Researchers from Princeton University and Vienna University of Technology have demonstrated a laser system that paradoxically turns off, rather than becoming brighter, when more power is added.
The system is comprised of two lasers measuring roughly the width of a human hair that are separated by a distance 50 times smaller than the lasers themselves. In the lab, researchers added electric current to one laser, causing it to emit light; however, when the second laser was pumped with current, instead of also turning on and emitting light, the whole system shut off.
“This is not the normal interference that we know,” Princeton assistant professor of electrical engineering Hakan Türeci said, referring to the phenomenon of light or sound waves from separate sources cancelling each other out. Instead, in this case, the cancellation comes from the careful distribution of energy loss within an overall system that is being amplified.
“Loss is something you normally are trying to avoid,” Türeci added. “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 accomplishment will likely lead to new ways of controlling the interaction between electronics and light to improve communications and information processing.
Türeci has been studying mathematical models for a number of years, during which he established a mathematical framework for understanding the unique properties and complex interactions that are possible in extremely small lasers in 2008. This work opened the door to manipulating gain or loss—in particular, their spatial distribution—within a laser system to increase efficiency.
According to a Princeton University blog post, the researchers’ ideas for taking advantage of loss stems from their examination of “non-Hermitian” matrices, mathematical constructs in which a normally symmetric table of values becomes asymmetric. The work is also related to certain concepts of quantum physics, in which “the fundamental symmetries of time and space in nature can break down, even though the equations used to describe the system continue to maintain perfect symmetry.”
In recent years, Türeci and his collaborators at Vienna worked to show how the mathematical anomalies at the center of the work, called “exceptional points,” could be manifested in an actual system. The team published a 2012 paper in the journal Physical Review Letters demonstrating a computer simulation of a laser system that turns off as energy is added. Now, in their newest paper published in the journal Nature Communications, the researchers have created an experimental realization of their theory using a light source known as a quantum cascade laser.
The researchers believe the results could be of particular use in better understanding and controlling how optical devices on a single computer chip—known as a “lab-on-a-chip” device—interact. In addition, taking advantage of the way loss and gain are distributed within tightly coupled laser systems could lead to the development of new types of extremely accurate sensors.
“Our approach provides a whole new set of levers to create unforeseen and useful behaviors,” Türeci said.
