This shows the ‘light trap’ setup, which consists of a partially transparent mirror, a thin, weak absorber, two converging lenses and a totally reflecting mirror. Normally, most of the incident light beam would be reflected. However, due to precisely calculated interference effects, the incident light beam interferes with the light beam reflected back between the mirrors, so that the reflected light beam is ultimately completely extinguished. Image: TU Wien.
This shows the ‘light trap’ setup, which consists of a partially transparent mirror, a thin, weak absorber, two converging lenses and a totally reflecting mirror. Normally, most of the incident light beam would be reflected. However, due to precisely calculated interference effects, the incident light beam interferes with the light beam reflected back between the mirrors, so that the reflected light beam is ultimately completely extinguished. Image: TU Wien.

Whether in photosynthesis or in a photovoltaic system, if you want to use light efficiently, you have to absorb it as completely as possible. This can be challenging, however, if the absorption is to take place in a thin layer of material that normally lets a large part of the light pass through.

Now, researchers from the Vienna University of Technology (TU Wien) in Austria and the Hebrew University of Jerusalem in Israel have discovered a surprising trick that allows a beam of light to be completely absorbed even by the thinnest of layers. They report this trick in a paper in Science.

The researchers built a ‘light trap’ around a thin layer of material using mirrors and lenses. This trap is able to steer a light beam in a circle and then superimpose the beam on itself, such that it blocks itself and can no longer leave the system. This means the light has no choice but to be absorbed by the thin layer – there is no other way out.

This absorption-amplification method was the result of a fruitful collaboration between the two teams. The approach was suggested by Ori Katz from the Hebrew University of Jerusalem and conceptualized with Stefan Rotter from TU Wien; the experiment was carried out by the team in Jerusalem, while the theoretical calculations came from the team in Vienna.

“Absorbing light is easy when it hits a solid object,” says Rotter, who is in 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 available, and you want the light to be absorbed exactly in this layer.”

There have already been attempts to improve the absorption of materials, such as by placing a material between two mirrors. Light is then reflected back and forth between the two mirrors, passing through the material each time and thus having a greater chance of being absorbed. However, for this purpose, the mirrors must not be perfect – one of them must be partially transparent, otherwise the light cannot penetrate the area between the two mirrors. But this also means that whenever the light hits this partially transparent mirror, some of the light is lost.

One way to prevent this from happening is to use the wave properties of light in a sophisticated way. “In our approach, we are able to cancel all back-reflections by wave interference,” explains Katz.

“In our method, too, the light first falls on a partially transparent mirror,” adds Helmut Hörner from TU Wien, who dedicated his thesis to this topic. “If you simply send a laser beam onto this mirror, it is split into two parts: the larger part is reflected, a smaller part penetrates the mirror.”

The part of the light beam that penetrates the mirror is now sent through the absorbing material layer and then returned to the partially transparent mirror by lenses and another mirror. “The crucial thing is that the length of this path and the position of the optical elements are adjusted in such a way that the returning light beam (and its multiple reflections between the mirrors) exactly cancels out the light beam reflected directly at the first mirror,” explain Yevgeny Slobodkin and Gil Weinberg, the graduate students who built the system in Jerusalem.

The two partial beams overlap in such a way that the light blocks itself. On its own, the partially transparent mirror would reflect a large part of the light, but this reflection is rendered impossible by the other part of the beam travelling through the system before returning to the partially transparent mirror.

Therefore, the mirror, which used to be partially transparent, now becomes completely transparent for the incident laser beam. This creates a one-way street for the light: the light beam can enter the system, but it can no longer escape because of the superposition of the reflected portion and the portion guided through the system in a circle. The light has no choice but to be absorbed, resulting in the entire laser beam being swallowed up by a thin layer that would otherwise allow most of the beam to pass through.

“The system has to be tuned exactly to the wavelength you want to absorb,” says Rotter. “But apart from that, there are no limiting requirements. 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.”

Not even air turbulence and temperature fluctuations can harm the mechanism, as was shown in experiments conducted at the Hebrew University in Jerusalem. This proves that it is a robust effect that promises a wide range of applications – for example, the presented mechanism could be well suited for perfectly capturing light signals that are distorted during transmission through the Earth’s atmosphere. This new approach could also be of great practical use for optimally feeding light waves from weak light sources (such as distant stars) into a detector.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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