Physics: Perfect Trap lets no light escape

In science and technology, as much light as possible should often be absorbed in order to gain energy from it or to examine the incident light as precisely as possible. This is relatively easy when dealing with massive objects, such as photovoltaic systems. However, the complete absorption of light becomes more complicated when it comes to wafer-thin and therefore translucent materials.

Use light efficiently

You can also see that in everyday life Stefan Rotter explained by the Institute for Theoretical Physics of the Technical University (TU) Vienna. An example of good absorption is black clothing. The human retina, on the other hand, absorbs the light less well. When it gets darker in the evening, visibility also gets worse – although there should actually still be a little light. However, since the retina is very thin and translucent, much of the incoming light is lost and cannot be used optimally.

Animals that are nocturnal or hunt at night have an advantage. Cats need to use what little light there is so efficiently that they can see at night. “The animals manage this by having a mirrored layer behind their retina. This means that the light is thrown back once more and the beam can be absorbed once more by the retina,” Rotter explains to science.ORF.at. This is also the reason for the glowing eyes of the cats at night.

role model nature

Rotter has been dealing with the absorption of light for a long time and wanted to use the cats’ approach to optimally use light in science and technology. Together with the experimental physicist Ori Katz from the Hebrew University of Jerusalem, he had an idea for a light trap that can completely absorb a ray of light even in the thinnest layers – much better than cats can. Rotter: “A ray of light only comes through the retina of the cat twice, but with our method the ray is reflected once more and once more and hits the material until it is completely absorbed.”

The theoretical calculations for the “perfect light trap” came from Vienna and were then experimentally realized by the team in Jerusalem.

One-way street for light

The trap is made up of precisely arranged mirrors and lenses, which let a beam of light through a partially transparent mirror into the interior, but then direct it in a circle for so long that it finally overlaps with itself. The initially partially transparent mirror becomes completely transparent for the incident laser beam.

The result is, so to speak, a one-way street for the light. If the trap is precisely tuned to the wavelength of the light beam, it will prevent itself from exiting the trap once more. “The light then has no other way out than to be completely absorbed by the thin material inside the trap,” says Rotter.

Robust System

In addition to perfect absorption, the trap also has other advantages, such as its robustness. “The system has to be tuned exactly to the wavelength that you want to absorb,” says Rotter. “But apart from that, there are no specifications. The laser beam does not have to have a specific shape. It can also be more intense in some places than others.”

Even air turbulence or temperature fluctuations cannot harm the mechanism, as the researchers were able to show in the experiments in Jerusalem. As a result, the trap might have many uses.

Wide range of applications

Among other things, the mechanism would be well suited to perfectly capturing even light signals that are distorted during transmission through the earth’s atmosphere. Light from weak light sources, such as from distant stars, might also be optimally fed into a detector.

The light trap has only just been tested in the laboratory – it might still be some time before it can actually be used in practice. “For the first time, we managed to build a trap that captures the light perfectly and doesn’t let it out once more. With the restriction that the trap has so far only worked at a clearly defined frequency,” says Rotter. The method from Vienna is ideal for this frequency, but it is not yet usable for other light. Rotter would therefore like to optimize the trap for other light frequencies in future studies.