Controlling Random Lasers

Random lasers, with their very irregular angular emission pattern, are difficult to tune. But a team at Vienna University of Technology has theoretically shown that they can be controlled by actively shaping the spatial pump distribution, giving these exotic light sources the potential to become useful.

With their light emission pattern governed by random scattering, the light emitted by a random laser is as unique as a fingerprint. An understanding of the mechanisms behind the random lasing occurred only recently.

Random lasers are powered by a beam of light from above. Random scatterers inside the laser (yellow dots) lead to chaotic emission of light in all directions. Images courtesy TU Vienna.

In a conventional laser, light is reflected back and forth between two mirrors, amplifying it until a laser beam is formed that exits on one of the two sides. The required energy is optically pumped from an external light source.

A random laser, however, "works without any mirrors,” said professor Stefan Rotter of the Institute of Theoretical Physics at TU Vienna. “It consists of a granular material in which light is randomly scattered and forced onto complicated paths.” The light is amplified along these paths; the position at which it eventually exits the laser depends on the random inner structure of the laser material.

“The essential point is the way in which the random laser is pumped,” Rotter said. “Our idea is to pump the laser not uniformly, but rather with a specific pattern, which is optimized such as to generate exactly the laser beam we want.”

The pumping pattern selectively stimulates certain regions of the random laser, which cooperatively produce light emission in a well-defined direction.

Matthias Liertzer (left) and professor Stefan Rotter.

The team used extensive computer simulations to determine the right pumping pattern for the desired laser beam. “We start with a randomly generated initial pumping pattern and calculate the resulting laser emission. The pumping pattern is then adjusted step by step until the laser sends out light in exactly the desired direction,” Rotter said.

Because all random lasers are different, this optimization process must be carried out for each device individually — but once the solution is known, the same laser beam can be created again and again. In principle, one also could steer the laser beam from a given direction to any other direction by changing the pumping pattern appropriately.

Here, the beam of light is sent through a mask, so that different regions of the laser are pumped with different amounts of energy. This selective pumping leads to the laser emitting a beam of light exactly in the right direction.

Rotter’s team is cooperating with a group in Paris, where random lasers are fabricated and studied in the lab. Together, they want to test their findings experimentally. If the experimental results match their numerical calculations, it would constitute a major step toward practical applications of random lasers, they said.

The work appears in Physical Review Letters.

For more information, visit: www.tuwien.ac.at