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Gold Particle Crystal Catches the Light

New quasi particle with amazing properties discovered

Physicists from the University of Bonn have discovered a new quasi particle and studied it in detail. The work was done in collaboration with the Max Planck Institute of Solid State Research in Stuttgart and the University of Moscow (A. Christ et al., PhysRev. Lett. 91, 183901). For this they used a crystal consisting of small gold wires, which they used to trap the light particles, as it were. In such 'photonic crystals' the light can be said to link the gold wires with each other; with them, therefore, microscopically small light conductor component parts could be produced, for example, which could then be used for telecommunications and other purposes.

Sending light from one place to another is not really a problem - via glass fibres it can be done across thousands of kilometres. It only becomes difficult when very small light conductors are required. For instance, one single glass fibre cable can transport many thousands of different phone calls simultaneously. To unscramble this mass of data we still need a great deal of space-consuming technology even today. If we wish to make this technology more compact, we need completely different light conductors than we currently have at our disposal.

'When we are talking about nanoscale sizes, glass fibres are simply too large, and it would be better to use photonic crystals,' explains Professor Harald Giessen of the Bonn Institute of Applied Physics. The principle is simple: if nanostructured gold is exposed to a laser, the electrons in the precious metal begin to slosh around to and fro with the frequency of light - 'just like water in a glass,' as Professor Giessen puts it. During this process the particle of gold stores the energy with which it has been fed, but it can also release it again in the form of light. This works particularly well when the particles of gold are very small - in the Bonn experiment they only measured 100 nanometres, i.e. the 300th of a hair's breadth. If a large number of these 'nanoscale gold wires' are placed on a light conductor (basically a kind of glass fibre) at suitable distances, the light can pass from gold wire to gold wire.

The reason for this is an effect which physicists call 'coupling'. 'If, for example, you take two pendulums of differing lengths, each one of them will possess its own duration of oscillation. If, however, the two pendulums are connected with a spring, i.e. if they are coupled together, the two pendulums will oscillate with a different duration,' Professor Giessen explains: the two pendulums no longer behave like individual systems, acting as a new, joint system. The change in the duration of oscillation compared with the unconnected system reflects the strength of the coupling.

'In physics it is pretty often the case that light and matter join together to form something new,' Professor Giessen continues. 'If we take an atom, for example, and place it between two mirrors where light particles, i.e. photons, can pass backwards and forwards, the joint system consisting of atom and photon will acquire completely new properties.' This joint system is then called 'quasi particle' by physicists. The research team also observed one such quasi particle, known as a polariton, in their photonic crystal. 'The laser beam is transformed again into light rather than being stored in the gold particle as electronic energy, and is thus passed on to the next gold particle,' Professor Giessen explains. 'The polariton is basically two things at the same time - both electronic energy and light.' The coupling in the photonic crystal is the strongest which has been hitherto observed. 'This is why the crystal can also catch the light so well and pass it on.'

The metallic photon crystals have only come under the scrutiny of the research team in recent months. Experts are already predicting a great future for them in
nano-optics. Theoreticians are also forecastting that these new kinds of material may have a negative refraction index. This means that light is refracted by them in the opposite direction to when it is refracted by glass. This would result in a whole series of new types of phenomena and applications.

Contact person:
Professor Harald Giessen
Institute of Applied Physics of the University of Bonn
Tel.: ++49-228-733459
E-mail:
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