By coupling two microlasers together, one would expect more light to be emitted than from a single pumped laser alone. But coupling such lasers can, paradoxically, lead to a "blackout" where they shut each other off. The finding could prove significant for technologies that couple electronics and photonics. Scientists at the Vienna University of Technology (TU Vienna), working with colleagues at Princeton and Yale universities in the US and ETH Zurich, discovered the "laser blackout" effect by using new methods developed at TU Vienna to solve the complicated equations that describe the problem. They found that when one laser is shining and the laser next to it is turned on gradually, complex interactions between the two can lead to a total shutdown of light emission. Surprisingly, pumping the second laser too does not necessarily increase the brightness of the coupled system, and supplying more energy can even reduce the brightness, until both lasers become dark. Two coupled microlasers with light beams (Images: TU Vienna) "When we saw that the two lasers can switch each other off completely, due to the coupling between them, we knew: Either we made a mistake, or this is a spectacular result," said professor Stefan Rotter of TU Vienna. The interplay between the lasers is more complicated than lightwaves interfering with each other and canceling each other out, the researchers said. "This effect is not just about wave interference. It is a combination of interference and light amplification, which can lead to seemingly paradoxical effects," said Matthias Liertzer. Together with Rotter, he studied the behavior of coupled microlasers using computer simulations at TU Vienna's Institute for Theoretical Physics. Matthias Liertzer (left) and Stefan Rotter of Vienna University of Technology's Institute for Theoretical Physics. “The phenomenon is based on what mathematicians call exceptional points [special intersections of surfaces in complex spaces],” Rotter said. “The appearance of such exceptional points in our laser equations can lead to a laser blackout. In this way, we could connect a rather abstract mathematical structure to a measurable phenomenon.” The effect was confirmed in independent calculations by Yale collaborators. In experiments with microlasers, electrical engineers at TU Vienna are now working to verify the theoretical predictions. The findings were published in Physical Review Letters. For more information, visit: www.tuwien.ac.at/en