Lasers that switch off when their pump power is increased could one day make viable optical switches and logic circuits. Researchers at the Vienna University of Technology, led by professor Stefan Rotter, have demonstrated the phenomenon using a pair of coupled microdisk quantum cascade lasers emitting at terahertz frequencies. The work builds on mathematical models devised by Hakan Türeci, assistant professor of electrical engineering at Princeton University. An electron microscope image shows two microdisk quantum cascade lasers placed 2 µm apart. Images courtesy of the Vienna University of Technology. “Loss is something you normally are trying to avoid,” Türeci said. “In this case, we take advantage of it and it gives us a different dimension we can use — a new tool — in controlling optical systems.” The study involved manipulating regions of gain and loss within the coupled laser system. In experiments, one laser began emitting light when pumped, but when the other was pumped the system switched off. Deeper understanding of gain and loss distribution could make microlasers more efficient, and lead to new types of highly accurate sensors and lab-on-chip applications, the researchers said. The mathematical concepts demonstrated could also be applied to electronics, they said. Manipulating minute areas of gain and loss within individual lasers (shown here as peaks and valleys), researchers were able to create paradoxical interactions between two nearby lasers. “Our approach provides a whole new set of levers to create unforeseen and useful behaviors,” Türeci said. The work was funded by the Vienna Science and Technology Fund and the Austrian Science Fund, as well as by the National Science Foundation through a grant for the Mid-Infrared Technologies for Health, the Environment Center (MIRTHE) at Princeton and by DARPA. The research was published in Nature Communications (doi:10.1038/ncomms5034). For more information, visit www.tuwien.ac.at or www.princeton.edu/engineering.