Physicists build random anti-laser

Scientists in Austria have built the inverse of a laser, an anti-laser.

Lasers turns energy into a specific light frequency. The device developed by researchers at the Vienna University of Technology does the opposite, absorbing a specific color of light and scattering nearly all of the energy.

The anti-laser technology -- described this week in the journal Nature -- may offer applications in a variety of electronic and optical fields.

"Every day we are dealing with waves that are scattered in a complicated way -- think about a mobile phone signal that is reflected several times before it reaches your cell phone," Stefan Rotter, a professor at TU Vienna's Institute for Theoretical Physics, said in a news release. "The so-called random lasers make use of this multiple scattering. Such exotic lasers have a complicated, random internal structure and radiate a very specific, individual light pattern when supplied with energy."

Rotter and his colleagues used the logic of the laser and worked backwards, constructing a model that showed the inner structure of an anti-laser device could be designed to absorb a specific light frequency.

"Because of this time-reversal analogy to a laser, this type of absorber is called an anti-laser," said Rotter. "So far, such anti-lasers have only been realized in one-dimensional structures, which are hit by laser light 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 specially tailored wave. That way, the concept can be used for a wide range of applications."

More than just a light absorber, an anti-laser works to effectively dissipate the energy of the lightwaves it swallows up.

"There is 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 the partial waves can get out at the end," Rotter said.

Researchers at TU Vienna teamed with scientists at the University of Nice in France to confirm the mathematical logic of the anti-laser and develop a strategy for turning the concept into an actual device.

The anti-laser consists of an absorbing antenna inside a microwave chamber. The chamber is surrounded by Teflon cylinders. The cylinders reflect the scattered light waves back toward each other, creating a complex frequency pattern.

Tests proved the device works as theorized.

"First we send microwaves from outside through the system and measure how exactly they come back," said researcher Kevin Pichler. "Knowing this, the inner structure of the random device can be fully characterized. Then it is possible to calculate the wave 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 percent of the incident signal."

According to the device's creators, the technology could have a variety of applications, including uses in communication technologies and medicine.

"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," Rotter said. "Also in medicine, we often deal with the task of transporting wave energy to a very specific point -- such as shock waves shattering a kidney stone."

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