Intensity Increases With Loss in Microlaser

Increasing energy loss in a laser system could actually enhance its energy, performance and efficiency.

A team from Washington University in St. Louis has discovered a way to reverse, or potentially eliminate, energy loss in laser systems by adding to that loss to increase light intensity.

“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,” said Dr. Lan Yang, an associate professor of electrical and systems engineering. “Instead of the standard method of adding more energy into the system, we’re offering a more energy-efficient method.”

Through several experiments, the researchers employed two tiny, directly coupled silica microtoroid resonators, each joined to a different fiber-taper coupler that aids in guiding light from a laser diode to photodetectors.

In the upper image, two directly coupled whispering-gallery silica resonators are illustrated with two equally distributed supermodes. In the lower image, the introduction of a nanostructure in one resonator makes the fields asymmetrical, resulting in Raman lasing (red line). Courtesy of J. Zhu, B. Peng, S.K. Ozdemir, L. Yang/Washington University.

In estimating the intensity of light in the two microresonators, the researchers found an initial decrease in the total intensity of the resonators; there was then an increase and subsequent rebirth of the strong light intensity as the energy loss was increased.

“The loss added beyond a critical value increased the total light intensity and its distribution between the resonators,” said graduate student Bo Peng.

The energy loss was delivered to one of the microresonators via a chromium-coated silica nanotip. Its position within one of the resonator’s evanescent fields was controlled by a nanopositioner that operates at a 20-nm resolution. Another nanopositioner controlled the coupling strength between the resonators by tuning their distance.

Additionally, the researchers were able to achieve two nonlinear phenomena: the thermal effect, caused by a redistribution of internal energy in a system, and a Raman gain in silica despite increasing loss.

“When we steer the system through the exceptional point, the symmetric distribution of the fields between two resonators become asymmetric,” said research scientist Dr. Sahin Kaya Ozdemir. “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 researchers’ findings could lead to new techniques for controlling and reversing the effects of loss in various other physical systems, including photonic crystals, plasmonic structures and metamaterials.

“Normally loss is considered bad, but we actually take advantage of this and reverse the bad effect. We used the laser to show it,” Yang said.

The work was funded by the Presidential Early Career Award for Scientists and Engineers, the U.S. Army Research Office, the U.S. Department of Energy, the Riken iTHES Project, the MURI Center for Dynamic Magneto-Optics, the Grant-in-Aid for Scientific Research, the Vienna Science and Technology Fund and the Austrian Science Fund.

The research was published in Science (doi: 10.1126/science.1258004).

For more information, visit www.wustl.edu.

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