perfect light trap

Physicists develop perfect light trap

The figure shows the “light trap” setup, consisting of a partially transparent mirror, a thin, weak absorber, two converging lenses, and a total reflection mirror. Typically, most of the incident light beam is reflected. However, due to precisely calculated interference effects, the incident beam interferes with the reflected beam from the mirror, causing the reflected beam to eventually extinguish completely. The energy of the light is completely absorbed by the thin and weak absorber. Credit: TU Wien

Whether in photosynthesis or photovoltaic systems: if you want to use light efficiently, you have to absorb it as completely as possible. However, this would be difficult if the absorption occurred in thin layers of material that normally let through most of the light.

Now, a research team from TU Wien and the Hebrew University of Jerusalem has discovered a surprising trick to fully absorb the light beam even in the thinnest layer: they used mirrors to create a “light trap” around the thin layer , a lens in which the beam is directed into a circle and then superimposed on itself – precisely in such a way that the beam blocks itself and can no longer leave the system. So the light has no choice but to be absorbed by the thin layer – there is no other way out.

This absorption-amplification method, now published in the scientific journal scienceis the result of a fruitful collaboration between the two teams: the method was proposed by Prof. Ori Katz at the Hebrew University of Jerusalem and by Prof. Stefan Rotter at TU Wien; the experiment was carried out by a laboratory team in Jerusalem, and the theoretical calculations were derived from Vienna team.

Thin layer is transparent to light

“When light hits a solid object, it is easy to absorb light,” says Professor Stefan Rotter from the Institute of Theoretical Physics at TU Wien. “A thick black wool jumper can easily absorb light. But in many technical applications, you only have A thin layer of material is available, and you want the light to be completely absorbed by that layer.”

There have been attempts to improve the absorbency of the material: for example, the material can be placed between two mirrors. Light bounces back and forth between the two mirrors, passing through the material each time, so it has a greater chance of being absorbed. However, the mirrors cannot be perfect for this purpose – one of them must be partially transparent, otherwise light cannot penetrate the area between the two mirrors at all. But it also means that whenever light hits this partially transparent mirror, some light is lost.

light blocks itself

To prevent this, the wave properties of light can be used in complex ways. “In our method, we were able to eliminate all back-reflections by wave interference,” said Professor Ori Katz of the Hebrew University of Jerusalem. Helmut Hörner from TU Wien, who devoted his paper to this topic, explains: “In our method, again, the light first falls on a partially transparent mirror. If you just send the laser beam to this On a mirror, it’s divided into two parts: the larger part is reflected, and the smaller part penetrates the mirror.”

This part of the beam that passed through the mirror now passes through the layer of absorbing material and then returns to the partially transparent mirror with the lens and another mirror. “The key is to adjust the length of this path and the position of the optics so that the returning beam (and its multiple reflections between mirrors) exactly cancels the beam that was reflected directly on the first mirror” Building the system in Jerusalem graduate students Yevgeny Slobodkin and Gil Weinberg said.

The two partial beams overlap in such a way that the light blocks itself, so to speak: while a separate partially transparent mirror will actually reflect most of the light, since the other part of the beam passes through the mirror, this reflection is impossible. Before the system returns to the partially transparent mirror.

Thus, mirrors that were partially transparent in the past are now fully transparent to the incident laser beam. This creates a one-way street for light: the beam can enter the system, but it can no longer escape due to the superposition of the reflected part and the part guided through the system on the circumference. So light has no choice but to be absorbed – the entire laser beam is swallowed by a thin layer that would otherwise allow most of the beam to pass through.

a powerful phenomenon

“The system has to be tuned precisely to the wavelengths you want to absorb,” says Stefan Rotter. “But other than that, there are no restrictions. The laser beam doesn’t have to have a specific shape, it can be more intense in some places than in others – almost perfect absorption is always achieved.”

Even air turbulence and temperature fluctuations do not harm the mechanism, as experiments conducted at the Hebrew University of Jerusalem showed. This proves to be a powerful effect that holds promise for a wide range of applications – for example, the proposed mechanism could even be perfectly suited to perfectly capture light signals that are distorted during transmission through Earth’s atmosphere. The new method also has great practical use for optimally feeding light waves from weak light sources, such as distant stars, to detectors.

What color is the mirror? Explain mirrors and how they work.

More information:
Yevgeny Slobodkin et al., Large-scale degenerate coherent perfect absorbers for arbitrary wavefronts, science (2022). DOI: 10.1126/science.abq8103.

Courtesy of Vienna University of Technology

Citation: Physicists develop a perfect optical trap (25 Aug 2022) Retrieved 26 Aug 2022 from

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