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Scientists at TU Vienna develop ‘random anti-laser’

13 Mar 2019

Laser concept is time-reversed: light source becomes perfect light absorber; device could offer applications in mobile and medical.

The concept of the laser can be time-reversed with the perfect light source becoming the perfect light absorber, according to a team of scientists at the Technical University of Vienna (TU Wien), Austria, who say they have found a way to build such an anti-laser, based on random scattering.

The anti-laser method was developed by the team in Vienna with the help of computer simulations, and confirmed by experiments in cooperation with the University of Nice. The team says this “opens up new possibilities for all areas in science and engineering where wave phenomena play an important role.” The method has been published in Nature.

Random structures that absorb waves

"Waves that are being scattered in a complex way are really all around us, such as a mobile phone signal that is reflected several times before it reaches your cell phone," commented Prof. Stefan Rotter from the Institute for Theoretical Physics of TU Wien.

"We can make practical use of this multiple scattering in random lasers. Such exotic lasers are based on a disordered medium with a random internal structure that can trap light and emit a complicated, system-specific laser field when supplied with energy."

With mathematical calculations and computer simulations, Rotter's team showed that this process can also be reversed in time. Instead of a light source that emits a specific wave depending on its random inner structure, it is also possible to build the perfect absorber, which completely dissipates an incoming wave with the right wavelength and spatial pattern in accordance with its characteristic internal structure.

Rotter explained, “The easiest way to think about this process is in terms of a movie showing a conventional laser sending out laser light, which is played backwards.”

"So far, anti-lasers have only been realized in one-dimensional structures onto which laser light was directed from opposite sides. Our approach is much more general: we were able to show that even arbitrarily complicated structures in two or three dimensions can perfectly absorb a suitably tailored wave, promising a much wider range of applications."

Perfect wave absorber

The main result of the research project is that for every object that absorbs waves sufficiently strongly, a certain wave front can be found, which is perfectly absorbed by this object. "It would be wrong to imagine, however, that this is all about the absorber just being strong enough to swallow every incoming wave," explained Rotter.

"Instead, the [anti-laser] effect is due to a complex scattering process in which the incident wave splits into many partial waves, which then overlap and interfere with each other in such a way that none of these partial waves can get out at the end," he added.

In this sense, the absorber used in the anti-laser can be rather weak; for example, a simple antenna that absorbs electromagnetic waves can already do this.

To test their calculations, the team worked together with the University of Nice. Kevin Pichler, the first author of the Nature article, currently working in Rotter’s team, spent several weeks with Prof. Ulrich Kuhl at the University of Nice to implement the theory in a microwave experiment.

Laser design

The Random Anti-Laser built consists of a microwave chamber with a central absorbing antenna, surrounded by a random arrangement of Teflon-coated cylinders. Like stones in a pool of water, on which water waves are deflected, these cylinders scatter microwaves and create a complicated wave pattern.

Pilcher explained, "First we send microwaves onto the system from the outside and measure how exactly they come back. This knowledge then allows us to characterize the random structure and to calculate the wave front that is completely swallowed by the central antenna at the right absorption strength. In fact, when implementing this protocol in the experiment, we find an absorption of approximately 99.8% of the incident signal."

While anti-laser technology is still in its early stage, it is easy to think of potential applications. "Imagine, for example, that you could adjust a cell phone signal exactly the right way, so that it is perfectly absorbed by the antenna in your cell phone," says Stefan Rotter. "Also in medicine, we often deal with the task of delivering wave energy to a very specific point - such as shock waves shattering a kidney stone."

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