqueous dispersions with roughly 200-nanometer polymer spheres, which flow together and form a homogeneous, protective film when the liquid evaporates. These can arrange themselves into a regular lattice and form a crystal (Figure 2). This would allow a stable, three-dimensional crystal to be developed in which a specific structure could then be registered.

The first functioning components from this new technology are expected by the end of 2008. The long-term goal is to use three-dimensional photonic crystals as components in the telecommunication industry. The project is half funded by the European Union. The 'NewTon' project was presented here as an example of the fundamental investigations being carried out in many locations. Many renowned institutions throughout the world are carrying out research work in this field. Manufacturers of components for telecommunication applications would profit from the commercial deployment of photonic crystals. They are smaller than electronic components, which would also make the devices smaller and cheaper – with absolutely no compromise in performance. However, a photonic processor, a light computer, is still a vision of the future. But it can be expected that as the technology becomes more economic, photonic crystals will eventually revolutionize telecommunications throughout the world.

Natural fluorite crystal Source Messe München

Example of a photonic crystal, produced by two-photon polymerization Source: J. Serbin, A. Ovsianikov, B. Chichkov, Projekt 'NewTon'