Energy loss in optical systems, such as lasers, is a chief hindrance to their performance and efficiency and it occurs on an ongoing, frustrating basis.
To help laser systems overcome loss, operators often pump the system with an overabundance of photons, or light packets, to achieve optical gain.
But now scientists have demonstrated a more effective—yet counterintuitive—way to reap energy gains: they add loss to the laser system. In other words, they’ve invented a way to win by losing.
“Too much of something can be really detrimental,” says Sahin Kaya Ozdemir, a research scientist at Washington University in St. Louis. “If you pump in more energy to get more laser intensity, and it’s too strong, you can get a sudden drop in the laser performance.
“It is counterintuitive and paradoxical to see that the lasing starts and its output power becomes higher and higher when more loss is introduced, that is, less pumping is used,” adds Ozdemir. “This turns the conventional textbook understanding of lasers upside down.”
The results were published recently in the journal Science.
Three Experiments
In the first experiment, an international team showed they could change the coupling between two microresonators by changing their distance and introduce on-demand loss controllably to one of them.
In a second experiment, by varying the loss, they manipulated the coupling regimes and estimated the intensity of light in the two resonators and surprisingly found an initial decrease in total intensity of the two resonators followed by an increase, and finally a rebirth of strong light intensity as the loss was increased.
“The loss added beyond a critical value increased the total light intensity and its distribution between the resonators,” says Bo Peng, a graduate student.
In a third experiment, the researchers report achieving two nonlinear phenomena, the Thermal Effect and a Raman gain in silica despite increasing loss.
“Light intensity is a very important parameter in optical systems, and here we have provided a new route to increase light intensity by modulating loss in the system,” says Lan Yang, an associate professor in electrical and systems engineering. “Instead of the standard method of adding more energy into the system, we’re offering a more energy-efficient method.”
Beyond Lasers
Yang says that in addition to lasing improvements, their findings could lead to new schemes and techniques for controlling and reversing the effects of loss in various other physical systems, such as in photonic crystal activities, plasmonic structures, and metamaterials.
The experimental system that the researchers used consists of two tiny directly coupled silica microtoroid (doughnut-shaped) resonators, each coupled to a different fiber-taper coupler that aids in guiding light from a laser diode to photodetectors; the fiber is tapered in the middle so that light can between the fibers and the resonators.
Yang says the concept will work in any coupled physical system.
Loss is delivered to one of the microresonators by a tiny device, a chromium-coated silica nanotip, whose position within the evanescent field (leaked-out light) of one of the resonator was controlled by a nanopositioner that operates at a minuscule 20-nanometer resolution.
“Chromium is used because it’s a strongly absorbing material at a wavelength of 1550 nanometers, and it gives a good dose of loss,” Peng explains.
The ‘Exceptional Point’
Another nanopositioner controls the coupling strength between the resonators by tuning their distance.
The loss-gain phenomenon occurs near a feature called the exceptional point, which has a dramatic effect on a system’s properties. The exceptional point has contributed to a number of counterintuitive activities and results in recent physics studies.
“When we steer the system through the exceptional point, the symmetric distribution of the fields between two resonators become asymmetric,” Ozdemir says. “Asymmetric distribution leads to field localization, increasing the light intensity in one of the resonators, in this case the resonator with less loss.
“As a result, all nonlinear processes, which depend on the intensity of light, in that subsystem become affected.”
“The beauty of this work is in how we came to provide new schemes and techniques to engineer a physical system by controlling loss,” Yang adds. “Normally, loss is considered bad, but we actually take advantage of this and reverse the bad effect. We used the laser to show it.”
The Presidential Early Career Award for Scientists and Engineers, Army Research Office, US Department of Energy, RIKEN iTHES Project, MURI Center for Dynamic Magneto-Optics, Grant-in-Aid for Scientific Research, Vienna Science and Technology Fund, and the Austrian Science Fund supported the project, which also included researchers from RIKEN in Japan and the Vienna University of Technology in Austria.
Source: Washington University in St. Louis. Republished from Futurity.org under Creative Commons License 3.0.
*Image of “industrial laser“ via Shutterstock
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