Coupled lasers cancel each other out

Ashley N. Paddock, ashley.paddock@photonics.com

The discovery that coupling two microlasers shuts them both off instead of emitting more light could prove significant for technologies that combine electronics and photonics.

The “laser blackout” effect, discovered by scientists at the Vienna University of Technology (TU Vienna), working with colleagues at Princeton and Yale universities in the US and at ETH Zurich, uses new methods developed at TU Vienna to solve the complicated equations that describe the problem.

“We were interested in what happens when each of the lasers in the coupled pair is pumped independently of the other one,” Matthias Liertzer of TU Vienna said in an interview with Photonics Spectra. “Although such setups have already been realized experimentally, we are not aware of any thorough theoretical investigation that has [been] performed with regard to pumping the disks individually.

“It was during these investigations that we noticed the effect of the laser turning off, although the overall pump strength in the system is increased. Since this behavior is quite counterintuitive, we first checked whether there was no mistake in our calculations, but soon realized that the observed effect can be linked to the occurrence of an ‘exceptional point’ in the underlying lasing equations.”

Two coupled microlasers with light beams. Courtesy of TU Vienna.

Exceptional points occur when two resonator modes trapping light for a comparatively long time amplify enough to be brought above threshold and begin to lase.

“Researchers have studied exceptional points in lasers before; however, they needed to change the shape of the laser in order to observe the influence of such a point,” Liertzer said. “This is experimentally cumbersome and can only be performed using mechanically deformable laser cavities. We go beyond this limitation by demonstrating that the effects of an exceptional point can be seen by a suitable variation of the applied pump.”

The team discovered 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 does not necessarily increase the brightness of the coupled system, and supplying more energy can reduce the brightness until both lasers become dark.

The interplay between the lasers is more complicated than lightwaves interfering with one another and canceling each other out, the researchers say.

“This effect is not just about wave interference,” Liertzer said. “It is a combination of interference and light amplification, which can lead to seemingly paradoxical effects.”

Next, the scientists will collaborate with colleagues from the Photonics Institute at TU Vienna on an experimental realization of the effect. After that, they plan to investigate whether exceptional points can be found in more intricate structures such as random lasers, Liertzer said.

He believes that their paper, which was published in Physical Review Letters (doi: 10.1103/PhysRevLett.108.173901), will bring together two active communities.

“On the one hand, there is the community of non-Hermitian physics, where exceptional points, gain/loss structures, etcetera, are at the center of attention,” he said. “On the other hand, there is the very large laser community with over 50 years of experience and a vast number of applications where lasers are employed. With our paper, we bridge these two communities by showing how the interesting non-Hermitian physics of exceptional points carries over to lasers.”