Photonics: The plasmons never vibrate more freely

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Photonics: The plasmons never vibrate more freely
Photonics: The plasmons never vibrate more freely

Never swing the plasmons more freely

Optical components could make computers and microchips much faster. So far, however, it has not been possible to produce structures that can keep up with the extremely tiny dimensions of microelectronics. A Danish-French research team seems to be able to do this to some extent. If the largest possible amount of data needs to be sent quickly from one place to another, fiber optic technology comes into play. The Heinrich Hertz Institute of the Fraunhofer Society recently reported a new record: They transmitted 2.56 trillion bits per second with a fiber optic cable over a distance of 160 kilometers. The researchers were able to smuggle the contents of 60 fully recorded DVDs through the cable in just one second.

In view of these achievements, many developers dream of optical components that also perform their services on microchips. But there is a catch: The infrared light, which is used for example in the telecommunications industry for information transmission, has a wavelength of around 1.5 micrometres. This means that the crests of the waves oscillate much farther than they could be forced into the tiny circuit paths that today make up a good semiconductor chip. These are often only a few hundred nanometers wide. With good reason, electrons still flow from transistor to transistor today. So far it makes no sense to try to connect each of these active components with glass fibres.

But if Sergey Bozhevolnyi from Aalborg University in Denmark has his way, that will soon change. Together with colleagues from the French University Louis Pasteur in Strasbourg, he used an ion beam to mill V-shaped indentations a little more than a micrometer wide in a wafer-thin gold substrate. These grooves now conduct light signals, although they are thinner than the wavelength of the infrared light used by the researchers. The magic word is channelized, plasmonic polaritons. These are, as the scientists put it, "young members of an ever-growing family of collective surface vibrations" excited by electromagnetic waves like those of light.

Even normal, visible light with a wavelength of between 400 and 750 nanometers, for example, penetrates a metal such as gold to a depth of around ten nanometers. There it forces the freely moving electrons on its surface to synchronize. To put it simply, this physical state can be described as follows: light on the outside – swaying electrons on the inside. With a suitable direction of radiation and at certain frequencies, which depend on the mobility of the electrons in the respective material, an electromagnetic wave practically adheres to the surface of the metal due to the repeated immersion of the light used in the metal. Within this area, on the other hand, the light can move relatively freely, especially if suitable paths are mapped out for it.

To show that these structures can also be used as optical switching elements, the scientists created various structures, including beam splitters and rings, which resonate at certain frequencies and therefore – depending on their geometry – amplify certain wavelengths. but oppress others. The experimenters believe that this technology could one day be used to build microchips that work directly with light instead of with the comparatively sluggish flow of electrons. You only need these charge carriers as a kind of seesaw for the light.

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