In which medium does light move quickly

laser : Beams of light as fast as a scooter

Nothing is faster and as difficult to grasp as light. Basically it is nothing more than electromagnetic fields that come along in tiny, massless wave packets. According to Einstein's theory of relativity, the speed of light is actually the greatest speed possible. Bodies with mass, regardless of whether they are spaceships or elementary particles, can only approach this speed, but never reach it.

But how can you slow down light, how can you capture the rapid waves? It has been known for centuries that translucent media with a high refractive index, such as certain glasses, can slow light down to half or even a third of its original speed. The entire optics, as we know them from glasses, magnifying glasses, cameras and telescopes, are based on this. But even with tailor-made materials, light can hardly be delayed more than a factor of 100. This is considerable, but in view of the enormous speed of light it is by far not sufficient for many applications.

The light was slowed down by a factor of 15 million

Researchers around the world are therefore working on slowing down light more and more and bringing it as close as possible to a standstill. A team of scientists at Imperial College in London has now designed a special material that, so to speak, throws the anchor and is supposed to slow down light by a factor of 15 million. Then it would move at only 20 meters per second, about as fast as a motor scooter. As the scientists report in the journal “Physical Review Letters”, they devised their own medium for this, a “nanoplasmonic waveguide”. This consists of different layers and is able to capture light waves. This does not work with all light, but only with light waves that vibrate at a certain frequency.

"The trick was to design the nanoplasmonic waveguide in such a way that only the desired light waves are allowed," says Ortwin Hess, head of the London research group. “We achieve this through a corresponding sequence of the various layers.” So far, however, the physicists have only researched the material theoretically. In the next step, the “light brake” is to be built in the laboratory; the researchers are working together with other groups on this.

The medium consists of three wafer-thin layers: In the middle there is a silicon layer surrounded by two layers of indium oxide. These layers measure only a few hundred nanometers and are therefore only about a two-hundredth of a human hair. The medium is designed for infrared light, such as that emitted by remote controls. When such light enters the waveguide, the medium behaves contradictingly thanks to the different material properties: the light wave is deflected back at the edges, while it tries to travel forward in the middle of the medium.

Hess compares this to a water pipe, which has the unusual property of allowing water to flow back at the edges while it flows forward in the middle. "As a result, the water practically flows in a circle, the liquid no longer moves from its place." What is difficult to imagine with water can certainly be done with light waves, says the researcher.

The indium oxide-silicon combination is insensitive to surface roughness, which is otherwise often a problem with optical components. It also keeps the light waves as they are without expanding them or changing their frequency.

Physicists hope for novel nanolasers

The physicist Ulf Kleineberg from the University of Munich also finds this material combination very promising: "It is particularly interesting that the inevitable losses of light when interacting with the nanostructure do not destroy the effect and that the simulated nanostructures can already be produced today."

The trick still only works with short laser pulses. In addition, a large part of the light is lost during the braking maneuver. For applications that are supposed to run particularly efficiently, one would therefore have to find suitable amplifier media. The light waves do not survive in the medium for too long either, but disappear again after a fraction of a second. However, the service life is long enough for possible applications in optoelectronics. The researchers are thinking of the next generation of communication technology, in which data processing takes place in nano dimensions.

Stopped light has an enormous effect on matter. It could therefore be used to build new types of nanolasers or start chemical reactions that would otherwise not take place or only ineffectively. “We're still at the very beginning,” says Hess. "We are slowly beginning to understand what it means when light becomes so slow - and what we can do with it."

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